boston regional nanomagnetism workshop...g-2 dzyaloshinskii-moriya interaction influence on...

Boston Regional

Nanomagnetism Workshop

Sponsored by:

IEEE Magnetics Society and Northeastern University

One-day Workshop on Magnetism at the Nanoscale

Location and Date:

Northeastern University

Boston, MA.

April 4th, 2014

Logo design: Ian McDonald & Nina Bordeaux

Agenda

i

Reception – Coffee – Poster set up (9:15 AM – 10:00 AM)

Morning Session (10:00 AM – 12:30 PM)

Time Topic Speaker Affiliation

10:00 – 10:10 Greetings Prof. Laura H. Lewis Chem. E., NEU

Focus: Nanomagnetism from Perspective of Fundamental Science [Moderator: Dr. Radhika Barua]

10:12 – 10:24 Inducing Ferromagnetism in Topological Insulators Using Heterostructures of TI and Magnetic Insulator

Dr. Ferhat Katmis Physics, MIT

10:24 – 10:36 Domain Walls Driven by Interfacial Phenomena: Pushing the Boundaries of Magnetics

Dr. Satoru Emori ECE, NEU

10:36 – 10:48 Exchange Bias of Ion Beam Etched NiFe/IrMn Nanostructures Mr. Frank Liu MSE, MIT

10:48 – 11:00 SnTe-EuS-SnTe Trilayers Grown by MBE on Si: A Materials Characterization Report.

Mr. Badih Assaf Physics, NEU

Break (11:00 AM – 11:20 AM)

11:24 – 11:36 Quantum Anomalous Hall Effect in the Magnetic Topological Insulator Films

Dr. Chang Cui-Zu Physics, MIT

11:36 – 11:48 Square Array of BiFeO3-CoFe2O4 Mutiferroic Nanocomposite Templated by Triblock Terpolymer

Dr. Hong Kyoon Choi

MSE, MIT

11:48 – 12:00 Magneto-Ionic Control of Interfacial Magnetism Mr. Uwe Bauer MSE, MIT

12:00 – 12:12 Correlation of Electronic Structure and Magnetic Properties in Fe-doped Titania Nanotubes

Dr. Pegah M. Hosseinpour

Chem. E., NEU

Lunch and Poster Session (12:15 PM – 2:00 PM)

A-1 Maximizing Hysteretic Losses in Magnetic Ferrite Nanoparticles via Model-Driven Synthesis and Materials Optimization

Mr. Ritchie Chen MSE, MIT

A-2 Tailoring AlFe2B2 Magnetism-Structure Relationships for Magnetocaloric Applications

Mr. Brian Lejune Chem. E., NEU

B-1 Templated Self-assembly of Perovskite/Spinel Nanocomposite Thin Films Mr. Nicholas Aimon MSE, MIT

B-2 Towards Exchange-Biased Permanent Magnets: Structure-Property Correlations in FeMn Alloys

Mr. Ian McDonald Chem. E., NEU

C-1 Interlayer Coupling in Ultra-Thin L10-FePt/MgO/[Co/Pd]30 Magnetic Tunnel Junctions

Dr. Pin Ho MSE, MIT

C-2 Half-Metallic Antiferromagnets: A New Class of Materials for Spintronic Devices

Ms. Michelle Jamer Physics, NEU

D-1 Large-area Periodic Magnetic Microstructures for Controlling Magnetic Micro-particles

Ms. Minae Ouk MSE, MIT

Boston Regional Nanomagnetism Workshop

Date: April 4th, 2014 Location: Room 378, 140 The Fenway Building

Northeastern University, Boston, MA.

AGENDA

Agenda

ii

D-2 Understanding L10-phase Formation in the FeNi System through the Study of FePd(Ni) Compounds

Ms. Ana Montes-Arango

Chem. E., NEU

E-1 Nanoscale-driven Crystal Growth of Hexaferrite Architecture for Magnetoelectrically Tunable Microwave Semiconductor Integrated Devices

Ms. Bolin Hu ECE, NEU

E-2 Sub-100 nm Magnetic Wires with Low Edge Roughness Ms. Saima Siddiqui MSE, MIT

F-1 Thermomagnetic Behavior of L10 FeNi (Tetrataenite) from Meteorites Ms. Nina Bordeaux Chem. E., NEU

