c1cs15047b
DESCRIPTION
rtertteTRANSCRIPT
This article was published as part of the
Molecule-based magnets themed issue
Guest editors Joel S. Miller and Dante Gatteschi
Please take a look at the issue 6 2011 table of contents to
access other reviews in this themed issue
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online / Journal Homepage / Table of Contents for this issue
3336 Chem. Soc. Rev., 2011, 40, 3336–3355 This journal is c The Royal Society of Chemistry 2011
Cite this: Chem. Soc. Rev., 2011, 40, 3336–3355
Molecular spintronicsw
Stefano Sanvito*
Received 23rd February 2011
DOI: 10.1039/c1cs15047b
The electron spin made its debut in the device world only two decades ago but today our ability
of detecting the spin state of a moving electron underpins the entire magnetic data storage industry.
This technological revolution has been driven by a constant improvement in our understanding on
how spins can be injected, manipulated and detected in the solid state, a field which is collectively
named Spintronics. Recently a number of pioneering experiments and theoretical works suggest that
organic materials can offer similar and perhaps superior performances in making spin-devices than
the more conventional inorganic metals and semiconductors. Furthermore they can pave the way
for radically new device concepts. This is Molecular Spintronics, a blossoming research area aimed
at exploring how the unique properties of the organic world can marry the requirements of
spin-devices. Importantly, after a first phase, where most of the research was focussed on exporting
the concepts of inorganic spintronics to organic materials, the field has moved to a more mature
age, where the exploitation of the unique properties of molecules has begun to emerge. Molecular
spintronics now collects a diverse and interdisciplinary community ranging from device physicists
to synthetic chemists to surface scientists. In this critical review, I will survey this fascinating,
rapidly evolving, field with a particular eye on new directions and opportunities. The main
differences and challenges with respect to standard spintronics will be discussed and
so will be the potential cross-fertilization with other fields (177 references).
1. Introduction
Most electronic devices, either for logic or sensing applications,
operate on the principle of detecting the variation of an
electrical current with an external stimulus. This can be the
gate potential in a transistor or the presence of the object to
detect in a sensor. Regardless of its nature the stimulus acts to
produce a change in the device electrostatic profile. When the
stimulus has a magnetic origin, then the detection is more
complicated, simply because a magnetic field is less efficient
then an electric one at driving the electron motion. For
example it takes a magnetic field of 103 T to produce a Lorentz
force equal to that of an electric field of 0.1 V nm�1 (assuming
that the electron moves at 105 m s�1). This means that for
magnetic detection a property different from the electrical
charge must be used. This is the spin.
The importance of the spin degree of freedom for the electron
motion in transition metal magnets was acknowledged a long
time ago by Mott,1 who first expressed the ‘‘two fluids’’
concept. This establishes that the current in a magnet is carried
by two fluids (interacting), characterized by electrons having
opposite spin directions. Such a concept was exploited only
50 years later with the discovery of the giant magneto-resistance
(GMR) effect in magnetic multilayers,2,3 which effectively
demonstrated that the electrical resistance of a magnetic device
can be modified by changing its magnetic texture. This signified
the beginning of the field of magneto-electronics or, as more
School of Physics and CRANN, Trinity College, Dublin 2, Ireland.E-mail: [email protected]; Fax: +353-1-6711759;Tel: +353-1-8963065w Part of the molecule-based magnets themed issue.
Stefano Sanvito
Stefano Sanvito completed hisundergraduate studies in Milan(Italy), before moving to theUniversity of Lancaster (UK),where he obtained a PhD intheoretical Physics. In 2002 hejoined the School of Physics atTrinity College Dublin as aLecturer, after having spenttwo successful years as apostdoctoral fellow at theUniversity of California SantaBarbara (USA). In 2006 hebecame associated Professorin Physics and in 2009 DeputyDirector of the Center for
Research on Adaptive Nanostructures and Nanodevices (CRANN).Stefano Sanvito leads the Computational Spintronics Group, alarge and dynamical theoretical/computational research groupthat investigates elementary properties of materials and of nano-devices using computer simulations. In particular the groupdevelops and maintains the code Smeagol, a state of the artpackage for materials specific electron transport calculations.
Chem Soc Rev Dynamic Article Links
www.rsc.org/csr CRITICAL REVIEW
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 3336–3355 3337
commonly known, of Spintronics.4 At present, spintronics
encompasses a rather large and diverse range of phenomena
and experimental techniques. However, broadly speaking,
they all address the problem of injecting, manipulating and
detecting spins in the solid state environment.
Historically, spin-related phenomena have been first studied in
transition metals and subsequently in inorganic semiconductors.5,6
More recently considerable attention has been dedicated to less
conventional materials sets. These for instance include ferroelectric
insulators, presenting a spontaneous electrical polarization,7
and magnetic semiconductors, offering spin-selectivity to a tunnel
current.8 Among the new materials, organic molecules have
recently emerged as a completely revolutionary platform for
spintronics. This field, inspired by pioneering spin-transport
experiments9 and early theoretical predictions10 has already taken
the evocative name of Molecular Spintronics.
The rationale for spintronics to go from metals to inorganic
semiconductors is that of creating a single materials platform
where to integrate information-processing, traditionally
implemented in non-magnetic semiconductors, with data-storage,
commonly the domain of magnetic metals.11 Semiconductors,
in general, are more versatile than metals, since their
electronic properties can be modified by little changing their
electronic structure (the carrier density for instance). Further-
more the spin-relaxation time is usually much longer in
semiconductors than in metals12 and in a semiconductor one
can hope of controlling the spin–orbit interaction and perform
spin-manipulation13 (this last possibility has turned out to be
rather challenging in practice). The addition of other materials
to the spintronics portfolio is also driven by specific expectations.
For instance the idea of using ferroelectric insulators underpins
the prospect of making multi-functional devices, where the
magnetic information can be read and/or stored electrically.
But, what is the rationale for going organic?
In general, organic molecules can be prepared in a practically
infinite range of types and combinations, their properties can
be finely tuned and their degree of purity may have no equals
in the inorganic world. Furthermore molecules are synthesized
at low temperature and in the same conditions they can also be
processed. However, the most important feature is that their
electronic properties and their functionalities span an extremely
large range. For instance the conductivity of organic polymers
can be engineered over fifteen orders of magnitude14 and they
can display both magnetism15 and ferroelectricity16 at room
temperature. Furthermore, since most of the organic materials
are made of elements populating the upper rows of the
periodic table, both spin–orbit and hyperfine interaction, the
two principal ways for spins to interact with the environment,
are weak. As such organic materials appear as an incredibly
versatile and unique playground for exploring new spintronics
concepts and/or for implementing existing ones.
In this review I will discuss the most recent developments in
the field of molecular spintronics. In particular I will highlight
the differences with conventional spintronics in inorganic
metals and semiconductors, so that the reader will form an
opinion on what organic materials have different to offer to
spintronics. I will start by discussing some general concepts
common to both inorganic and organic spintronics and review
the main mechanisms that spins have available to interact with
the external world. Then, I will first overview the most recent
advances in large-scale devices and finally move to single
molecule junctions. Throughout the discussion I will also
make connections to other research fields which share with
molecular spintronics experimental techniques and concepts.
Little discussion will be dedicated to carbon-only inorganic
macromolecules such as graphite, graphene and carbon nano-
tubes. For these we refer the reader to the abundant recent
literature.17
In closing this introduction I wish to mention that a number
of comprehensive reviews on different aspects around molecular
spintronics have appeared recently. These include spin-transport
in organic semiconductors18,19 and in single molecules,20,21
quantum computing with molecules,22–24 molecular materials,25
organic radicals on surfaces,26 and spin-polarized scanning
tunnel microscopy (SP-STM).27
2. From spintronics to organic spintronics
There are a number of concepts, related to how spins move in
a solid state environment, which are common to any spin-devices
and can be applied to any materials classes. These define the
entire field of spintronics and will be the subject of the first half
of this section. Then I will discuss how specific materials
properties affect the dynamics of spins in materials, a discussion
which will reveal how peculiar organic molecules are with
respect to any other media. Finally I will overview the most
recent experiments and theories on large-area organic devices.
As such, this section describes how the ideas of conventional
spintronics have been exported to devices made of organic
semiconductors, i.e. it describes the evolution from spintronics
to Organic Spintronics.
2.1 Basic concepts
The general idea behind spintronics is that of detecting the
response of spins to an external stimulus and, by using such a
platform, implementing logic (either classical or quantum),
memory and sensing capabilities. Therefore, one has first to
understand how spins evolve in time in a solid state environment,
in particular when these are associated to moving electrons.
An ideal spintronics experiment consists of three main actions
(see Fig. 1). First one has to generate a (non-equilibrium) spin
population within a given material (injection), i.e. spins must
be prepared in the desired initial configuration. Then an
external stimulus should be applied to alter the initial spin
population in a controllable way (manipulation) and finally
the result of the manipulation should be detected (detection).
Of course the manipulation step can be eliminated and one
may simply look at how spins evolve, i.e. how they reach
equilibrium again. Importantly for each one of these steps a
number of experimental strategies are possible, depending on
the specific nature of the materials set used. Optical pump–
probe techniques based on Kerr/Faraday rotation are the
standard for inorganic semiconductors,28 while for metals
both electrical29 and optical methods30 can be used. In organic
molecules, with a very few exceptions, only electrical injection/
detection is possible, so that only this aspect will be discussed
in some details here.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
3338 Chem. Soc. Rev., 2011, 40, 3336–3355 This journal is c The Royal Society of Chemistry 2011
The prototypical electric spin-device is the spin-valve,31
which consists of two magnetic materials, usually metals,
sandwiching a non-magnetic one (middle panel of Fig. 1).
An electron current drives a non-equilibrium spin population
from the injector to the non-magnetic material. If this reaches
the detector the resistance of the entire device will depend on
the mutual orientation between the magnetization vectors
of the injector and the detector. This occurs essentially because
the individual resistances of the two spin-channels, in Mott’s
spirit,1 are different, so that a different equivalent spin-resistance
circuit is associated to the different magnetic orientations of the
electrodes magnetizations.32
In general the relative conductance of the two spin-channels
of a magnetic material defines its spin-polarization, P. This is a
property related to the electronic structure of the material
itself and to the experimental technique used to measure such a
polarization. In general one can write33
Pn ¼N"v
n" �N#v
n#
N"vn" þN#vn#; ð1Þ
where Ns is the material’s density of state (DOS) at the Fermi
level (EF) for spin s (s = m for majority electrons and s = k
for minority) and vs is the spin-dependent Fermi velocity. The
Fermi velocity is weighted differently depending on the particular
experiment, with n = 0 for photoemission measurements and
for tunneling across amorphous barriers, n = 1 for ballistic
transport and n = 2 for diffusive conduction. Note that, as a
consequence of the definition of Pn, the same material may
have a substantially different polarization depending on the
specific experiment carried out or on the specific length-scale
examined.34
Typically the place where spins are injected is different from
the one where they are detected so that electrons spins must be
driven across the non-magnetic material. In such a transfer
process they will continuously attempt to reach their equilibrium
state by interacting with the environment. At equilibrium in a
non-magnetic material there is no spin imbalance so that
during their motion electrons lose their initial spin-polarization.
Two quantities characterize the interaction of spins with their
surrounding: the spin-relaxation time, tS, and the spin-relaxation
length, lS. The spin-relaxation time is the average time that an
electron spin takes before changing its original direction. Note
that, in analogy with nuclear magnetic resonance,35 also in
spintronics one can define the longitudinal, T1, and the
transverse, T2, spin-relaxation times. However in a spin-valve
experiment there is no direct access to these two quantities
separately, since the injected electrons do not have a defined
phase relation (they are not coherent). Hence the more general
definition of tS must be used. Instead the average distance
travelled by a spin defines the spin-relaxation length. Clearly
tS and lS are simply related by lS = mtS, where m is the average
electron velocity.
The magnitude of lS relatively to the spin-valve non-
magnetic spacer thickness, L, determines the ability of the
spin-valve to work. In fact if lS o L, the spin-polarization of
the injected current will be completely lost by the time the
electrons reach the detector. As such the resistance of the
entire device will not depend on the direction of the magneti-
zation vectors of the electrodes. In contrast if lS 4 L some
spin-polarization will survive the motion in the non-magnetic
spacer and the total resistance will depend on the spin-valve
magnetic state. This simple concept however suffers of another
obstacle, present even if lS = N. This is known as the
resistance mismatch problem.36
The equivalent resistance of a spin-valve is obtained by
adding in series the resistances of the electrodes (spin dependent)
and that of the spacer (spin independent). As such, if the
resistance of the spacer is much larger than that of the
electrodes, the total resistance of the device will be only weakly
spin dependent. Unfortunately this is the case of inorganic
semiconductor spacers contacted by magnetic metals, so that
the direct spin-injection from metals to semiconductors has
been traditionally problematic and one usually needs to
include large spin dependent barriers in the device stack.37
Since the typical resistivity of an organic semiconductor is
significantly larger than those of its inorganic counterparts an
even more severe problem is expected in organic spin-valves.
This is however not the case, as we will see later on.
A final mention goes to tunneling junctions. In this case the
spacer is a wide-gap material and the electrons are not injected
but simply tunnel between the two electrodes. The MR is
determined by the electronic structure of the entire junctions.
When the tunnel barrier is amorphous the MR is controlled by
the spin-polarization of the electrodes,38 while for crystalline
barriers the precise symmetry of the wave-function of the tunneling
Fig. 1 Different experimental strategies for studying spin-dynamics
in semiconductors. In inorganic semiconductors both the creation of a
spin-polarized wave-packet and its detection at a different position can
be performed optically (upper panel), by using respectively circularly
polarized light (injection) and Kerr (or Faraday) rotation (detection).
In the case of spin valves (middle panel) both detection and injection
are performed by magnetic metals. Finally hybrid strategies do exist,
for instance where the injection is electrical and the detection is optical
(lower panel).
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 3336–3355 3339
electrons dominate the transport.39–41 In tunnel junctions
made with organic materials no fully crystalline structure
can be made and most of the properties are dominated by
the interface between the electrodes and the relevant molecules
(see next sections).
2.2 Materials and devices considerations
Electron transport in organic materials is substantially different
from that in inorganic semiconductors and spin transport is no
exception. The conventional band picture, where electrons or
holes move as quasi-free particles (the effects of the band-
structure can be incorporated into the effective mass describing
the bands curvature) is replaced by one where electron hopping
between localized molecular orbitals provides the channel for
the electrons to move across the material. Thus, the typical
mobility of 450 cm2 V�1 s�1 for p-doped Si must be compared
with that of rubrene, one of the most conducting organic
semiconductors, which is only around 10 cm2 V�1 s�1.
The electrical response of a typical vertical device (a spin-valve
for example) made with organic materials depends on how
extended the transport channel is with respect to the coherence
length of the hopping process. If the device dimensions are
comparable with the coherence length, then the main transport
mechanism will be tunneling. In this case the conductance is
only weakly temperature dependent, it is proportional to
the device area and it decays exponentially with the barrier
thickness.42 Otherwise, when the coherence length is shorter
than the device size, the transport mechanism will be incoherent
electron diffusion. Also in this case the conductance is proportional
to the device area, but now it scales as a power low with both
the temperature and the layer thickness.43 Hence, establishing
the transport mechanism should always be the first step
in the characterization of an organic spin-valve,44 since there
is no spin-injection in the tunneling limit in contrast to the
diffusive one.
Let us now turn our attention to the spin-transport properties
of organic materials. These are summarized in Fig. 2, where we
present the lS–tS diagram for a representative sample of
different materials. The overwhelming message emerging from
the picture is that organic materials occupy the top-left corner,
i.e. they are characterized by long spin-diffusion times but
rather short spin-diffusion lengths. The relationship between
lS and tS can be easily rationalized by fact that the mobility in
organic materials is usually rather poor (note that for many
data in Fig. 2, tS is not directly measured but simply inferred
from the values of lS and m). However, why is the spin-
relaxation time typically so long?
Long tS’s in organic materials are well known in the electron
paramagnetic resonance (EPR) community,55 and they are
understood in terms of the absence of any efficient mechanism
for spin-relaxation. Spin–orbit interaction, critical in inorganic
semiconductors, is rather weak in organic media, mostly
because it scales as Z4 with the atomic number, Z, and because
most organic molecules are made of elements in the upper
rows of the periodic table. Just as an example, one should
recall that spin–orbit interaction produces a splitting of the
valence band of GaAs of 340 meV, while in C-diamond this
is only 13 meV.19 Notably spin–orbit interaction plays a
ubiquitous role in spintronics. On the one hand it is usually
the largest source of spin-dephasing, at least in inorganic
semiconductors, and on the other hand it is the main interaction
allowing one spin-manipulation and optical spin-detection.12
Thus, such a lack of significant spin–orbit interaction makes
polarized-light pump–probe optical techniques ineffective for
organic materials and most of the standard optical charac-
terization tools designed for the inorganic world cannot be
adopted for molecules.56
Also hyperfine interaction is rather inefficient, since there
are only a few nuclei of light elements possessing a nuclear spin
(except for H) and the typical molecular orbitals responsible
for electron transport are p-type delocalized mostly over
C atoms. Still hyperfine interaction appears to be as a key
ingredient for explaining the almost universal presence of the
low-field room-temperature MR in organic materials57 known
as the organic magneto-resistance (OMAR) effect.58
A further source of spin-scattering is provided by para-
magnetic impurities. The efficiency of such a spin-scattering
channel depends on the overlap between the electronic wave-
functions of the hopping electrons and that of the impurity.
This is expected to vary from material to material and general
rules are difficult to draw. Also in this case H is the most
notable scattering-center as well as spin-radicals, which might
be abundant in molecular materials.55 Note that scattering
to impurities overall contributes to the spin-relaxation of
conducting electrons proportionally to both the impurity
concentration and the electron density.
A natural question arising now is on how the physical–
chemical properties of molecular materials affect their spin-
transport characteristics. In general the electron transport
Fig. 2 Spin-relaxation time, tS, against spin-diffusion length, lS, for
different classes of materials. Note that non-magnetic organic materials
occupy the upper left corner, i.e. they are characterized by long tS andshort lS. The picture is adapted from ref. 44. Data in the figure
correspond to the following references: a,45 b,12 c,46 d,47 e,48 f,49 g,50
h,51,52 i,53 m,9 n.54 G. Szulczewski, S. Sanvito and J. M. D. Coey,
Nature Materials, 2009, 8, 693–695. Copyright (2009) by the Nature
Publishing Group.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
3340 Chem. Soc. Rev., 2011, 40, 3336–3355 This journal is c The Royal Society of Chemistry 2011
in organic media is highly sensitive to the molecules packing,
to their mutual interaction, to doping and to the system
dimensionality. Relatively high mobility is usually guaranteed
by p-conjugation and increases as the material’s crystallinity
improves. Currently the highest mobility ever recorded for an
organic field effect transistor is for rubrene single-crystals60
and it is of the order of 40 cm2 V�1 s�1. Much less however is
known about the influence of the physical–chemical properties
of molecular materials on their spin-transport. Apart from the
already mentioned relationship between lS and tS, which
essentially establishes that the spin-diffusion length in organic
materials is mobility limited, little is known about the role of
crystallinity and chemical composition over tS. In the case of
conducting polymers there is a significant body of information
obtained with EPR,55 which enables one the study of both spin
and spinless nonlinear excitations. The ratio of these quasiparticles
depends sensitively on various properties of the polymer and
the dopants introduced into it. As such both lS and tS becomes
sensitive to the polymer physical–chemical properties. Importantly
conducting polymers are quasi-1D objects and most of their
characteristics are determined by their dimensionality, so that
the known EPR results do not directly transfer to 3D molecular
crystals.
In summary organic materials are characterized by their
ability of sustaining long-living spin-states, since all the possible
mechanisms for spin-scattering are weak. This means that in
general there is not a single dominant interaction limiting tS,but a combination of all the possible scattering events
determines the dynamics. For instance spin–orbit interaction
has been recognized as the main source of spin-dephasing in
ultra-pure carbon nanotubes,59 but strong hyperfine coupling
can also be present in 13C-enriched ones.61 Similar experiments
have been carried out in either protonated or deuterated
polymers with the similar conclusion that hyperfine coupling
can play a crucial role in spin-dephasing.62 At the same time
however there is also a significant body of evidence in favor of
spin–orbit interaction.
One can then look ahead and ask what organic molecules
can bring to spintronics, or in other words, what range of
devices organic spintronics can embrace. Clearly whatever is
the application, this should benefit from the long tS but
should not be limited by the short lS. As such one should
envision applications where spins are manipulated or detected
not too far from the point of injection. This essentially means
making nano-scale devices where the typical channel length is
smaller than 20 nm. An intriguing prospect is that of coherent
spin-manipulation by using electron spin resonance (ESR)
either electrical or optical detected,63 which may lead to new
logic devices. At present only a few proposals for spin-logic
have been brought forward,64 but the situation might rapidly
change. Intriguingly the same ESR techniques can be used
as diagnostic tools for closely related technologies such as
organic light emitting diodes and photovoltaic cells just to
name two of them. A second option is that of looking at
single molecule devices. Here the issue of the injection and
spin-diffusion length becomes immaterial but one has to
understand how spin-manipulation can be carried out when
the electrons spend only a tiny amount of time on the active
region of the device (the molecule). The first class of devices is
discussed in the next section while single molecule junctions
are reviewed in section 3.
2.3 Early experimental progress discussed by concepts
The first demonstration of MR in an organic spin-valve was
provided by a lateral device and dates to almost a decade ago.9
The electrodes material of choice was La0.7Sr0.3MnO3,
while the organic layer was composed of sexithiophene (T6).
The typical mobility of T6 is of the order of 10 cm2 V�1 s�1
(p-type conduction), so that the appearance of a MR at room
temperature for channels with lengths ranging between 100 nm
and 200 nm was a big surprise. In fact in this situation of a
poorly conductive channel the resistance mismatch argument36
predicts essentially no MR. Notably a spin-valve can still
support MR even in the presence of large resistance mismatch
if the electrode is a perfect half-metal.36,65 This is, in principle,
the case for La0.7Sr0.3MnO3. However, one has to realize that
the half-metallicity of the current (100% spin-polarization) is
strictly valid only at low temperature even for band half-
metals. Furthermore it generally decreases as the temperature
gets larger. Recent experiments66,67 correlate the detection of
the MR to the persistence of the surface magnetism of
La0.7Sr0.3MnO3. In one case66 the MR is completely lost
at about 220 K, while in another it survives up to room
temperature,67 with the difference between the two experiments
being rooted in the different surface preparation. In any case
both experiments showMR at temperatures where the injected
current has a polarization smaller than 100%, i.e. they are in
defiance of the resistance mismatch argument. Such an early
experiment then posed the question of whether or not injection
was really occurring.
A second problem originating from this early experiment is
that there was no direct correlation between the MR and the
magnetic switching of the electrodes’ magnetization. This
is what defines the spin-valve operation. In particular the
resistance vs. magnetic field curve of a spin-valve should
display a characteristic ‘‘butterfly’’ shape, with abrupt changes
in resistance as the magnetic field sweeps across the coercive
field of the electrodes [see Fig. 3 top panel]. A clear demon-
stration of the spin-valve effect was provided two years later
for a vertical device using La0.7Sr0.3MnO3 and Co as the two
electrodes’ materials and tris(8-hydroxyquinoline)aluminium(III)
(Alq3) as organic spacer. In this case a significant MR was
found at low temperature for an organic layer thickness of
about 100 nm and persisting, although severely reduced, up to
thicknesses of about 200 nm. Considering that the typical
mobility of Alq3 (10�5 cm2 V�1 s�1, n-type) is significantly
lower than that of T6, the possibility of spin-injections appeared
even more puzzling.
An additional puzzling aspect of this second early experi-
ment was the fact that the MR had a negative sign. If one uses
the standard definition of MR, MR ¼ Rð0Þ�RðHÞRð0Þ , where R(H) is
the device resistance in a magnetic field H, then negative MR
means that an increase in magnetic field from H = 0 produces
a decrease in the device resistance. Since the two abrupt
changes in resistance in the R–H plot are attributed to the
switching of the magnetization of one of the two electrodes
towards the external field, one then concludes that the resistance
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 3336–3355 3341
is lower when the magnetization vectors of the two electrodes
are anti-parallel to each other. This is in contrast to several
experimental observations in inorganic spin-valves,68,69
although it can be accounted for by a particular band alignment
between the organic layer and the electrodes.70
In fact by assuming tunneling transport one can express the
MR in terms of the electrode spin-polarizations, P1 and P2,
MR ¼ Rð0Þ � RðHÞRð0Þ ¼ 2P1P2
1þ P1P2; ð2Þ
where it is understood that the H = 0 state corresponds to a
perfect antiparallel alignment of the magnetization vectors of
the two electrodes, while R(H) is taken for a perfect parallel
configuration.38 If the spin-polarizations of the two electrodes
have opposite signs (one electrode is a spin-majority conductor
while the other is a spin-minority one), then a negative MR
will be expected. Such an interpretation however is in contrast
to the experimental evidence for majority spin injection at the
Co/Alq3 and Co/Al2O3/Alq3 interfaces42 and to the widely
accepted fact that La0.7Sr0.3MnO3 is a majority conductor.
Since negative MR has been found in many La0.7Sr0.3MnO3/
Alq3/INS/Co junctions (INS =Al2O3, LiF)51,67,71–73 a common
explanation must be found. Two elements may contribute
to solving the puzzle. Firstly one has to note that the spin-
polarization entering the Julliere’s formula [eqn (2)] is not a
well defined quantity. In particular it does not depend solely
on the electronic structure of the electrodes, but also on that of
the organic channel and on how the two materials interact
with each other. Secondly, eqn (2) is strictly valid only for
tunneling transport and so it may be inappropriate to use it for
a spin-injection problem. Both these hypotheses have been
recently verified. In support of the first idea a recent well-
controlled tunneling experiment on La0.7Sr0.3MnO3/Alq3/Co
junctions with nanoscale cross-section has shown large positive
MR at low temperature,74 interpreted in terms of the particular
bonding structure between Alq3 and the electrodes. At the
same time a model requiring majority spin-injection at both
the Co and La0.7Sr0.3MnO3 electrodes, but including the
electronic structure details of the Alq3 layer and its hopping
conductance, allows one to explain the negative MR.67
Although I will return to the two issues of the sign of the
MR and of the bonding between the organic and the magnetic
electrodes in the next sections, here I wish to stress that such a
controversy unveils our incomplete understanding around the
microscopic origins of the MR in organic materials. One
crucial aspect is that constructing a level-energy diagram for
an organic/inorganic interface is usually quite challenging. Let
us see in some detail what are the problems that might arise.