F-2 Formation and Current Effects on 360° Domain Walls in Magnetic Nanowires

Dr. Larysa Tryputen MSE, MIT

G-1 Transofmation Kinetics in Epitaxial FeRh Thin Films Dr. Melissa Loving Chem. E., NEU

G-2 Dzyaloshinskii-Moriya Interaction Influence on Stochastic Spin Orbit Torque Switching

Mr. Seong-Hoon Woo

MSE, MIT

H-1 Detection of Field and Current Effects on 360˚ Domain Walls by Anisotropic Magnetoresistance Measurements

Mr. Jinshuo Zhang MSE, MIT

H-2 Resonant modes of coupled magnetic nanodisks Mr. Maximilian Albert

U. of Southampton

Afternoon Session (2:00 PM – 3:00 PM)

Focus: Nanomagnetism: Towards Magnetic Devices [Moderator: Dr. Pegah M. Hosseinpour]

Time Topic Speaker Affiliation

2:00 – 2:12 Tailoring the FeRh Magnetostructural Response: Simultaneous Effects of Pressure and Magnetic Field

Dr. Radhika Barua Chem. E., NEU

2:12 – 2:24 Magnetothermal Multiplexing Mr. Michael Christiansen

MSE, MIT

2:24 – 2:36 Quantification of Strain and Charge Co-Mediated Magnetoelectric Coupling on Ultra-thin Permalloy/PMN-PT Interface

Mr. Tianxiang Nan ECE, NEU

2:36 – 2:48 Nanoscale Magnetic Materials for Energy-Efficient Spin-Based Transistors and Logic

Ms. Jean Currivan MSE, MIT; Physics, Harvard

2:48 – 3:00 Fabrication of Magnetically Hard Cobalt Carbide Nanoparticles via Wet Chemical Synthesis

Dr. Mehdi Zamanpour

Interdiscip. Eng., NEU

3:00 – 3:12 Integration of Self-Assembled Nanocomposite on Si substrate Dr. Dong Hun Kim MSE, MIT

Break (3:15 PM – 3:30 PM)

3:30 to 4:30 Topological Effects in Nanomagnetism: From Perpendicular Recording to Monopoles

Prof. Hans-Benjamin Braun University College Dublin, Ireland

End of the Workshop

Keynote Speaker, IEEE Distinguished Lecture [Moderator: Professor Laura H. Lewis]

Greetings from the Organizers

iii

Greetings from the Organizers of the Boston Regional Nanomagnetism Workshop

“When you engage with people, you build your own insight into what’s being discussed. Someone else’s understanding complements yours, and together you start to weave an informed interpretation. You tinker until you can move on.” - Marcia Conner, Author of the book, “The New Social Learning”

In keeping with the spirit of this quotation, Northeastern University is pleased to

host the first Boston Regional Nanomagnetism Workshop (RNW) on April 4th 2014.

The primary objective of this forum is to facilitate exchange of ideas and active

collaborations between research groups working in the field of fundamental and

applied magnetism in the Greater Boston area. The Workshop features a series of

oral talks and poster presentations by graduate students and post-doctoral research

associates from local universities namely Northeastern University, Harvard

University and Massachusetts Institute of Technology (MIT). The key-note speech for

the event will be given by the IEEE Magnetics Society 2014 Distinguished Lecturer,

Prof. Hans-Benjamin Braun, University College Dublin (UCD), Ireland.

Abstracts of the talks and posters are available in this booklet and online at:

http://www.northeastern.edu/nanomagnetism. To enable long term interaction

between the attendees of the Workshop, contact information of the authors of the

abstracts is provided in this document as well. We hope that you have a wonderful

time networking with your peers today!

Sincerely, Radhika Barua, Ph. D.

Pegah M. Hosseinpour, Ph. D.