The first step in producing a level diagram is that of aligning
the Fermi level of the electrode with the valence band of
the semiconductor, or for extremely localized states, with the
molecule highest occupied molecular orbital (HOMO). This is
in itself a difficult task since chemical reactions might occur at
the interface and the level pinning might be determined by
hybrid interface states.75 Similarly interfacial electrical dipoles
may significantly alter the vacuum level alignment76 and
so may electron charging.77 Intriguingly such difficulties in
determining the alignment of the occupied states across the
junction also highlight the abundance of possibilities that the
organic world has to offer to spintronics. One can for example
envision situations where interfacial dipole engineering is
performed in order to improve the spin-injection efficiency
or to produce currents with a spin-polarization superior to
that of the magnetic metals forming the device. Likewise,
dipoles can be used to engineer the magnetic properties of a
metallic surface, as already obtained by using intense electric
fields.78 Notably it was recently demonstrated that functional
group engineering can be used to manipulate also the coercive
field of a permalloy,82 an effect that can be exploited for
the construction of future devices with tailored magnetic
properties.
The final step in drawing the level diagram consists in
aligning the organic semiconductor conduction band (or the
lowest unoccupied molecular orbital—LUMO) with respect to
the electrodes’ EF. The problem here is that one cannot simply
add the optical-gap to the HOMO, since the exciton binding
energy is crucial for photon absorption but bears little relevance
for electron transport. Importantly, misplacing the LUMO
may result in an erroneous assignment of the character
(electron or hole) of the injected electrons. Sometimes, but
this analysis is rarely carried out, a combination of direct and
inverse photo-emission can help in deducing the transport
HOMO–LUMO gap.83 Advanced electronic structure theory
based on first principles methods can also be a powerful tool
in hand. However, its use is complicated by the fact that a
correct description of the quasi-particle states is guaranteed
only when many-body methods (such as the GW scheme)
Fig. 3 Magneto-resistance of the La0.7Sr0.3MnO3/Alq3/Co spin-valve
reported in ref. 51. The top panel present the R–H plot defining the
magneto-resistance, the bottom left panel is the device scheme while
the bottom right one shows the dependence of the MR with the
layer thickness. Z. H. Xiong, D. Wu, Z. Valy Vardeny and J. Shi,
Nature (London), 2004, 427, 821–824. Copyright (2004) by the Nature
Publishing Group.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
3342 Chem. Soc. Rev., 2011, 40, 3336–3355 This journal is c The Royal Society of Chemistry 2011
are combined with an energy theory (such as density functional
theory—DFT).84 In this case however the computational over-
heads are significant and only systems comprising a limited
number of atoms can be described.
Before closing this section I wish to make some final
remarks on the role played by the interface between the
organic material and the magnetic electrode in determining
the spin-transport properties of spin-valves. In general it is
commonly accepted that maintaining good vacuum conditions
improves the device performance and increases the MR. This
fact is usually associated with a better quality of the electrode
itself. It is also true that in many vertical spin-valves incorporating
organic molecules the deposition of an inorganic insulator
(typically Al2O3) in between the bottom electrode and the
organic layer greatly enhances the MR.42 The reason for this is
not clear. Certainly organic materials in direct contact with a
metallic electrode may form a dipole layer at the interface.
This is widely documented for tunnel junctions79 and organic
light emitting diodes.80 One of the consequences of the dipole
layer is the formation of gap states, which may participate in
the electron conduction. Importantly gap and surface states
can be drastically modified by surface treatment. For instance
it has been recently demonstrated that surface treatment
in a rubrene-based field effect transistor may reduce the
electron trapping time at the interface by more than one order
of magnitude.81 This suggests that the quality of the interface
may significantly affect the transport properties of these
devices.
However, it is indeed much less clear how the same effects
influence the spin-transport properties, for instance the spin-
polarization. For inorganic semiconductors it has been demon-
strated theoretically that tunnel contacts can significantly
improve the spin injection.37 As such the manipulation of
the interface barrier certainly plays a role. To date however
our understanding is still incomplete and in particular it is
not known how the specific surface chemistry affects the spin-
polarization of the current.
2.4 Recent experimental advances and new directions
The early experiments described in the previous section kicked
off the field of organic spintronics, but at the same time left a
number of fundamental issues without a definite solution. The
search for these solutions is the main focus of the most recent
literature. In particular three conceptual questions remain:
1. Can tunneling be ruled out and can spin-injection be
uncontroversially demonstrated in organic junctions?
2. What is the main source of spin-relaxation?
3. What does determine the MR sign?
2.4.1 Tunneling or spin injection?. Demonstrating with
certainty whether the transport is dominated by tunneling or
spin injection is a rather subtle issue. The results of a recent
experiment by Barraud et al.74 help in putting the discussion
on a quantitative ground. The experiment consists in fabricating
Alq3-based tunnel junctions by nano-indenting a large-area
device with an atomic force microscope tip. In this particular
case the electrodes are respectively La0.7Sr0.3MnO3 and Co, so
that the junctions are composed of the same materials used in
earlier works.51,67 The only difference is that the nano-indentation
process allows one to define vertical devices with a rather
confined cross section (a few nm in diameter), and most
importantly to monitor the device resistance as a function of
the Alq3 thickness so that tunneling can be established with
certainty. Then, a huge MR is found at low temperature
and surprisingly this has a positive sign. I will come back to
the issue of the sign later, for the moment I will focus the
discussion on the resistances involved in the experiment.
The typical layer resistance of a large-area organic spin-valve
is about 25 kO mm�2, while the resistance of a single tunnel
junction with an Alq3 thickness of 2 nm is of the order of 108 O(see ref. 74). With these numbers in hand we can estimate that
the presence of tunnel junctions of this type, separated from
each other by about 20 mm, in an infinite resistive layer will
produce the same layer resistance of a typical large-area
organic spin-valve. The detection of current hot spots of a
few nanometres in size at such a low density is not an easy
task. It is then legitimate to ask whether the large-area
experiments measure spin-injection or simply spin-tunneling
through hot spots.
This is not simple to establish in organic materials. In
contrast to inorganic semiconductors in fact standard optical
pump–probe methods cannot be applied since they all rely on
spin–orbit interaction. Also the typically large resistivity in
organic materials seems to preclude the detection of either the
spin Hall or the Hanle effect, and more in general the use of
four probe non-local measurements. Thus one has little access
to a direct spatially resolved measure of the spin propagation
within the organic medium and has to depend on a less
direct characterization. Let me describe what are the options
available.
Firstly one has to conduct a thorough electrical charac-
terization and establish the dominant transport mechanism. In
particular the study of the dependence of the I–V curve on the
temperature and on the organic layer thickness is emerging as
a standard element of metrology for these junctions, although
it is not always conducted. When the analysis is carried out the
results appear broadly consistent with each other and point to
the fact that the MR is drastically reduced as one moves from
the tunneling to the diffusive transport regime. This has been
demonstrated true for a number of materials combinations:
(i) in CoFeB/Al2O3(1.5 nm)/Alq3/Co junctions85 the transition
from direct to two-step tunneling occurs for an Alq3 thickness
of about 2 nm, where the room temperature MR drops from
35% to B5% (see Fig. 4); (ii) in Fe/Rubrene/LaAlO3/
La0.7Sr0.3MnO3 junctions86 thicknesses in excess of 20 nm give
diffusive transport and small low-temperature MR (45%),
which disappears at about 100 K; (iii) similar results to those
obtained in Fe/Rubrene/LaAlO3/La0.7Sr0.3MnO386,87 have
been reported for Co/Al2O3/Rubrene/Fe;88 (iv) the persistence
of a low-bias room temperature 12% MR at the onset of the
diffusive transport has been demonstrated in CoFeB/MgO/
Alq3/Co.89 At the same time also some more negative results
have been reported with the complete absence of MR in the
diffusive transport regime for Fe/Alq3/Co junctions.90
Has the question been finally answered, i.e. has spin-injection
been demonstrated? It is important to realize that, although an
electrical characterization is a necessary condition to claim
spin injection, it is not a sufficient one. In fact one can still
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 3336–3355 3343
have hot tunneling spots dominating the spin-transport over a
background of non-spin-polarized diffusive conduction. Thus
additional characterization must be performed. At present two
rather different methods have been used. These are respectively
two-photon photo-emission91 and muon spin resonance.92 The
key aspect is that both schemes possess some level of spatial
resolution in the spin-detection within the organic layer, so
that detailed evidence of the persistence of the spin-injection
can be collected. So far these two schemes have provided
ground for the spin injection in copper phthalocyanine91 and
Alq3,92,93 with typical spin-diffusion lengths in the few nano-
metre range. Hence, the next question is about what suppresses
the spin-polarization of the current in organic materials.
2.4.2 What is the main source of spin-relaxation?. First of
all let me recall how the spin diffusion length, lS, is commonly
extracted from a spin-valve measurement. The general idea is
to assume that there is little spin scattering at the inter-
face between the organic material and the electrodes and that
the spin polarization of the injected electrons is attenuated
exponentially as e�(d)/lS, where d is the layer thickness. This
leads to a modified version of the Julliere’s formula, which
now reads
MRðdÞ ¼ Rð0Þ � RðHÞRð0Þ ¼ 2P1P2e
�ðdÞ=lS
1þ P1P2e�ðdÞ=lS: ð3Þ
Eqn (3) is then used to fit the MR as a function of length. This
scheme however presents conceptual and practical problems,
which make the estimates of lS rather uncertain.94 Firstly the
Julliere’s formula is valid for tunneling but not necessarily for
diffusive transport. This means that the exponential decay of
the spin-polarization might not be appropriate for organic
materials. Secondly, there is now a substantial body of work
suggesting that the interfaces play a fundamental role in the
spin injection by acting either on the structural, the electronic
or both structural and electronic properties of the magnetic
electrodes. As such the spin polarization Pi becomes an ill
defined quantity and little can be said about lS.
Still, even with these uncertainties, the modified Julliere’s
formula can be used to obtain a semi-quantitative under-
standing of the spin-diffusion length, which typically in organic
materials is around a few nanometres. The question is then,
what is the limiting factor determining such a particular length
scale. Also in this case the answer is not trivial. As mentioned
before both the spin–orbit and the hyperfine interactions
are expected to be small in organic media. On the one hand
this means that the spins will interact weakly with their
environment and on the other hand that both the interactions
may contribute equally. Let us have a quick overview on the
current experimental situation.
There is one main argument supporting the spin–orbit
interaction as the main source of spin relaxation in organic
materials.95 This is based on a number of experiments92 reporting
a fast decrease of lS with temperature. Since the energy scale of
the nuclear spins dynamics is tiny, one should not expect a
severe temperature dependence of the hyperfine contribution
to lS. As a consequence the hyperfine coupling is ruled out and
spin–orbit interaction should be considered as the main cause
for the spin relaxation. The specific mechanism for the spin
de-phasing is then identified as the Elliott–Yafet96 one.52 The
identification is based on an observed enhancement of lS with
system confinement, which is consistent with the inverse
dependence of lS on the mobility predicted by the Elliott–Yafet
mechanism. A second piece of supporting evidence is provided
by the dependence of lS over the electric field.95 Although
this analysis is compelling, there are also several experi-
ments where lS varies only slowly with the temperature,66,67
which then brings the hyperfine interaction back into the
race.
In organic materials the primary channel for hyperfine
interaction is opened by the hydrogen protons via super-
exchange between the conducting p orbitals of carbon and
the s electrons of H.97 A key experiment demonstrates the
relevance of hyperfine interaction for the spin-diffusion in
organic materials.62 This consists in measuring both magnetic
resonance (optically detected) and the MR of a polymer in
either the hydrogenated or the deuterated form. Since the
deuterium nuclear magnetic moment is about 1/4 of that of
hydrogen, deuteration is expected to narrow the linewidth of
the photoluminescence peak as a function of an external
magnetic field. Furthermore deuterated polymers are expected
to give a significantly larger MR than the hydrogenated ones
in spin-valves of similar thicknesses. This indeed was observed
in the recent work of Nguyen et al.62 and in those of a few
other groups.98,99 As such, the importance of hyperfine inter-
action for the electron spin dynamics in organic materials
appears to be established with some certainty. However, also
in this case there is negative experimental evidence. In fact a
recent measurement on the response of an Alq3-based organic
light-emitting diode to a magnetic field suggests no variation
of the signal with deuteration, indicating little hyperfine
coupling with protons.100
In conclusion, although there is now significant experimental
evidence for both a hyperfine and a spin–orbit related spin
Fig. 4 Room-temperature resistance (circles, right axis) and MR
(squares, left axis) for variable Alq3 thicknesses. The junction is
CoFeB/Al2O3(1.5 nm)/Alq3/Co. The figure is from ref. 85. J. J. H. M.
Schoonus, P. G. E. Lumens, W. Wagemans, J. T. Kohlhepp,
P. A. Bobbert, H. J. M. Swagten and B. Koopmans, Phys. Rev. Lett.
2009, 103, 146601. Copyright (2009) by The American Physical Society.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
3344 Chem. Soc. Rev., 2011, 40, 3336–3355 This journal is c The Royal Society of Chemistry 2011
diffusion mechanism, there is still no consensus on what
is the primary interaction responsible for the spin diffusion.
This may simply point to the fact that the dominant mechanism
is materials specific, i.e. it may vary from material to material.
An interesting observation along this direction is the fact that
to date there is no single device displaying at the same time
both spin-valve behaviour and OMAR. More investigation is
certainly still needed on this particular aspect.
2.4.3 What does determine the MR sign?. Let us now return
to the discussion about the sign of the MR. This is an issue
that has puzzled the field of organic spintronics since its birth.
The puzzle stems from the fact that the same materials
combination, namely La0.7Sr0.3MnO3/Alq3/Co, can produce
either negative51,67 or positive74 MR. This fact is incompatible
with the Julliere’s formula, unless one assumes that the sign of
the spin-polarization of the injected electrons at one of the
two interfaces may change from device to device. Chemical
bonding can help in achieving this and the argument proceeds
as follows.
The spin-polarization entering the Julliere’s formula for
tunneling effectively corresponds to the electrodes DOS spin-
polarization (organic barriers are likely to be amorphous so
that the Julliere’s formula in the pure tunneling limit holds33).