Co-Organizers, Boston Regional Nanomagnetism Workshop

http://www.northeastern.edu/nanomagnetism

Keynote Speaker

1

Biography: Professor Hans-Benjamin Braun is currently Associate

Professor for Theoretical Physics at University College Dublin

(Ireland). After studies in Physics and Mathematics he received

his diploma degree from the University of Basel (Switzerland) and in 1991 he earned his PhD in

Theoretical Physics at ETH in Zurich. After postdoctoral research at the Physics Department and

the Center for Magnetic Recording Research at the University of California at San Diego he was

awarded a NSERC International Fellowship to work at Simon Fraser University in Vancouver

(Canada). Subsequently he returned to Switzerland to take up a position as Senior Scientist at

the Paul Scherrer Institute (PSI). He joined the Faculty of the School of Physics at University

College Dublin (UCD) in 2004, where he founded and leads the group in Condensed Matter

Theory supported by the Science Foundation of Ireland. Professor Braun developed the theory

for non-uniform thermally activated magnetization reversal in nanowires which now forms the

basis for the design of perpendicular magnetic recording media. Well before it was recognized

experimentally, he theoretically predicted quasi one-dimensional behavior in magnetic

nanowires and he introduced the now widely used notion of domain wall chirality. His work led

to the prediction of the spontaneous emergence of spin currents in quantum spin chains, an

effect that he and his collaborators subsequently observed via spin polarized neutron

scattering. Furthermore he proposed and interpreted a series of experiments on

nanolithographic arrays that led to the discovery of emergent monopoles in artificial spin ice

together with colleagues from PSI and UCD. In addition to numerous publications in top

research journals he also authored popular articles for the French and German versions of

Scientific American and he holds two patents.

Keynote Speaker

Professor Hans-Benjamin Braun

School of Physics, University College Dublin, Ireland.

(IEEE Magnetics Society 2014 Distinguished Lecturer)

Keynote Speaker

2

Keynote Presentation

Topological Effects in Nanomagnetism: From Perpendicular Recording to Monopoles

Prof. Hans-Benjamin Braun, University College Dublin (School of Physics), Ireland

Similar to knots in a rope, the magnetization in a material can form particularly robust

configurations. Such topologically stable structures include domain walls, vortices and

skyrmions which are not just attractive candidates for future data storage applications

but are also of fundamental importance to current memory technology. For example,

the creation of domain wall pairs of opposite chirality delimits the thermal stability of

bits in present high anisotropy perpendicular recording media. The ever increasing

demand for higher data storage density forces us to understand topological defects at

ever decreasing length scales where thermal and quantum effects play an increasingly

important role.

This talk will be adapted to the interests of the audience and will start with an

overview over topological defects in magnetic systems. As a practical application it is

shown how thermal domain wall nucleation affects the design of perpendicular

magnetic recording media. In a second part, it is demonstrated how the geometric

Berry’s phase allows micromagnetics to be extended to include quantum effects. As an

important consequence it will be shown how the chirality of a classical domain wall

translates into quantum spin currents which in turn can be used for information

transport. All concepts will be illustrated by state of the art experiments, which

encompass the techniques of polarized neutrons and synchrotron x-rays. The final part

of the talk will discuss how magnetic monopoles emerge as topological defects in

densely packed arrays of nanoislands which effectively interact as dipoles, a system also

known as ‘artificial spin ice’. In contrast to conventional thin films, where magnetization

reversal occurs via nucleation and extensive domain growth, magnetization reversal in

2D artificial spin ice is restricted to an avalanche-type formation of 1D strings. These

objects constitute classical versions of Dirac strings that feed magnetic flux into the

emergent magnetic monopoles. It is demonstrated how the motion of these magnetic

charges can be individually controlled experimentally and used to perform simple logic

operations.

References:

[1] H.B. Braun, "Topological effects in nanomagnetism: from superparamagnetism to

chiral quantum solitons", Adv. Phys. 61, 1–116 (2012).

[2] E. Mengotti, L.J. Heyderman, A. Fraile Rodriguez, F. Nolting, R.V. H Hügli and H.B.

Braun, "Real space observation of Dirac strings and magnetic monopoles in artificial

kagome spin ice” Nature Physics 7, 68–74 (2011).

Oral Presentations

(Morning Session)

Oral Presentations (Morning Session)

4

Inducing Ferromagnetism in Topological Insulators using Heterostructures of TI and Magnetic Insulator

F. Katmis1,2, V. Lauter3, B. A. Assaf4, P. Wei2, D. Heiman4, and J. S. Moodera1,2

1Department of Physics, MIT, Cambridge, MA, USA 2Francis Bitter Magnet Laboratory, MIT, Cambridge, MA, USA

3NSSD, Oak Ridge National Laboratory, Oak Ridge, TN, USA 4Department of Physics, Northeastern University, Boston, MA, USA

The short-range nature of magnetic proximity coupling with a ferromagnetic insulator

(FI) induces ferromagnetic interactions on a topological insulator (TI) surface state. In

the present study we investigate Bi2Se3/EuS (bi-layer) heterostructures and elucidate

the mechanism to induce the ferromagnetic order onto the surface of Bi2Se3 thin films

by using EuS as a FI layer. SQUID measurements demonstrated a magnetic moment that

is excessive for the EuS film alone, thus indicating that EuS induces a significant

magnetic moment on the surface of the Bi2Se3 film. Polarized neutron reflectometry

(PNR) allows for a direct measurement of the magnetization depth profile in the bi-layer

films. Using PNR we reveal that EuS induces a significant magnetic moment in TI films.

These results indicate that the interface region of TI becomes magnetized. Thus, it

creates broken time-reversal symmetry and should appear as a magnetic signature in

electrical transport. Both the ferromagnetism of EuS and coupling between EuS and

Bi2Se3 has to be strong to induce surface magnetism. These findings contribute to

emergent quantum coherent phenomena; the local time-reversal symmetry breaking is

essential for observing predicted new effects such as quantized topological

magnetoelectric response.


5

Domain Walls Driven by Interfacial Phenomena: Pushing the Boundaries of Magnetics

Satoru Emori1,2, and Geoffrey S. D. Beach1

1Massachusetts Institute of Technology, Cambridge, USA 2 Northeastern University, Boston, USA

A domain wall in a ferromagnetic material is a boundary between differently

magnetized regions, and its motion provides a convenient scheme to control the

magnetization state of the material. Domain walls can be confined and moved along

nanostrips of magnetic thin films, which are proposed platforms for next generations of

solid-state magnetic memory-storage and logic devices. In these devices, domain walls

must be moved by electric current, rather than by magnetic field, to achieve scalability

and lower-power operation. Recent studies have reported efficient domain-wall motion

driven by current in out-of-plane magnetized multilayer films with strong spin-orbit

coupling. In particular, extraordinary current-driven domain-wall motion has been

observed in atomically-thin ferromagnets sandwiched between a nonmagnetic heavy

metal and an insulator.

Through experimental studies on various sputtered magnetic multilayers, we

elucidate the mechanism of such anomalous domain-wall dynamics driven by the spin

Hall effect: a charge current in the nonmagnetic heavy metal generates a spin current,

which exerts a torque on spins in the adjacent ferromagnet. This spin Hall torque drives

domain walls forward if the domain-wall spins are parallel to the nanostrip axis with a

fixed chirality. We reveal that the Dzyaloshinskii-Moriya interaction, arising from spin-

orbit coupling and asymmetric interfaces, stabilizes homochiral domain walls in ultrathin

ferromagnets. Our findings not only provide a route to bolster current-driven domain-

wall dynamics, but also enable new chiral magnetic textures in magnetic

heterostructures for device applications.


6

Exchange bias of ion beam etched NiFe/IrMn nanostructures

F. Liu and C.A. Ross

Massachusetts Institute of Technology, USA

Exchange bias between ferromagnet/antiferromagnet (FM/AFM) layers creates a bias

field that is promising for locally pinning the magnetic orientation of ferromagnetic

layers in nanodevices. However, in structures where part of the ferromagnetic layer is

pinned and another part is unpinned, such as a magnetic wire with exchange pinning at

the ends, it is necessary to develop a fabrication process that provides adequate local

pinning. One such method is to deposit the bilayer stack then selectively remove the

AFM material by ion beam etching. However, to minimize the amount of etching and

protect the unbiased parts of the structure, an alternative method will be described in

which the FM is deposited and patterned, then a second lithography step is done to

open windows where the AFM is required. A short etch followed by optionally

deposition of a thin FM then an AFM layer completes the process. This presentation

compares results from these processing methods. Fig. 1 shows the effect on the

hysteresis loops of ion beam etching of a NiFe 10 nm/IrMn 20 nm bilayer. 10 nm NiFe

was deposited using DC magnetron sputtering, then ion beam etched for varying

amounts of time before 20 nm IrMn was deposited. Thickness, coercivity, and exchange

bias were obtained using vibrating sample magnetometry. Results from patterned

nanostructures will be presented including the effect of feature size on exchange bias

and coercivity.

Figure. A comparison between hysteresis loops of 120 nm diameter dots and that of

continuous films under different periods of ion beam etching time.