However hybrid surface states may form as the result of the
interaction between the molecular orbitals of the organic
media and the electrodes’ magnetic surfaces. If these appear
at the Fermi level, they may alter the spin-polarization of the
tunnel electrons to a point that Pmay coincide with that of the
hybrid states themselves. The cartoon of Fig. 5 helps in
understanding this point.
In panel (a) the DOS of the electrodes and that of the
molecule in isolation are both presented, where for the sake of
simplicity I assume that only the HOMO is responsible for
the conduction. With this particular level alignment EF of
the electrodes is not resonant with any molecule states, the
transport is tunnel-like and the spin-polarization of the tunnel
carriers, P, coincides with that of the electrodes. When the two
materials are brought together interaction takes place and
two new main features appear. Firstly, the molecule DOS
broadens, reflecting the fact that the lifetime of the molecular
orbitals becomes finite as electrons can be transferred to and
from the electrodes. Such a broadening is spin-selective as it
depends on how strongly a particular molecular orbital interacts
with the extended electronic states of the metal. Since in
magnetic metals the Fermi surfaces of majority and minority
spins are different also the interaction with the molecular
orbitals may be different. In the cartoon for instance [panel
(b)] it is the minority spin-band that interacts more strongly
and consequently the broadening is larger for that spin direction.
Note that such a spin-dependent level broadening brings
only the minority component of the HOMO at the electrodes’
EF, so that P will change sign with respect to the situation of
panel (a).
The second effect is a shift of the molecule DOS with respect
to the electrodes’ EF. Since such a shift can also be spin-
dependent one may end up with a new spin-polarized molecular
orbital at EF. This case is presented in panel (c). Now the
largest DOS at the Fermi level for the hybrid state has
majority character and the sign of P changes again to return
to that of the situation described in panel (a). Importantly
if P is determined by an hybrid state the strength of the
interaction between the molecule and the electrodes also sets
the energy scale of the MR. Thus it is not surprising that in the
experiments of Barraud et al.74 the MR halves at only 25 mV
and disappears completely at 180 K.
The exciting prospect arising from the fact that the MR of
an organic device can be modified by molecular bonding is
that such a modification may be engineered. One can then
envisage specific custom made chemistry aimed at improving
the performances of organic spin-valves and of spin-devices in
general. This is an extremely tantalizing opportunity which has
given birth to the suggestive new field of Spinterface science.101
A final comment should be made on the case where the
transport is diffusive. In this situation spin-polarized tunneling
is only the first step of the transfer of a spin from one electrode
to the other. This is followed by diffusion and by a second
tunneling process to the drain. However, apart from possible
spin relaxation, no other event can change the spin-polarization
of the injected electrons. Furthermore, even if inelastic
processes occur, the vast majority of the electrons will exit
the organic media approximately at the same energy they
enter, since little electric field builds up in the junction. As
such one expects that the arguments provided above will still
hold true for the spin-injection limit.
2.4.4 Hybrid devices. In the introduction I have pointed
out that one of the benefits of using molecules for spin
Fig. 5 Schematic of the formation of an hybrid state between a
magnetic metal and a molecule: (a) When the magnetic metal (left) and
the molecule (right) are well separated, the overall DOS is simply the
superposition of the individual DOS of the two spin components
(blue represents the spin-up DOS and red the spin-down DOS)—that
is, a broad spin-polarized DOS for the metal and a series of discrete
energy levels for the molecule (here only the HOMO is represented). In
this case, the DOS of the metal alone determines the spin-polarization
of the tunnelling current. (b,c) When the molecule is brought into
contact with the metal the DOS gets modified in two ways: the energy
levels broaden (b) (the broadening is exaggerated in the figure) and
their position shifts in energy (c). In both cases new peaks in the DOS
might appear at the EF of the electrodes, arising from new hybrid
interfacial states. It is this new DOS that determines the spin-
polarization of the injected current, which can be dramatically different,
and even reversed, compared with the polarization of the electrodes
(as in b). Stefano Sanvito, Nature Physics, 2010, 8, 562–564. Copyright
(2010) by the Nature Publishing Group.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 3336–3355 3345
applications is that molecules come with an immense range of
properties. However in all the discussion up to now the
organic media have simply replaced their organic counterparts
but no additional functionalities have been exploited. Some
very recent experiments however have explored this second
possibility and in particular have looked at magnetic and
configurational bistable molecules.
Robust magnetism well above room temperature occurs
rarely in organic materials, mainly because the typical density
of the magnetic ions is small and the magnetic interaction
is relatively short-ranged. A notable exception is that of
vanadium(TCNE: tetracyanoethylene)x, V(TCNE)x (x B 2),15
which orders ferromagnetically up to around 400 K. The
magnetic order originates from the interaction of the unpaired
p* orbitals of the (TCNE)� anion with the spins of V2+,
resulting in a magnetic moment of 1 mB per formula unit.
V(TCNE)x is also believed to be a half-metal so that it appears
as an ideal spin-injector for organic spin-valves. The idea is
also particularly attractive since the resistivity of V(TCNE)x is
considerably larger than that of a typical 3d magnetic metal,
so that the resistance mismatch obstacle36 should play only a
minor role.
Two experiments have successfully attempted to use V(TCNE)xas an injector of spin-polarized carriers. In the first102 the
spin-polarized electrons injected from V(TCNE)x in a GaAs/
AlGaAs light emitting diode are recombined to produce
polarized light. From the analysis of the light polarization as
a function of an external magnetic field it is found that
majority electrons are injected but the injection efficiency
remains quite low. Importantly the magnitude of the spin-
polarization of the current appears to be rather insensitive to
the temperature and to the bias.
In the second experiment103 a V(TCNE)x/rubrene/LaAlO3/
La0.7Sr0.3MnO3 spin-valve is made and the MR is recorded
(it is the order of 2%). The R–H plot shows the typical spin-
valve butterfly shape with the switching fields corresponding
to the electrodes coercive fields, i.e. the spin-valve MR effect is
established with certainty. Intriguingly the MRmagnitude first
increases with temperature up toB100 K and then it decreases
until vanishing at a temperature between 170 K and 220 K,
depending on the precise device stack. Such a non-monotonic
behaviour of the MR with temperature is ascribed respectively
to the temperature dependence of surface magnetization
of La0.7Sr0.3MnO3 for T 4 100 K and to the V(TCNE)xresistance increase as T decreases (for T o 100 K). Again the
efficiency of the injection seems to be rather temperature
independent in the range investigated.
These two results are extremely interesting since they
provide the first demonstration of spin-injection from an
organic magnet. As such they pave the way for all organic
spin-devices. These can be fabricated with inexpensive chemical
methods and potentially they can bring additional function-
alities to their inorganic counterparts. Further development
however is hampered by the limited choice of organic magnets
with Curie temperatures well above room temperature,
although the progress in the field in the last decade has been
significant.
A second fruitful research line is that aimed at producing
multi-functional junctions, i.e. junctions which react to stimuli
of different origin (say electrical and magnetic). Many examples
exists in the near field of molecular electronics and concern
devices made with molecules that can switch between alternative
geometrical configurations presenting different electronic
properties.104 This means that the I–V curve of the device
changes abruptly when the molecule is driven into the various
states.105 Generally the molecules are photo-switchable so that
most of the research is driven by the prospect of making smart
molecular opto-electronic devices. However a similar switching
activity can be triggered by an intense static electric field or by
an electrical current. In particular this can alter the morpho-
logy of the transport medium and change its resistance. Such a
principle has been used to manipulate the spatial distribution
of oxygen vacancies in conducting oxides leading to the
discovery of the memristor.106 A memristor is a essentially a
resistor showing hysteresis, i.e. it is the fourth basic circuit
element.107
A recent experiment has taken these concepts and
applied them to organic spintronics.48 Again the device is a
La0.7Sr0.3MnO3/Alq3/Co spin-valve. The main feature however
is that in addition to the standard spin-valve MR effect,
there is an irreversible switch of the I–V curve both at positive
and negative bias (see Fig. 6). The crucial point is that the
MR is different along the different irreversible branches
of the I–V curve,108 meaning that the junction is program-
mable in different conducting states, each one of them
presenting a different magnetic response. The potential for
this device platform is quite vast since both multi-state
memories and memristive logic devices can be envisioned.
Again this line of investigation is still at an early stage, but
progress is expected to arrive in a near future. In particular a
potentially revolutionary research line may open following the
recent development of both room temperature ferroelectric
molecular crystals16,109 and of magnetically controllable
organic ferroelectrics.110
Fig. 6 Current/voltage room temperature characteristics for a
La0.7Sr0.3MnO3/Alq3/Co spin-valve displaying voltage memory effects.
Note the irreversible switching both at positive and negative bias. The
spin-valve presents different MR over the different irreversible branches
of the I–V. L. E. Hueso, I. Bergenti, A. Riminucci, Y. Zhan and
V. Dediu, Adv. Mater., 2007, 19, 2639–2642. Copyright (2007) by
WILEY-VCH Verlag GmbH & Co. KGaA.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
3346 Chem. Soc. Rev., 2011, 40, 3336–3355 This journal is c The Royal Society of Chemistry 2011
3. From organic spintronics to single-molecule
spintronics
The discussion presented up to this point, concerning mainly
macro- and mesoscopic devices, has revealed the potentialities
but also the complexity of organic spintronics. One clear
message is that the study of spin-transport in organic devices
suffers from the lack of accurate microscopic characterization
tools. Partially for this reason and partially for creating a
natural extension of the blossoming field ofmolecular electronics111
to spin phenomena, the last few years have witnessed a growing
interest in the study of spin-transport in single molecules.
These include both the implementation of standard spintronics
devices (for instance the spin-valve) at the single molecule
level, but also the study of electron transport through three
terminal molecular devices112 and through molecular objects
possessing internal spin degrees of freedom. This field, parallel
to that of organic spintronics, takes the collective name of
Molecular Spintronics.10
Interestingly, organic and molecular spintronics nicely
complement each other, with the advantages of one field being
the drawbacks of the other. Thus, while single molecule
measurements are intrinsically local, so that they are easy to
relate directly to the geometry and the electronic structure
of a device, they have usually low yield, since the electrical
contact between the molecules and the electrodes is difficult to
establish. In contrast in large-area devices many molecules are
contacted at the same time so that the device properties are
relatively uniform from device to device, but the microscopic
understanding of the device operation is often incomplete.
This holds true also for theory and modeling. Large-area
devices are usually described by macroscopic transport theories
implemented on parametric model Hamiltonians.57,85,113–115
These require a number of adjustable parameters, which can
often be well inferred from the experimental data. In contrast
experiments conducted with single molecules are more suitable
for first principles transport theory, which provides a quantitative
description without any free parameters.116
In this final part of the review I have taken a rather
unorthodox approach in presenting the field. Firstly, I have
looked at STM measurements for molecules on surfaces.
These are extremely important since they provide spatially
local information about the spin-transport, so that one can
understand fully how spins cross not just the molecule, but
its constituent parts. As such STM measurements may be
extremely informative on the microscopic physics/chemistry of
large scale devices. Then I have considered devices made with
single molecule magnets. Note that to date the experiments on
these have been quite limited so that an exhaustive treatment is
not possible. For this reason I have preferred simply to
introduce the most relevant concepts. This is indeed a personal
perspective. In doing so I have not discussed experi-
ments relating to the Kondo effect117,118 for which extensive
reviews have been already written.119 Finally I have decided to
introduce spin crossover molecules for which little published
experimental work addressing electron transport exists.
Therefore the last part concerns mostly theoretical considera-
tions and somehow has the goal of stimulating experimental
activity.
3.1 SP-STM and Spinterface
One of the most direct ways to measure the spin-polarization
of the electron current emerging from a magnetic surface
through a single molecule is provided by SP-STM.27 This
essentially consists in performing a spin-valve experiment
where one of the electrodes is the scanning tip and the second
one is the magnetic substrate hosting the molecule. State of the
art SP-STM has a spatial resolution finer than the typical
molecule bond-length so that the atomic details of the spin-
injection can be investigated. The only disadvantage of
SP-STM with respect to other transport techniques for single
molecules is that the samples need to be prepared in ultra-high
vacuum (UHV), i.e. it is poorly compatible with wet chemistry.
Fortunately many planar molecules are compatible with UHV
deposition (porphyrins, phthalocyanines, salens, etc.). These
can be prepared in a large variety of configurations and can
incorporate transition metal centers. As such they form an
important class of materials attracting a rapidly growing
interest.120–126
SP-STM can provide a direct proof of the manipulation of a
spin-current by chemical bond tailoring, i.e. it is a preferential
tool of Spinterface science. As in a standard magnetic tunnel
junction the current in SP-STM depends on the mutual
orientation between the magnetization of the tip and that of
the substrate. SP-STM however possesses additional spatial
and spectral resolution, i.e. the tunneling current changes with
bias and with the position of the tip with respect to the
molecule. As such it allows one to obtain a spatial and energy
mapping of the spin tunneling process. It is then possible to
determine which particular molecular orbital contributes the
most to the net spin polarization of the current. Intriguingly,
since different molecular orbitals may present DOS with
opposite spin-polarization (in particular if hybrid with the
electronic states of the substrate) and they may be spatially
distributed over different portions of the molecule, opposite
spin-polarization may be detected for the same molecule at
different tip positions. This was theoretically predicted some
time ago127 and recently demonstrated experimentally.128,129
Indeed the recent data from Atodiresei et al.128 and
Brede et al.129 show clearly not only that the spin polarization
of the current emerging from an organic molecule (either
Phthalocyanine128 or Co-Phthalocyanine129) can be opposite
to that of the magnetic substrate on which the molecule is
adsorbed [two monolayers of Fe on W(110)], but also that
different regions of the molecule can sustain different current
spin-polarization. The orbitals responsible for both the chemical
bond and the conductivity of the Fe surface have dz2 symmetry.