7

SnTe-EuS-SnTe Trilayers Grown by MBE on Si: A Materials Characterization Report

B. A. Assaf1, F. Katmis2, 3, P. Wei2, B. Satpati4, J. S. Moodera2, 3, D. Heiman1

1Physics. Dept. Northeastern University, Boston, MA 02115, USA 2 Francis Bitter Magnet Lab, MIT, Cambridge, MA 02139, USA

3Physics. Dept. MIT, Cambridge, MA 02139, USA 4Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 64, India

The realization of functional devices, combining a topological material with a

ferromagnet is of capital importance, as it allows one to probe and manipulate

topological surface states without altering the bulk [1,2]. We thus report the growth and

characterization of heterostructures consisting of a 2nm-thick ferromagnetic insulator –

EuS – buried between two 10nm-thick films of topological crystalline insulator (TCI)

SnTe. Trilayers are grown by MBE on Si. X-ray diffraction, atomic force microscopy,

cross-sectional transmission electron microscopy and X-ray absorption spectroscopy are

used to characterize the structural properties of the films. SQUID magnetometry and

transport measurements are used to investigate the magnetic properties of the trilayers

and the possible appearance of proximity-induced magnetism onto the surface states of

SnTe. Our results show that the magnetic properties of EuS are not dramatically altered

and the EuS layer remains strongly ferromagnetic. A change in the lattice constant of

EuS is observed along with a slight reduction of the moment per Eu2+. These changes

could either be a result of strain or Te diffusion into EuS. It is interesting that proximity-

induced magnetism is only observed in the trilayer having the largest interface

roughness. These results provide the basis for future studies on the behavior of TCI

surface states under the effect of proximity-induced magnetism.

Work supported by NSF-DMR-0907007 and partly by NSF-DMR-1207469, ONR-N00014-

13-1-0301, and MIT MRSEC under NSF-DMR-0819762.

References:

[1] P. Wei, et al. Phys. Rev. Lett. 110, 186807 (2013).

[2] C.H. Li, et al. Nature Nanotech. 16, 218 (2014).


8

Quantum Anomalous Hall Effect in the Magnetic Topological Insulator Films

Cui-Zu Chang

Francis Bitter Magnetic Lab, MIT, Cambridge, U.S.A ([email protected])

Anomalous Hall effect (AHE) was discovered by Edwin Herbert Hall in 1880. In this talk,

we report the experimental observation of the quantized version of AHE, the quantum

anomalous Hall effect (QAHE) in thin films of Cr-doped (Bi0.1Sb0.9)2Te3 magnetic

topological insulator. At zero magnetic field, the gate-tuned anomalous Hall resistance

exhibits a quantized value of h/e2 accompanied by a significant drop of the longitudinal

resistance. The longitudinal resistance vanishes under a strong magnetic field whereas

the Hall resistance remains at the quantized value. The experimental realization of the

QAHE paves a way for developing low-power-consumption electronics.

References:

[1] Cui-Zu Chang et al. Adv. Mater. 25, 1065 (2013).

[2] Cui-Zu Chang et al. Science 340, 167 (2013)


9

Square Array of BiFeO3-CoFe2O4 Mutiferroic Nanocomposite Templated by Triblock Terpolymer

Hong Kyoon Choi, Nicolas Aimon, Dong Hun Kim, Xue Yin Sun, Caroline Ross

Department of Materials Science and Engineering, Massachusetts Institute of

Technology, Cambridge, MA, USA

BiFeO3/CoFe2O4 (BFO/CFO) composite grown on SrTiO3 (STO) substrate forms self-

assembled columnar nanostructure. This BFO/CFO nanocomposite shows

magnetoelectric coupling induced from epitaxial strain along to the vertical interface. In

order to apply this multiferroic columnar nanostructure for devices application,

controlling the location of pillars in ordered array is desirable. In this work, we present

an effective process to fabricate square array BFO/CFO nanocomposite by using ABC

linear triblock terpolymer as a template. As described in Figure 1, square array of pits on

Nb:STO substrate were generated by pattern transfer form PI-b-PS-b-PFS triblock

terpolymer. CFO nuclei and thin BFO film were then selectively grown in pit and mesa

respectively by taking advantage of different wetting behavior of etch phases. Finally

thick BFO/CFO nanocomposite was deposited from guidance of thin BFO/CFO layer.

Compare to previously reported top down patterning methods, this patterning process

based on block copolymer can provide smaller periodicity of square symmetry over

large area with short process time.