These are spin split with the current being dominated by
minority electrons. The bond with the molecule is between
the Fe dz2 and the Phthalocyanine pz orbitals and brings
additional majority DOS at the Fermi level (via the energy
level shift and broadening mechanism described in section 2.4.3).
As a consequence the spin-polarization of the composite
molecule+substrate is reversed with respect to that of the
Fe substrate (see Fig. 7).
The ability of STM to investigate surface magnetism however
does not stop here, as STM can also be used to extract detailed
information on the magnetic excitations. This is enabled
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 3336–3355 3347
by spin-flip inelastic tunnel spectroscopy (SF-IETS).130,131
SF-IETS is based on a principle common to all inelastic
spectroscopy techniques, namely that a change in the electrical
conductance occurs every time the bias voltage reaches the
critical value characteristic of a given excitation. At that
particular bias a second channel adds to the elastic tunneling.
This is given by the inelastic process, where an electron
exchanges energy with the molecule, exciting it. Whether
or not the opening of an inelastic channel increases the current
depends sensibly on the details of the junction. In the case
the molecular excitations are phonons, propensity rules have
been established correlating the symmetry of the various
vibrational modes and of their coupling to the electrodes to
the transmission.132,133
In SF-IETS the molecular excitations have spin origin and
they involve the flipping of the spin direction of the current-
carrying electrons. At variance with the phonon case, a spin-
transition must satisfy some selection rules, so that the theory
becomes more informative. However the energy scale of the
magnetic excitations is typically in the sub-meV range, i.e. it is
more than one order of magnitude lower than that of the
molecular vibrations. As such SF-IETS requires severely
low temperatures. SF-IETS was pioneered at IBM with
experiments conducted on magnetic ions deposited over
metallic surfaces covered by a thin insulating film.130 Landmark
results include the measurement of the magnetic coupling134
and of the magnetic anisotropy135 of single atom chains. Most
recently the same technique has been employed for planar
magnetic molecules deposited on surfaces. The investigation of
the details of the super-exchange mechanism in Co phthalo-
cyanine atomic layers deposited on Pb136 and of the charging
state of the same molecules137 are some examples of this
research. It is important to note in this context that SF-IETS
works best in the limit of weak electronic coupling between the
magnetic center and the electrodes. In the opposite situation
the transport, at least at low bias, is dominated by the Kondo
effect. This should deserve a review in itself,119 here we just
wish to mention that the Kondo effect in magnetic molecules is
well established and can be tuned by STM manipulation of
the molecule itself.138 Also it is important to remark that a
quantitative parameter-free theory of the Kondo effect has
begun to emerge.139
3.2 Single molecular magnets devices
Although STM is a powerful tool for understanding the basic
mechanism of the magnetic interaction between a molecule
and a substrate, it is not a device fabrication platform. In
contrast two- and three-terminal junctions incorporating
magnetic molecules are closer to real devices since both the
spin and charging state of the molecule can be altered. These
are usually fabricated by combining breaking junction type
technologies with wet chemistry and, as such, one does not
need any longer UHV growth conditions. However, when
moving away from planar molecules and UHV, other pro-
blems emerge.
A rather fundamental one is the fact that, despite many
classes of magnetic molecules can be chemically synthesized,140
most of them are extremely fragile away from solution and
often react on a metallic surface.141,142 Furthermore some
molecules can lose entirely their magnetic moment by
coupling with the electrodes, even if they remain intact.143
This creates a substantial uncertainty since it is difficult to
establish whether the molecule entering a device is the same
one that was intentionally designed for that device. Rapid
progress however has been made in constructing robust
magnetic molecules and it has been already demonstrated
that members of the Fe4 family can survive on surfaces and
preserve both their spin-state and most of the magneto-crystalline
anisotropy.144,145
Let us now assume that two- and three-terminal devices
incorporating magnetic molecules which preserve their single
molecular magnet (SMM) properties, can be made. The next
important question is: how should these devices operate? In
particular one has to understand which among the molecule
degrees of freedom must be used for operating the device
(reading, writing and manipulating information in logic
devices or reacting to an external stimulus in sensors). For
magnetic molecules the spin is clearly the degree of freedom of
choice, but then the question becomes, which property to use.
This is of course a crucial choice, since different properties rely
Fig. 7 The geometry and electronic structure of a free Co-Phthalocyanine
(a) and of the same molecule adsorbed on an Fe surface (b). Results in
(c) are for Co-Phthalocyanine adsorbed on Fe when the calculations
include van der Waals interaction (this situation describes better the
experimental finding). Note that, while in (a) the spin-polarization of
the DOS is dominated by the substrate minority dz2 orbitals, an hybrid
state forms in (c) whose spin polarization is opposite, i.e. it is mainly
due to majority spins. J. Brede, N. Atodiresei, S. Kuck, P. Lazic,
V. Caciuc, Y. Morikawa, G. Hoffmann, S. Blugel and R.Wiesendanger,
Phys. Rev. Lett., 2010, 105, 047204. Copyright (2010) by The American
Physical Society.Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
3348 Chem. Soc. Rev., 2011, 40, 3336–3355 This journal is c The Royal Society of Chemistry 2011
on different interactions, which in turn set the energy scale
(and thus the temperature) of the device operation.
Consider for instance the case of a spin-valve. The two key
ingredients to make the device functioning are the ability of
the electrodes to produce a spin-polarized current and the
possibility of altering the mutual orientation of the electrodes’
magnetizations vectors. The first property depends on the
exchange interaction and therefore survives up to tempera-
tures close to the material’s Curie temperature. The second
one relies on the magneto-crystalline anisotropy, hence on the
spin–orbit coupling. If one now wishes to scale the spin-
valve concept down to the molecular level a problem will
immediately appear, namely that the magneto-crystalline
anisotropy in magnetic molecules is usually small (the typical
energy barriers are in the few Kelvin range), mostly because
only a handful of atoms carry the magnetic moment. As a
consequence the molecule is likely to behave as a paramagnetic
object at any reasonable temperatures and the resulting
current will not be spin-polarized.
Also in this case, however, recent progress seems to indicate
a more positive outlook. For instance many theoretical studies
have pointed to transition metal multidecker clusters (TM–P
with TM= Sc, Ti, V, Ni, and P = cyclopentadienyl, benzene)
as possible sources of spin polarized electrons.143,146–149 An
alternative and intriguing prospect is also that of constructing
devices with magnetic molecules whose magnetic anisotropy
produces little or no magnetic moment but a definite toroidal
moment.150 There are predictions for such molecules to be able
to switch the spin-polarization of an injected current151 and
still to remain protected from dipolar-interaction, one of
the main sources of spin-relaxation in molecular crystals
(note that here I refer to the magnetic spin-relaxation of the
molecules and not to the spin-diffusion of current carrying
electrons).
A second and possibly more promising strategy for fabricating
devices by using SMMs is that of exploiting the robust
exchange interaction instead of the tiny magneto-crystalline
anisotropy. This essentially means to use the spin state of the
molecule as the physical property to read, write and manipulate.
Clearly one then has to be able of addressing (reading, writing
and manipulating) the spin state of the molecule without
the need of maintaining the spin-quantization axis fixed,
i.e. without the need of strong magneto-crystalline anisotropy.
I will first consider the problem of reading the magnetic state,
while a discussion over the possible strategies for writing and
manipulating will be reviewed in the remaining sections. The
general idea is to convert spin information into molecular
orbital information. These are then detected by an electrical
current. In practice one wants to demonstrate that the different
spin-states of a molecule present different frontier molecular
orbitals or different coupling to the electrodes, both features
that might affect an electrical current. Again, because of
the unlikely possibility of fixing the molecule anisotropy axis,
the electrical read out of the molecules’ state needs to be
done without using a spin-polarized current, i.e. by using
non-magnetic electrodes.
The possibility for such a non-spin-polarized electrical read
out of the magnetic state of a single molecule magnet was
recently explored theoretically for a prototypical two terminal
device incorporating a Mn12 molecule.152 In particular it
was demonstrated that the S = 10 ground state (GS) can
be distinguished from a spin-flip state (SFS) obtained by
reversing the magnetic moment of both a Mn3+ and a
Mn4+ ion relatively to their GS orientation (the total spin
projection of the SFS is 9). This is possible because the frontier
molecular orbitals of a particular magnetic state re-hybridize
differently under-bias, so that the I–V presents low-bias
negative differential resistances (NDRs) characteristic of the
molecule spin state.
In order to understand this concept in some more detail
consider Fig. 8(a) where the DOS for the two magnetic states
are presented. If one neglects the spin polarization, as for a
paramagnetic molecule, the DOS of the GS and the SFS
appear rather similar, so that one may expect similar I–Vs.
This however is not the case, as demonstrated by the calculated
I–V curve shown in Fig. 9.
The presence of NDR in the I–V curve is the result of orbital
polarization under bias. Consider for instance the spatial
Fig. 8 DOS around the Fermi level (vertical line at 0) for
[Mn12O12(CH3COO)16(H2O)4] in the ground state (a) and the spin-flip
state (b). The three small plots show isosurfaces of the HOMO wave-
function for the GS at different bias: top = 300 mV, center = 0,
bottom=�300 mV. Note the drastic polarization of the wave-function
in the electric field. C. D. Pemmaraju, I. Rungger and S. Sanvito, Phys.
Rev. B, 2009, 80, 104422. Copyright (2009) by The American Physical
Society.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 3336–3355 3349
distribution of the HOMO wave-function as a function of the
bias, presented in the three small panels of Fig. 8. As the bias
is swiped from positive to negative the HOMO electronic
wave-function polarizes in order for the molecule to maintain
its local charge neutrality. Such a molecular orbital polari-
zation changes the degree of overlap between the HOMO and
the extended states of the electrodes, thus changing the
electronic coupling and hence the molecular state life-time. If
such a polarization is significant and the electronic coupling
with one of the two electrodes is drastically reduced, then the
current will decrease despite an increase in the applied bias.
This mechanism generates the NDR.
The quantum mechanical way to polarize a given molecular
orbital is to hybridize it with other available molecular orbitals.
These should be sufficiently close in energy and have an
appropriate orbital and spin-symmetry. Since in the GS the
HOMO multiplet is composed by states having the same spin,
while in the SFS some states have the opposite spin direction,
the number of molecular orbitals available for re-hybridization
is different and hence the NDR appears at a different bias. This
means that the response of a spin-state to an electric field is
driven by the dielectric response of its molecular orbitals, i.e.
spin information is translated into orbital information. Note
moreover that such an argument does not apply only to SMMs,
but more generally to situations where the HOMO is charac-
terized by a multiplet of closely spaced orbitals. This is for instance
the case of the non-magnetic dithienylethene molecules.105
Is there any experimental proof of the above mechanism?
It is quite difficult to answer to this question at the moment
since only a few transport experiments on SMMs have been
conducted to date. Still early measurements for Mn12incorporated in a three-terminal device153,154 reveal the
presence of low energy features (NDRs satellite to Coulomb
blockade peaks not involving different charging states), which
might be ascribed to orbital effects. At present however the
evidence is too little to call for a definitive explanation
and alternative theories based on selection-rule forbidden
transitions between different spin states have been brought
forward and are at present equally possible.155,156 In summary,
it appears that the demonstration of the electrical read out of
spin-states of a molecule has been provided both experi-
mentally and theoretically. Whether or not one will be able
to assign with certainty a given spin-state to a specific finger-
print in the transport and whether this will be controllable still
remains an open question.
In closing this section I wish to briefly mention another
device protocol different from the SMM-based three-terminal
junction discussed so far. This is based on grafting SMMs on
either graphene or carbon nanotubes and in investigating their
magneto-transport response.21 The concept underpinning such
a device is that the interaction between the molecule and the
transport channel (either the nanotube or graphene) produces
an electrical response which is sensitive to the magnetic state of
the molecule (spin and spin orientation with respect to the
channel). Also in this case one has to identify the strength of
the interactions available. In general both the spin of the
molecules used and its magneto-anisotropy should be as large
as possible, and this is why bis(phthalocyaninato)terbium(III)
complexes have been utilized so far.157 This however is enough
only to stabilize the molecule against thermal fluctuation, then
one has to make the molecule interacting with the channel.
Proposals to date include the detection of the magnetic
flux originating from the molecule stray field21 and the use
of robust chemical bonds able to produce spin-dependent
Fano-like resonances in the channel conductance spectrum.158
The first case may provide a technological platform for ultra-
sensitive magnetometry while the second one for magnetic
field sensors.
3.3 Manipulation of the spin state of a molecule
The last step in demonstrating the validity of the concept of
single molecule spintronics consists in designing a strategy
for writing and eventually manipulating the possible states
of a device. In the case of SMMs devices this translates
into the ability of altering the spin of a SMM, i.e. in the
ability of manipulating the exchange interaction in a control-
lable way. Note that this rather fundamental aspect underpins
not only the potential for new logic devices and sensors but
also the use of magnetic molecules as elements for quantum
computation.24,159
A general, always available, strategy for altering the magnetic
properties of a molecule is that of acting chemically.
For instance it was recently demonstrated that the magnetic
exchange between two Cr7Ni rings can be modified by
changing the molecular linker coupling the rings.160 An intriguing
aspect of this protocol is that the electronic structure of
the constituent magnetic elements (the Cr7Ni rings) is little
Fig. 9 The I–V curves for both the GS and the SFS of a Mn12two-probe device. Note the spin-state characteristic NDRs. C. D.
Pemmaraju, I. Rungger and S. Sanvito, Phys. Rev. B, 2009, 80,
104422. Copyright (2009) by The American Physical Society.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
3350 Chem. Soc. Rev., 2011, 40, 3336–3355 This journal is c The Royal Society of Chemistry 2011
affected by the linker, so that the freedom in the design of the
desired compound is rather large. One can then envision the
use of active linkers, i.e. of linkers whose electronic structure
can be changed by a local probe such as a gate. Examples of
these are the one-electron reducible [Ru2+Ru3+(OCCMe3)4]+
linker still for Cr7Ni rings160 and [PMo12O40(VO)2]q�molecules.161
The chemical strategy has also been successfully employed for
manipulating the spin state 162 and the magnetic anisotropy of
planar magnetic molecules on surfaces.124
Such a chemical approach, which may be a valuable option
for sensor applications, is not well suited for the electronic
ones. These in fact require a reversible switching mechanism
with relatively fast access times, two requirements hardly
matched by a chemical reaction. As a consequence one has
to look at reversible spin-manipulation triggered by an easily
accessible external stimulus, for instance electromagnetic
radiation or a static electric field. In both case the transition
between two different states should have electronic origin
(to be fast), although it might be also accompanied by con-
figurational changes (to be stable). The next sections will
discuss some of the present possibilities for switching magnetic
molecules and the effects that such switch produces on the
electron transport.
3.3.1 Spin-crossover compounds and valence tautomerism.
Demonstrations that the spin-state of a molecule can be
changed by an external stimulus are abundant. In fact there
exists an entire class of molecules, named spin-crossover
compounds, whose magnetic ground state can be altered by
light, temperature or pressure.163 These molecules incorporate
a single transition metal ion and display an entropy driven
low-spin to high-spin transition accompanied by a geometrical
relaxation of the first coordination shell around the ion. The
microscopic mechanism driving the spin-crossover is illustrated
in the top panel of Fig. 10 for the prototypical case of FeII. In
the low-spin configuration the six 3d-electrons occupy the
t2g levels (note that the Fe coordination is approximately
octahedral) in a spin-zero state (1A1g). By increasing the
temperature the competing high-spin state, where two electrons
are promoted to the eg levels, (5T2g), becomes thermodynamically
more stable. Such a phase transition is driven by the larger
entropy of the high-spin state. In particular there are two
contributions to the entropy: one is provided by the spin and
the second by the molecule vibrations. The t2g orbitals have
bonding nature and the eg are non-bonding, so that the
promotion of electrons to the eg shell weakens the chemical
bond and softens the phonon mode between the magnetic ion
and the ligands.
A special case among spin-crossover compounds is represented
by molecules presenting valence tautomerism.164 These are
complexes in which the crossover is obtained by an inter-
conversion between redox isomers.165,166 The lower panel of
Fig. 10 schematically describes the inter-conversion process in
Co-dioxolene. In the low-spin state the eight valence electrons
are distributed to give a diamagnetic Co(III)-catecholate
configuration. The entropy driven phase transition leads
to paramagnetic high-spin Co(II)-semiquinonate, which is
obtained from the low-spin configuration by transferring one
electron from the o-dioxolene group to Co.
These two broad classes of molecules are extremely interesting
as potential materials platform for molecular spintronics for a
number of reasons. Firstly, usually the two states accessible by
the compound are both long-living (at least at low temperature),
so that the molecules can be maintained in the desired spin
state for relatively long times. This essentially means that such
molecules have an intrinsic non-volatile nature. Secondly, since
the crossover always involves an internal charge re-arrangement
and an atomic relaxation, the two different molecular states
present rather different electronic structures. As such a two-
terminal device incorporating crossover compounds is expected
to display different I–V curves for different spin-state, i.e. the
spin-state is likely to be electrically readable. Finally, the cross-
over transition appears to be sensitive to the local electrostatic
environment,167,168 so that an electric field may be used to
induce the transition.169 This means that a potential device
might be electrically switchable.
What are the challenges for spin crossover compounds as
active materials for single molecule spintronics? These in
general are similar to those facing SMMs. Again the molecule
stability on the surface is a primary concern and there is no
fundamental reason to believe that spin crossover molecules
will be less fragile than SMMs. Certainly the crossover activity
Fig. 10 Energy level schemes for the low-spin to high-spin transition
in various spin crossover molecules. In the top panel the conventional
spin-crossover of FeII is presented, where the transition is driven by the
transfer of two electrons from the t2g to the eg levels. The lower panel
describes valence tautomerism in Co-dioxolene complexes. In this case
an electron is transferred from the o-dioxolene group to Co.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 3336–3355 3351
is strongly dependent on the environmental conditions
(chemical and electrostatic conditions, wetting etc.), so that
a certain fragility appears intrinsic to these compounds.
This means that a molecule displaying spin crossover activity
in the single crystal phase may lose this property when
deposited on the surface, because one of the two states will
be no longer accessible. Secondly as the hysteresis in the
switching temperature is the result of the collective inter-
action among the molecules in a crystal, it is a property that
does not transfer to the single molecule world. Whether the
hysteresis will be possible for two-dimensional arrangement
of molecules on a surface, so that spin crossover compounds
may be used in large-area devices, is certainly not clear at
present.
To date the experimental activity on spin crossover
compounds as active elements for nano-junctions has been
quite limited although a few key results have already emerged.
However all the experiments showing spin crossover activity
do not involve single molecules but instead crystals or
nanoparticles, where the stability issues mentioned above
are less relevant. As such we still lack of an experimental
demonstration for a device made with spin crossover single
molecules. In any case it was first demonstrated that the
conductivity of a [Fe(qsal)2][Ni(dmit)2]3�CH3CN�H2O single
crystal170 has an hysteretic behaviour with temperature,
suggesting that the two spin states across the crossover
transition have a different resistivity. Secondly it was proved
that the spin state of FeII complexes containing pairs of planar
terdentate N ligands and immobilized on highly oriented
pyrolytic graphite is detectable by STM.171 Finally, very
recently it was shown that nanoparticles made of spin cross-
over molecules present a temperature hysteresis in their
conductivity, which also in this case is attributed to the
spin crossover transition.172 Intriguingly this last experiment
also demonstrated that the crossover can be induced by a
potential bias, i.e. by a static electric field. This finding paves
the way for electrically controlled spin devices at the molecular
level.
3.3.2 Electrostatic spin crossover effect. Another possibility
for manipulating electrically the spin of a molecule, alternative
to changing the spin of a single magnetic ion, is that of acting
on the exchange interaction between two or more magnetic
centers. This is the concept behind the electrostatic spin-
crossover effect (ESCE), first proposed for high electron
density molecular wires173 and then for polar molecules.174
The general idea of the ESCE is that the high-spin and the
low-spin state of a molecule can Stark shift differently if
they have different polarizabilities and, in the case of polar
molecules, also different permanent electrical dipoles. One can
then speculate that there exists a particular condition where a
high-spin to low-spin crossover is possible. The fundamental
question is how large is the crossover field.
In molecules with inversion symmetry the first order contri-
bution to the Stark effect vanishes and the difference between
the polarizabilities alone induces the crossover.173 Realistic
crossover electric fields, Ecross, are obtained only for molecules
with a large spin-contrast in the polarizability (i.e. the
polarizability of different spin-states must be very different).
This translates into a large charge density and a small
HOMO–LUMO gap, and brings two serious drawbacks.
First, one needs to pursue an extremely challenging strategy
for chemical synthesis in order to produce the desired molecules
(the ones proposed in ref. 173 were 10p benzene) and it is not
clear whether such chemical route will be ever available.
Secondly and most importantly, the small HOMO–LUMO
gap prevents the on-set of a large electric field in a real device,
so that even if 10p-benzene are made, one will probably never
be able to produce an electric field intense enough to switch the
molecule.
The use of polar molecules circumvents these problems. The
crucial idea is that a permanent electrical dipole can effectively
‘‘bias’’ the crossover field to smaller fields. In fact (see Fig. 11)
the energy change as a function of field, DEGS, is a parabolic
function, centered at E = 0 for non-polar molecules
(the second order Stark shift is proportional to 12EiaijEj, where
aij is the polarizability tensor). Then, Ecross is determined
by the interception between the parabolas associated to
the different spin-states (a spin singlet and a spin triplet in
Fig. 11.). The addition of a linear contribution to DEGS
(the first order Stark shift ~p�~E, with ~p the permanent electrical
dipole) shifts the center of the parabolas bringing their
interception closer to E = 0 at least for one of the two field
polarities. Hence the molecule electric dipole effectively
introduces a bias field Ebias, which reduces the external electric
field needed for the crossover. This mechanism brings a much
more promising strategy since one can first engineer the
magnetic molecule and then the specific electrical dipole, as
demonstrated for acetylene-bridged di-Cobaltocene(CoCp2)
molecules functionalized with different substituents.174
Such a field-induced crossover has important consequences
for the electrical transport. If one of these molecules is sand-
wiched in a two-terminal device in such a way that there is a
potential drop between the magnetic centers, then the strength
Fig. 11 Stark energy gain, DEGS, for the singlet and triplet state of a
magnetic molecule as a function of the applied electric field, E. In
panel (a) we represent a molecule with inversion symmetry, while in
(b) one where the symmetry is broken by an electrical dipole. Note that
the shift of the DEGS(E) parabola by Ebias generates a shift of the
crossover field to lower fields. J0ST is the exchange interaction energy at
zero field. N. Baadji, M. Piacenza, T. Tugsuz, F. D. Sala, G. Maruccio
and S. Sanvito, Nature Materials, 2009, 8, 813–817. Copyright (2009)
by the Nature Publishing Group.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
3352 Chem. Soc. Rev., 2011, 40, 3336–3355 This journal is c The Royal Society of Chemistry 2011
of the magnetic coupling will change with bias. Interestingly
for biases corresponding to the crossover field the different
spin-states become degenerate, i.e. on average the molecule
spends an identical amount of time in any one of them. This
essentially means that there is no magnetic energy scale at
Ecross and the current is predicted to become temperature
independent.175 Intriguingly the electrical control of a high-
spin to low-spin transition has been recently demonstrated
experimentally in a single Mn2+ ion coordinated by two
tetrapyridine ligands in a three-terminal device geometry.176
We wish to close this section by mentioning that some
theoretical work has been devolved to understand how the
magnetization direction of a magnetic molecule can be
manipulated with an external current, by essentially translating
the concept of spin-transfer torque to the molecular magnets
world.177 Importantly was shown that a spin-polarized current
can switch the magnetic moment of the molecule despite the
molecule intrinsic spin-relaxation.
4. Conclusions
In this contribution I have reviewed the most recent advances
in the emerging and fascinating fields of organic and single
molecule spintronics. In particular I have highlighted the main
difference between spintronics in extended organic semi-
conductors and that in single molecules. The picture emerging
is encouraging although the challenges ahead are still significant.
Devices made with extended organic semiconductors appear
now reproducible to a good degree and may show room
temperature spin-valve magnetoresistance. Controlling the
quality and the nature of the interfaces between the organic
and the inorganic elements of the device appears to be the
critical aspect in the fabrication process. This sensitivity however
can be turned into a strength and examples of improved
magnetoresistance tuned by chemical doping of the interfaces
are rapidly appearing.
In contrast single molecule devices remain highly unstable
and difficult to reproduce. However, when made, these may
show a range of intriguing properties not shared by large area
devices. In particular the use of functional molecules where
switching activity is present may give access to electrically
controlled single molecule spin devices.
In moving forward organic andmolecular spintronics have also
to face an additional challenge beyond the technical issues
mentioned above. This is the need of finding a range of specific
applications where the unique characteristics of organic materials,
namely the long spin diffusion times, are fully exploited.
Acknowledgements
This work is sponsored by Science Foundation of Ireland (grant
No. 08/ERA/I1759 and 07/IN.1/I945) and by CRANN. I would
like to thankNadjib Baadji for a critical review of the manuscript.
References
1 N. F. Mott, Proc. R. Soc. London, Ser. A, 1936, 153, 699–717.2 M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau,F. Petroff, P. Etienne, G. Creuzet, A. Friederich andJ. Chazelas, Phys. Rev. Lett., 1988, 61, 2472–2475.
3 G. Binasch, P. Grunberg, F. Saurenbach and W. Zinn, Phys. Rev.B, 1989, 39, 4828–4830.
4 I. Zutic, J. Fabian and S. Das Sarma, Rev. Mod. Phys., 2004, 76,323–410.
5 Semiconductor Spintronics and Quantum Computation, ed. D. D.Awschalom, D. Loss and N. Samarth, Springer-Verlag, BerlinHeidelberg, Germany, 2002.
6 D. D. Awschalom and M. E. Flatte, Nat. Phys., 2007, 3,153–159.
7 M. Gajek, M. Bibes, S. Fusil, K. Bouzehouane, J. Fontecuberta,A. Barthelemy and A. Fert, Nat. Mater., 2006, 6,296–302.
8 J. S. Moodera, T. S. Santos and T. Nagahama, J. Phys. Condens.Matter, 2007, 19, 165202.
9 V. Dediu, M. Murgia, F. C. Matacotta, C. Taliani andS. Barbanera, Solid State Commun., 2002, 122, 181–184.
10 A. R. Rocha, V. Garcia Suarez, S. W. Bailey, C. J. Lambert,J. Ferrer and S. Sanvito, Nat. Mater., 2005, 4, 335–339.
11 D. D. Awschalom and M. E. Flatte, Nat. Phys., 2007, 3,153–159.
12 J. M. Kikkawa and D. D. Awschalom, Nature, 1999, 397,139–141.
13 S. Datta and B. Das, Appl. Phys. Lett., 1990, 56, 665–667.14 C. K. Chiang, C. R. Fincher, Jr., Y. W. Park, A. J. Heeger,
H. Shirakawa, E. J. Louis, S. C. Gau and A. G. MacDiarmid,Phys. Rev. Lett., 1977, 39, 1098–1101.
15 J. M. Manriquez, G. T. Yee, S. Mclean, A. J. Epstein andJ. S. Miller, Science, 1991, 252, 1415–1417.
16 S. Horiuchi and Y. Tokura, Nat. Mater., 2008, 7, 357–366.17 W. J. M. Naber, S. Faez and W. G. van der Wiel, J. Phys. D:
Appl. Phys., 2007, 40, R205–R228.18 V. Alek Dediu, L. E. Hueso, I. Bergenti and C. Taliani,
Nat. Mater., 2009, 8, 707–716.19 S. Sanvito and A. R. Rocha, J. Comput. Theor. Nanosci., 2006, 3,
624–642.20 S. Sanvito, J. Mater. Chem., 2007, 17, 4455–4459.21 L. Bogani and W. Wernsdorfer, Nat. Mater., 2008, 7, 179–186.22 J. Lehmann, A. Gaita-Arino, E. Coronado and D. Loss, J. Mater.