Figure. (a) Schematic illustration of the fabrication process of a templated BFO/CFO nanocomposite. (b) Self-assembled square symmetry of holes from PI-b-PS-b-PFS triblock terpolymer. (c) Square array of pits patterned on Nb:STO substrate. (d) CFO nuclei selectively grown in patterned pits. (e) Thin layer of BFO/CFO nanocomposite. (f) Thick BFO/CFO nanocomposite grown from thin layer of BFO/CFO nanocomposite. Scale bars are 300nm.


10

Magneto-Ionic Control of Interfacial Magnetism

U. Bauer, S. Emori and G.S.D Beach

Massachusetts Institute of Technology, Cambridge, USA

Voltage control of magnetism could bring about revolutionary new spintronic memory

and logic devices. Here, we examine domain wall (DW) dynamics in ultrathin Co films

and nanowires under the influence of a voltage applied across a gadolinium oxide gate

dielectric that simultaneously acts as an oxygen ion conductor. We investigate two

electrode configurations, one with a continuous gate dielectric and the other with a

patterned gate dielectric which exhibits an open oxide edge right underneath the

electrode perimeter. We demonstrate that the open oxide edge acts as a fast diffusion

path for oxygen ions and allows voltage-induced switching of magnetic anisotropy at the

nanoscale by modulating interfacial chemistry rather than charge density. At room

temperature this effect is limited to the vicinity of the open oxide edge, but at a

temperature of 100˚C it allows complete control over magnetic anisotropy across the

whole electrode area, due to higher oxygen ion mobility at elevated temperature. We

then harness this novel ‘magneto-ionic’ effect to create unprecedentedly strong

voltage-induced anisotropy modifications of 5000 fJ/Vm and create electrically

programmable DW traps with pinning strengths of 650 Oe, enough to bring to a

standstill DWs travelling at speeds of at least 20 m/s.


11

Correlation of Electronic Structure and Magnetic Properties in Fe-doped Titania Nanotubes

Pegah M. Hosseinpour, Félix Jiménez-Villacorta and Laura H. Lewis

Department of Chemical Engineering, Northeastern University, Boston, MA

Iron-doped titania nanotube arrays are proper candidates for applications such as

photocatalytic and potential spintronic devices. Considering the effect of crystallinity,

magnetic properties and electronic structure on the functionality of titania-based

nanotubes, understanding the influence of the crystal structure and magnetic impurities

on the electronic structure of titania as a function of dopant composition and processing

conditions is of paramount importance. In this work, arrays of iron-incorporated titania

nanotubes are electrochemically synthesized followed by annealing at 450 °C in oxygen

to crystalize. The crystal structure, magnetic properties and electronic structure of these

nanotubes in the as-anodized and annealed states are characterized using x-ray

diffraction, SQUID magnetometry and x-ray absorption spectroscopy (XAS). Results

show that annealing the nanotubes yields crystallization into the anatase structure, and

that Fe-doped nanotubes have a slightly larger unit cell volume as compared to the pure

nanotubes. In addition, higher amount of near surface and bulk anatase formation and

larger crystallite size is observed in the Fe-doped nanotubes. Iron in the doped

nanotubes is in the ionized state and mostly with a local structure resembling that of the

α-Fe2O3. Furthermore, the magnetic moment of the Fe-doped nanotubes, larger than

that of the pure nanotubes, is suggested to have its origin in the diluted iron in form of

Fe3+. The attained information on the crystallography, electronic structure and magnetic

characteristics of the Fe-doped titania nanotubes results in a change in the functional

properties of these materials and highlights the path toward modification of these

structures for energy-related applications.

This work if funded by the National Science Foundation NSF (Grant No. DMR-0906608).

Poster Presentations


13

Maximizing Hysteretic Losses in Magnetic Ferrite Nanoparticles via Model-Driven Synthesis and Materials Optimization

R. Chen, M. Christiansen, and P. Anikeeva

Massachusetts Institute of Technology, Cambridge, MA

Using magnetic and structural nanoparticle characterization, we identify key synthetic

parameters in the thermal decomposition of organometallic precursors that yield

optimized magnetic nanoparticles over a wide range of sizes and compositions. The

developed synthetic procedures allow for gram-scale production of magnetic

nanoparticles stable in physiological buffer for several months. Our magnetic

nanoparticles display some of the highest heat dissipation rates, which are in qualitative

agreement with the trends predicted by a dynamic hysteresis model of coherent

magnetization reversal in single domain magnetic particles. By combining physical

simulations with robust scalable synthesis and materials characterization techniques,

our work provides a pathway to a model-driven design of magnetic nanoparticles

tailored to a variety of biomedical applications ranging from cancer hyperthermia to

remote control of gene expression.


14

Designing ‘Cool’ Magnetic Materials for Efficient Refrigeration:

Tailoring AlFe2B2 Magnetism-Structure Relationships for Magnetocaloric Applications

Brian Lejeune, Radhika Barua, and L. H. Lewis

Department of Chemical Engineering, Northeastern University, Boston, United States

The magnetocaloric effect (MCE) can be utilized for magnetic refrigeration to achieve

higher efficiencies than compression cooling cycles potentially reducing society’s energy

usage.1,2 The MCE is the reversible adiabatic temperature change of a magnetic material

upon the application or removal of a magnetic field.3 In AlFe2B2 it exceeds all other

intermetallic borides and is comprised of lightweight, inexpensive earth-abundant

elements in contrast to the prototypical MCE material Gd, providing motivation for this

research.4 The intermetallic compound AlFe2B2 serves as a test bed to provide insight

into the phase transition driving the MCE behavior of transition-metal borides. Synthesis

was achieved via arc melting elements in a 3 Al:2 Fe:2 B stoichiometric ratio. Attainment

of the layered orthorhombic crystal structure (a= 2.923Å b= 11.038Å c= 2.871Å) was

confirmed using Cu Kα x-ray diffraction. AlFe2B2 possesses a saturation magnetization

value of ~37.2 emu/g and exhibits a ferromagnetic to paramagnetic phase transition at a

Curie temperature (TC )~ 300 K.1,3 The presence of a 10 K thermal hysteresis (∆Tt) upon

heating and cooling through the magnetic phase transition confirms it is

thermodynamically first-order in nature. A 5 K/T magnetic field-dependence (dTC/dH )

shifts TC to higher values upon increasing the applied magnetic field. It is anticipated

that correlation of: crystal structure, microstructure, magnetism, and thermal behavior

will facilitate a better understanding of the magnetic phase transition underlying the

MCE response of the AlFe2B2 system, allowing for tuning the TC and the associated MCE.

References:

[1] ElMassalami et al. J Magn Magn Mater 323, 2133-2136 (2011).

[2] Gschneidner, K., Pecharsky, V. Physical review letters 12, 1-19 (1997).

[3] Tan et al. Journal of the American Chemical Society 135, 9553-9557 (2012).

[4] Franco, V. Annual review of materials research 42, 305-342 (2012).

[5] Lewis, L. Jiménez-Villacorta, F. Metall. Mater. Trans. A. 44, 2-20 (2013).


15

Templated Self-assembly of Perovskite/Spinel Nanocomposite Thin Films

Nicolas M. Aimon, Hong Kyoon Choi, XueYin Sun, Dong Hun Kim, Caroline A. Ross

Massachussetts Institute of Technology, Cambridge, MA, USA

Vertically aligned self-assembled nanocomposites of a perovskite and a spinel phases,

like BiFeO3 (BFO) and CoFe2O4 (CFO) have been extensively studied because of the

improved magnetoelectric coupling due to the increased interfacial area between the

piezoelectric and the magnetoelastic phases, as well as to the reduced substrate

clamping. Since the early reports of such nanostructures1, additional interesting

properties have been discovered at the interfaces between the two phases such as local

conduction2 and enhanced magnetization due to coupling between the CFO and BFO

spins3. For devices that leverage the magnetoelectric coupling and these interfacial

functionalities, the accurate control of the location of the pillars and interfaces will be

required, ideally using fabrication methods that take advantage of the auto-

organization. Our recent work4 describes how topographical features written in Nb-

doped SrTiO3, either using top-down lithography (Focused Ion Beam) or by etching

through a self-assembled mask (triblock terpolymer), can be used to selectively nucleate

the spinel phase in controlled locations, providing a handle on the location of the

growth of the pillars. Using this method, pillars of CFO, MgFe2O4, NiFe2O4, and mixed

composition CoxNi1-xFe2O4 were arranged in arrays of various symmetries (square,

rectangular and hexagonal) and arbitrary shapes. We will discuss the magnetic and

electrical properties of these templated films as measured by scanning probe

microscopy-based techniques, confirming their multiferroic character.