Chem., 2009, 19, 1672–1677.23 P. C. E. Stamp and A. Gaita-Arino, J. Mater. Chem., 2009, 19,
1718–1730.24 M. Affronte, J. Mater. Chem., 2009, 19, 1731–1737.25 J. Camarero and Eugenio, J. Mater. Chem., 2009, 19,
1678–1684.26 M. Mas-Torrent, N. Crivillers, V. Mugnaini, I. Ratera, C. Rovira
and J. Veciana, J. Mater. Chem., 2009, 19, 1691–1695.27 R. Wiesendanger, Rev. Mod. Phys., 2009, 81, 1495–1550.28 Optical Orientation, ed. F. Meier and B. P. Zachachrenya,
North-Holland, Amsterdam, The Netherlands, Modern Problemsin Condensed Matter Science series, vol. 8 edn, 1984.
29 Spin Electronics, ed. M. Ziese and M. J. Thornton, Springer-Verlag, Heidelberg, Germany, Lecture Notes in Physics series,vol. 569 edn, 2001.
30 A. Kirilyuk, A. V. Kimel and T. Rasing, Rev. Mod. Phys., 2010,82, 2731–2784.
31 B. Dieny, Phys. Rev. B, 1991, 43, 1297–1300.32 T. Valet and A. Fert, Phys. Rev. B, 1993, 48, 7099–7113.33 I. I. Mazin, Phys. Rev. Lett., 1999, 83, 1427–1430.34 J. M. D. Coey and S. Sanvito, J. Phys. D: Appl. Phys., 2004, 37,
988–993.35 A. Abragam, The principles of nuclear magnetism, Oxford
University Press, Oxford, UK, 14th edn, 2002.36 G. Schmidt, D. Ferrand, L. W. Molenkamp, A. T. Filip and
B. J. van Wees, Phys. Rev. B, 2000, 62, R4790–R4793.37 E. I. Rashba, Phys. Rev. B, 2000, 62, R16267–R16270.38 M. Julliere, Phys. Lett. A, 1975, 54, 225–226.39 W. H. Butler, X.-G. Zhang, T. C. Schulthess and
J. M. MacLaren, Phys. Rev. B, 2001, 63, 054416.40 J. Mathon and A. Umerski, Phys. Rev. B, 2001, 63, 220403.41 I. Rungger, O. Mryasov and S. Sanvito, Phys. Rev. B, 2009, 79,
094414.42 T. S. Santos, J. S. Lee, P. Migdal, I. C. Lekshmi, B. Satpati and
J. S. Moodera, Phys. Rev. Lett., 2007, 98, 016601.43 B. N. Limketkai, P. Jadhav and M. A. Baldo, Phys. Rev. B, 2007,
75, 113203.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 3336–3355 3353
44 G. Szulczewski, S. Sanvito and J. M. D. Coey, Nat. Mater., 2009,8, 693–695.
45 J. Bass and W. P. Pratt, J. Phys.: Condens. Matter, 2007, 19,183201.
46 B. Huang, D. Monsma and I. Appelbaum, Phys. Rev. Lett., 2007,99, 177209.
47 I. Appelbaum, B. Huang and D. Monsma, Nature, 2007, 447,295–298.
48 L. E. Hueso, J. M. Pruneda, V. Ferrari, G. Burnell, J. P.Valdes-Herrera, B. D. Simons, P. B. Littlewood, E. Artacho,A. Fert and N. D. Mathur, Nature, 2007, 445, 410–413.
49 K. Tsukagoshi, B. W. Alphenaar and H. Ago, Nature, 1999, 401,572–574.
50 N. Tombros, C. Jozsa, M. Popinciuc, H. T. Jonkman andB. J. van Wees, Nature, 2007, 448, 571–574.
51 Z. H. Xiong, D. Wu, Z. Valy Vardeny and J. Shi, Nature, 2004,427, 821–824.
52 S. Pramanik, C. G. Stefanita, S. Patibandla, S. Bandyopadhyay,K. Garre, N. Harth and M. Cahay, Nat. Nanotechnol., 2007, 2,216–219.
53 J. H. Shim, K. V. Raman, Y. J. Park, T. S. Santos, G. X. Miao,B. Satpati and J. S. Moodera, Phys. Rev. Lett., 2008, 100,226603.
54 B. Huang, H.-J. Jang and I. Appelbaum, Appl. Phys. Lett., 2008,93, 162508.
55 V. I. Krinichnyi, Synth. Met., 2000, 108, 173–222.56 S. Sanvito, Nat. Mater., 2007, 6, 803–804.57 P. A. Bobbert, T. D. Nguyen, F. W. A. van Oost,
B. Koopmans and M. Wohlgenannt, Phys. Rev. Lett., 2007, 99,216801.
58 O. Mermer, G. Veeraraghavan, T. L. Francis, Y. Sheng,D. T. Nguyen, M. Wohlgenannt, A. Kohler, M. K. Al-Suti andM. S. Khan, Phys. Rev. B, 2005, 72, 205202.
59 F. Kuemmeth, S. Ilani, D. C. Ralph and P. L. McEuen, Nature,2008, 452, 448–452.
60 J. Takeya, M. Yamagishi, Y. Tominari, R. Hirahara,Y. Nakazawa, T. Nishikawa, T. Kawase, T. Shimoda andS. Ogawa, Appl. Phys. Lett., 2007, 90, 102120.
61 H. O. H. Churchill, A. J. Bestwick, J. W. Harlow, F. Kuemmeth,D. Marcos, C. H. Stwertka, S. K. Watson and C. M. Marcus,Nat. Phys., 2009, 5, 321–326.
62 T. D. Nguyen, G. Hukic-Markosian, F. Wang, L. Wojcik,X.-G. Li, E. Ehrenfreund and Z. V. Vardeny, Nat. Mater.,2010, 9, 345–352.
63 J. M. Lupton, D. R. McCamey and C. Boehme, ChemPhysChem,2010, 11, 3040–3058.
64 H. Agarwal, S. Pramanik and S. Bandyopadhyay, New J. Phys.,2008, 10, 015001.
65 A. M. Bratkovsky, Phys. Rev. B, 1997, 56, 2344–2347.66 F. J. Wang, C. G. Yang, Z. V. Vardeny and X. G. Li, Phys. Rev.
B, 2007, 75, 245324.67 V. Dediu, L. E. Hueso, I. Bergenti, A. Riminucci, F. Borgatti,
P. Graziosi, C. Newby, F. Casoli, M. P. De Jong, C. Taliani andY. Zhan, Phys. Rev. B, 2008, 78, 115203.
68 S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki and K. Ando,Nat. Mater., 2004, 3, 868–871.
69 S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice,B. Hughes, M. Samant and S.-H. Yang, Nat. Mater., 2004, 3,862–867.
70 J. M. De Teresa, A. Barthelemy, A. Fert, J. P.Contour, F. Montaigne and P. Seneor, Science, 1999, 286,507–509.
71 S. Majumdar, H. S. Majumdar, R. Laiho and R. Osterbacka,J. Alloys Compd., 2006, 423, 169–171.
72 W. Xu, G. J. Szulczewski, P. LeClair, I. Navarrete, R. Schad,G. Miao, H. Guo and A. Gupta, Appl. Phys. Lett., 2007, 90,072506.
73 H. Vinzelberg, J. Schumann, D. Elefant, R. B. Gangineni,J. Thomas and B. Buchner, J. Appl. Phys., 2008, 103,093720.
74 C. Barraud, P. Seneor, R. Mattana, S. Fusil, K. Bouzehouane,C. Deranlot, P. Graziosi, L. Hueso, I. Bergenti, V. Dediu,F. Petroff and A. Fert, Nat. Phys., 2010, 6, 615–620.
75 J. Hwang, A. Wan and A. Kahn, Mater. Sci. Eng., R, 2009, 64,1–31.
76 I. Rungger, X. Chen, U. Schwingenschlogl and S. Sanvito, Phys.Rev. B, 2010, 81, 235407.
77 C. Toher and S. Sanvito, Phys. Rev. B, 2008, 77, 155402.78 T. Maruyama, Y. Shiota, T. Nozak, K. Ohta, N. Toda,
M. Mizuguchi, A. A. Tulapurkar, T. Shinjo, M. Shiraishi,S. Mizukami, Y. Ando and Y. Suzuki, Nat. Nanotechnol., 2009,4, 158–161.
79 W. F. Brinkman, R. C. Dynes and J. M. Rowell, J. Appl. Phys.,1970, 41, 1915–1921.
80 I. G. Hill, A. Rajagopal, A. Kahn and Y. Hu, Appl. Phys. Lett.,1998, 73, 662–664.
81 K. Marumoto, N. Arai, H. Goto, M. Kijima, K. Murakami,Y. Tominari, J. Takeya, Y. Shimoi, H. Tanaka, S. Kuroda,T. Kaji, T. Nishikawa, T. Takenobu and Y. Iwasa, Phys. Rev.B, 2011, 83, 075302.
82 J.-C. Tai, J.-C. Huang, Y.-M. Chang, K.-S. Li, J.-Y. Hong,S.-S. Wong, W.-C. Chiang and M.-T. Lin, Appl. Phys. Lett.,2010, 96, 262502.
83 S. Krause, M. and. A Scholl and E. Umbach, New J. Phys., 2008,10, 085001.
84 J. B. Neaton, M. S. Hybertsen and S. G. Louie, Phys. Rev. Lett.,2006, 97, 216405.
85 J. J. H. M. Schoonus, P. G. E. Lumens, W. Wagemans,J. T. Kohlhepp, P. A. Bobbert, H. J. M. Swagten andB. Koopmans, Phys. Rev. Lett., 2009, 103, 146601.
86 J.-W. Yoo, H. W. Jang, V. N. Prigodin, C. Kao, C. B. Eom andA. J. Epstein, Phys. Rev. B, 2009, 80, 205207.
87 J.-W. Yoo, H. W. Jang, V. N. Prigodin, C. Kao, C. B. Eomand A. J. Epstein, Synth. Met., 2010, 160, 216–222.
88 K. V. Raman, S. M. Watson, J. H. Shim, J. A. Borchers,J. Chang and J. S. Moodera, Phys. Rev. B, 2009, 80,195212.
89 G. Szulczewski, H. Tokuc, K. Oguz and J. M. D. Coey, Appl.Phys. Lett., 2009, 95, 202506.
90 J. S. Jiang, J. E. Pearson and S. D. Bader, Phys. Rev. B, 2008, 77,035303.
91 M. Cinchetti, K. Heimer, J.-P. Wustenberg, O. Andreyev,M. Bauer, S. Lach, C. Ziegler, Y. Gao and M. Aeschlimann,Nat. Mater., 2009, 8, 115–119.
92 A. J. Drew, J. Hoppler, L. Schulz, F. L. Pratt, P. Desai,P. Shakya, T. Kreouzis, W. P. Gillin, A. Suter, N. A.Morley, V. K. Malik, A. Dubroka, K. W. Kim, H. Bouyanfif,F. Bourqui, C. Bernhard, R. Scheuermann, G. J. Nieuwenhuys,T. Prokscha and E. Morenzoni, Nat. Mater., 2009, 8,109–114.
93 L. Schulz, L. Nuccio, M. Willis, P. Desai, P. Shakya, T. Kreouzis,V. K. Malik, C. Bernhard, F. L. Pratt, N. A. Morley, A. Suter,G. J. Nieuwenhuys, T. Prokscha, E. Morenzoni, W. P. Gillin andA. J. Drew, Nat. Mater., 2011, 10, 39–44.
94 S. Sanvito, Nat. Nanotechnol., 2007, 2, 204–206.95 S. Bandyopadhyay, Phys. Rev. B, 2010, 81, 153202.96 R. J. Elliott, Phys. Rev., 1954, 96, 266–279.97 H. M. McConnell, J. Chem. Phys., 1956, 24, 764–766.98 P. A. Bobbert, Nat. Mater., 2010, 9, 288–290.99 D. R. McCamey, K. J. van Schooten, W. J. Baker, S. Y. Lee,
S. Y. Paik, J. M. Lupton and C. Boehme, Phys. Rev. Lett., 2010,104, 017601.
100 N. J. Rolfe, M. Heeney, P. B. Wyatt, A. J. Drew, T. Kreouzis andW. P. Gillin, Phys. Rev. B, 2009, 80, 241201.
101 S. Sanvito, Nat. Phys., 2010, 6, 562–564.102 L. Fang, K. Deniz Bozdag, C.-Y. Chen, P. A. Truitt, A. J.
Epstein and E. Johnston-Halperin, Phys. Rev. Lett., 2011, 106,156602.
103 J.-W. Yoo, C.-Y. Chen, H. W. Jang, C. W. Bark, V. N.Prigodin, C. B. Eom and A. J. Epstein, Nat. Mater., 2010, 9,638–642.
104 S. J. van der Molen and P. Liljeroth, J. Phys.: Condens. Matter,2010, 22, 133001.
105 A. Odell, A. Delin, B. Johansson, I. Rungger and S. Sanvito, ACSNano, 2010, 4, 2635–2642.
106 D. B. Strukov, G. S. Snider, D. R. Stewart and R. S. Williams,Nature, 2008, 453, 80–83.
107 L. Chua, IEEE Trans. Circuit Theory, 1971, 18, 507–519.108 M. Prezioso, A. Riminucci, I. Bergenti, P. Graziosi, D. Brunel
and V. A. Dediu, Adv. Mater., 2011, 23, 1371–1375.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
3354 Chem. Soc. Rev., 2011, 40, 3336–3355 This journal is c The Royal Society of Chemistry 2011
109 S. Horiuchi, Y. Tokunaga, G. Giovannetti, S. Picozzi, H. Itoh,R. Shimano, R. Kumai and Y. Tokura, Nature, 2010, 463,789–792.
110 F. Kagawa, S. Horiuchi, M. Tokunaga, J. Fujioka andY. Tokura, Nat. Phys., 2010, 6, 169–172.
111 F. Chen, J. Hihath, Z. Huang, X. Li and N. J. Tao, Annu. Rev.Phys. Chem., 2007, 58, 535–564.
112 A. Osorio, T. Bjørnholm, J.-M. Lehn, M. Ruben and H. S. J. vander Zant, J. Phys.: Condens. Matter, 2008, 20, 374121.
113 S. J. Xie, K. H. Ahn, D. L. Smith, A. R. Bishop and A. Saxena,Phys. Rev. B, 2003, 67, 125202.
114 P. P. Ruden and D. L. Smith, J. Appl. Phys., 2004, 95,4898–4904.
115 M. Yunus, P. P. Ruden and D. L. Smith, Synth. Met., 2010, 160,204–209.
116 N. Baadji, S. Kuck, J. Brede, G. Hoffmann, R. Wiesendanger andS. Sanvito, Phys. Rev. B, 2010, 82, 115447.
117 J. Park, A. N. Pasupathy, J. I. Goldsmith, C. Chang, Y. Yaish,J. R. Petta, M. Rinkoski, J. P. Sethna, H. D. Abruna,P. L. McEuen and D. C. Ralph, Nature, 2002, 417, 722–725.
118 W. Liang, M. P. Shores, M. Bockrath, J. R. Long and H. Park,Nature, 2002, 417, 725–729.
119 G. D. Scott and D. Natelson, ACS Nano, 2010, 4, 3560–3579.120 A. Scheybal, T. Ramsvik, R. Bertschinger, M. Putero, F. Nolting
and T. A. Jung, Chem. Phys. Lett., 2005, 411, 214–220.121 H. Wende, M. Bernien, J. Luo, C. Sorg, N. Ponpandian,
J. Kurde, J. Miguel, M. Piantek, X. Xu, P. Eckhold, W. Kuch,K. Baberschke, P. M. Panchmatia, B. Sanyal, P. M. Oppeneerand O. Eriksson, Nat. Mater., 2007, 6, 516–520.
122 C. Iacovita, M. V. Rastei, B. W. Heinrich, T. Brumme, J. Kortus,L. Limot and J. P. Bucher, Phys. Rev. Lett., 2008, 101,116602.
123 M. Bernien, J. Miguel, C. Weis, M. E. Ali, J. Kurde, B. Krumme,P. M. Panchmatia, B. Sanyal, M. Piantek, P. Srivastava,K. Baberschke, P. M. Oppeneer, O. Eriksson, W. Kuch andH. Wende, Phys. Rev. Lett., 2009, 102, 047202.
124 P. Gambardella, S. Stepanow, A. Dmitriev, J. Honolka,F. M. F. de Groot, M. Lingenfelder, S. S. Gupta, D. D. Sarma,P. Bencok, S. Stanescu, S. Clair, S. Pons, N. Lin, A. P. Seitsonen,H. Brune, J. V. Barth and K. Kern, Nat. Mater., 2009, 8,189–193.
125 S. Javaid, M. Bowen, S. Boukari, L. Joly, J. B. Beaufrand,X. Chen, Y. J. Dappe, F. Scheurer, J. P. Kappler, J. Arabski,W. Wulfhekel, M. Alouani and E. Beaurepaire, Phys. Rev. Lett.,2010, 105, 077201.
126 S. Stepanow, A. Mugarza, G. Ceballos, P. Moras, J. C. Cezar,C. Carbone and P. Gambardella, Phys. Rev. B, 2010, 82,014405.
127 A. R. Rocha and S. Sanvito, J. Appl. Phys., 2007, 101,09B102–5.
128 N. Atodiresei, J. Brede, P. Lazicacute, V. Caciuc, G. Hoffmann,R. Wiesendanger and S. Blugel, Phys. Rev. Lett., 2010, 105,066601.
129 J. Brede, N. Atodiresei, S. Kuck, P. Lazicacute, V. Caciuc,Y. Morikawa, G. Hoffmann, S. Blugel and R. Wiesendanger,Phys. Rev. Lett., 2010, 105, 047204.
130 A. J. Heinrich, J. A. Gupta, C. P. Lutz and D. M. Eigler, Science,2004, 306, 466–469.
131 S. Loth, C. P. Lutz and A. J. Heinrich, New J. Phys., 2010, 12,125021.
132 A. Troisi and M. A. Ratner, Nano Lett., 2006, 6, 1784–1788.133 M. Paulsson, T. Frederiksen, H. Ueba, N. Lorente and
M. Brandbyge, Phys. Rev. Lett., 2008, 100, 226604.134 C. F. Hirjibehedin, C. P. Lutz and A. J. Heinrich, Science, 2006,
312, 1021–1024.135 C. F. Hirjibehedin, C.-Y. Lin, A. F. Otte, M. Ternes, C. P. Lutz,
B. A. Jones and A. J. Heinrich, Science, 2007, 317, 1199–1203.136 X. Chen, Y.-S. Fu, S.-H. Ji, T. Zhang, P. Cheng, X.-C. Ma,
X.-L. Zou, W.-H. Duan, J.-F. Jia and Q.-K. Xue, Phys. Rev.Lett., 2008, 101, 197208.
137 Y.-S. Fu, T. Zhang, S.-H. Ji, X. Chen, X.-C. Ma, J.-F. Jia andQ.-K. Xue, Phys. Rev. Lett., 2009, 103, 257202.
138 A. Zhao, Q. Li, L. Chen, H. Xiang, W. Wang, S. Pan, B. Wang,X. Xiao, J. Yang, J. G. Hou and Q. Zhu, Science, 2005, 309,1542–1544.
139 P. Lucignano, R. Mazzarello, A. Smogunov, M. Fabrizio andE. Tosatti, Nat. Mater., 2009, 8, 563–567.
140 D. Gatteschi, R. Sessoli and J. Villain, Molecular Nanomagnets,Oxford University Press, Oxford, 2006.
141 S. Voss, M. Fonin, U. Rudiger, M. Burgert, U. Groth andY. S. Dedkov, Phys. Rev. B, 2007, 75, 045102.
142 M. Mannini, P. Sainctavit, R. Sessoli, C. Cartier dit Moulin,F. Pineider, M.-A. Arrio, A. Cornia and D. Gatteschi, Chem.–Eur.J., 2008, 14, 7530–7535.
143 Z. Yi, X. Shen, L. Sun, Z. Shen, S. Hou and S. Sanvito, ACSNano, 2010, 4, 2274–2282.
144 M. Mannini, F. Pineider, P. Sainctavit, C. Denieli, E. Otero,C. Sciancalepore, A. M. Talarico, M. A. Arrio, A. Cornia,D. Gatteschi and R. Sessoli, Nat. Mater., 2009, 8, 194–197.
145 M. Mannini, F. Pineider, C. Danieli, F. Totti, L. Sorace,P. Sainctavit, M. A. Arrio, E. Otero, L. Joly, J. C. Cezar,A. Cornia and R. Sessoli, Nature, 2010, 468, 417–421.
146 J. Wang, P. H. Acioli and J. Jellinek, J. Am. Chem. Soc., 2005,127, 2812–2813.
147 H. Xiang, J. Yang, J. G. Hou and Q. Zhu, J. Am. Chem. Soc.,2006, 128, 2310–2314.
148 V. V. Maslyuk, A. Bagrets, V. Meded, A. Arnold, F. Evers,M. Brandbyge, T. Bredow and I. Mertig, Phys. Rev. Lett., 2006,97, 097201.
149 L. Wang, Z. Cai, J. Wang, J. Lu, G. Luo, L. Lai, J. Zhou, R. Qin,Z. Gao, D. Yu, G. Li, W. N. Mei and S. Sanvito, Nano Lett.,2008, 8, 3640–3644.
150 A. Soncini and L. F. Chibotaru, Phys. Rev. B, 2008, 77,220406.
151 A. Soncini and L. F. Chibotaru, Phys. Rev. B, 2010, 81,132403.
152 C. D. Pemmaraju, I. Rungger and S. Sanvito, Phys. Rev. B, 2009,80, 104422.
153 H. B. Heersche, Z. de Groot, J. A. Folk, H. S. J. van der Zant,C. Romeike, M. R. Wegewijs, L. Zobbi, D. Barreca, E. Tondelloand A. Cornia, Phys. Rev. Lett., 2006, 96, 206801.
154 M.-H. Jo, J. E. Grose, K. Baheti, M. M. Deshmukh, J. J. Sokol,E. M. Rumberger, D. N. Hendrickson, J. R. Long, H. Park andD. C. Ralph, Nano Lett., 2006, 6, 2014–2020.
155 C. Romeike, M. R. Wegewijs and H. Schoeller, Phys. Rev. Lett.,2006, 96, 196805.
156 C. Romeike, M. R. Wegewijs, M. Ruben, W. Wenzel andH. Schoeller, Phys. Rev. B, 2007, 75, 064404.
157 M. Lopes, A. Candini, M. Urdampilleta, A. Reserbat-Plantey,V. Bellini, S. Klyatskaya, L. Marty, M. Ruben, M. Affronte,W. Wernsdorfer and N. Bendiab, ACS Nano, 2010, 4,7531–7537.
158 P. Wei, L. Sun, E. Benassi, Z. Shen, S. Sanvito, S. Hou, Preprint,2011.
159 M. N. Leuenberger and D. Loss, Nature, 2001, 410, 789–793.160 G. A. Timco, S. Carretta, F. Troiani, F. Tuna, R. J. Pritchard,
C. A. Muryn, E. J. L. McInnes, A. Ghirri, A. Candini, P. Santini,G. Amoretti, M. Affronte andR. E. P.Winpenny,Nat. Nanotechnol.,2009, 4, 173–178.
161 J. Lehmann, A. Gaita-Arino, E. Coronado and D. Loss, Nat.Nanotechnol., 2007, 2, 312–317.
162 C. Wackerlin, D. Chylarecka, A. Kleibert, K. Muller, C. Iacovita,F. Nolting, T. A. Jung and N. Ballav, Nat. Commun., 2010,1, 94.
163 P. Gutlich and H. A. Goodwin, Spin Crossover in TransitionMetal Compounds, Springer-Verlag, Berlin Heidelberg, Germany,2004.
164 O. Sato, J. Tao and Y.-Z. Zhang, Angew. Chem., Int. Ed., 2007,46, 2152–2187.
165 P. Gutlich, H. A. Goodwin, D. N. Hendrickson and C. G. Pierpont,in Valence Tautomeric Transition Metal Complexes, SpringerBerlin/Heidelberg, 2004, vol. 234, pp. 786–786.
166 A. Beni, C. Carbonera, A. Dei, J.-F. Letard, R. Righini,C. Sangregorio and L. Sorace, J. Braz. Chem. Soc., 2006, 17,1522.
167 M. Kepenekian, B. L. Guennic and V. Robert, J. Am. Chem. Soc.,2009, 131, 11498–11502.
168 M. Kepenekian, B. Le Guennic and V. Robert, Phys. Rev. B,2009, 79, 094428.
169 A. Droghetti and S. Sanvito, 2011, arXiv:1101.4777v1.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online
This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 3336–3355 3355
170 K. Takahashi, H.-B. Cui, Y. Okano, H. Kobayashi, Y. Einagaand O. Sato, Inorg. Chem., 2006, 45, 5739–5741.
171 M. S. Alam,M. Stocker, K. Gieb, P.Muller,M.Haryono, K. Studentand A. Grohmann, Angew. Chem., Int. Ed., 2010, 49, 1159–1163.
172 F. Prins, M. Monrabal-Capilla, E. A. Osorio, E. Coronado andH. S. J. van der Zant, Adv. Mater., 2011, 23, 1545–1549.
173 M. Diefenbach and K. S. Kim, Angew. Chem., 2007, 119,7784–7787.
174 N. Baadji, M. Piacenza, T. Tugsuz, F. D. Sala, G. Maruccio andS. Sanvito, Nat. Mater., 2009, 8, 813–817.
175 S. K. Shukla and S. Sanvito, Phys. Rev. B, 2009, 80,184429.
176 E. A. Osorio, K. Moth-Poulsen, H. S. J. van der Zant, J. Paaske,P. Hedegard, K. Flensberg, J. Bendix and T. Bjørnholm,Nano Lett.,2009, 10, 105–110.
177 M. Misiorny and J. Barnas, Phys. Rev. B, 2008, 77, 172414.
Publ
ishe
d on
09
May
201
1. D
ownl
oade
d by
IN
DIA
N I
NST
ITU
TE
OF
TE
CH
NO
LO
GY
BO
MB
AY
on
20/1
0/20
15 1
2:19
:08.
View Article Online