References:

[1] Zheng et al., Science, 303 (5658) 661–3. (2004).

[2] Hsieh et al., Advanced Materials, 24 (33) 4564–8 (2012).

[3] Chen et al., Nanoscale, 5 (10) 4449–53 (2013).

[4] Aimon et al., Advanced Materials (2014).

Figure. Top view SEM image of a template nanocomposite, where the spacing between CFO

pillars was 80 nm and the film thickness 100 nm


16

Towards Exchange-Biased Permanent Magnets: Structure-Property Correlations in FeMn Alloys

Ian McDonald, Luke G. Marshall, Laura H. Lewis

Department of Chemical Engineering, Northeastern University, Boston, MA, USA

Efforts to realize novel types of permanent magnet materials have been motivated by

the need for greater control over technical magnetic properties in inexpensive and

readily-accessible materials. In particular, development and control of magnetic

anisotropies in such systems has shown to be a suitable route for the attainment of

enhanced energy products and performance. Towards this goal, FeMn-based alloys

exhibit a range of tailorable magnetic anisotropies and are thus promising candidates

for permanent magnetic materials. Depending upon the precise composition, alloys in

the Fe-Mn system can exhibit ferromagnetism or antiferromagnetism, facilitating the

development of enhanced coercivity donated by exchange anisotropy in multiphase

FeMn systems.

In this study, alloys of composition Fe100-xMnx (x = 10, 30) have been solidified in

a non-equilibrium manner using melt spinning to produce FeMn nanocomposites:

Fe90Mn10 is primarily ferromagnetic while Fe70Mn30 is primarily antiferromagnetic.

Evolution of the multiphase microstructure upon systematic annealing is correlated with

intrinsic and extrinsic magnetic attributes of these alloys, providing unique and

controllable testbed systems to study the impact of various anisotropies

(magnetocrystalline, exchange, shape, etc.) on the technical magnetic properties of

these nanocomposite FeMn alloys.

(This work is supported by ONR Grant # N00014-10-1-0553)


17

Interlayer coupling in ultra-thin L10-FePt/MgO/[Co/Pd]30 magnetic tunnel junctions

P. Ho1,2, G. C. Han3 , G. M. Chow1 , C. A. Ross2 and J. S. Chen1

1Department of Materials Science and Engineering, National University of Singapore,

Singapore 2Department of Materials Science and Engineering, Massachusetts Institute of

Technology, MA. 3A*STAR Data Storage Institute, Singapore, Singapore

Spin valves and magnetic tunnel junctions (MTJs) with perpendicular magnetic

anisotropy (PMA) are favorable for spin transfer torque magnetic random access

memory (STT-MRAM) as they promise a reduction in the critical current density and an

improvement in areal density while maintaining thermal stability. L10-FePt and Co/Pd

multilayers have been extensively studied as suitable candidates for the magnetic layers

due to their high PMA [1-3]. A device structure which consists of an ultra-thin (< 4 nm)

L10-FePt layer as the free layer and Co/Pd multilayers as the fixed layer has the potential

in fulfilling these requirements while minimizing high temperature deposition process.

In this work, we investigate the feasibility of such ultra-thin L10-FePt/MgO/[Co/Pd]30

MTJs, with emphasis placed on the study of interlayer coupling effects in these MTJs.

The interlayer coupling within the MTJ was attributed mainly to the magnetostatic

coupling and direct coupling due to pinholes [Figure 1]. The magnitude of the interlayer

coupling field Hint in the L10-FePt/MgO/[Co/Pd]30 MTJ was insignificant compared to the

L10-FePt based pseudo spin valves reported earlier [4-5]. The improved interlayer

decoupling in the MTJs was presumably due to the reduced interlayer diffusion with the

room temperature deposition of MgO spacer and top fixed [Co/Pd]30 layers. There was

also no sign of dipolar stray field coupling, owing to the single domain characteristic

property of the fixed [Co/Pd]30 layer. The MTJ films are further patterned into nano-

devices to provide an insight into the influence of interlayer coupling on the spin

transport properties.

References:

[1] P. Ho, G. C. Han, R. F. L. Evans, R.W. Chantrell, G. M. Chow, and J. S. Chen, Appl. Phys.

Lett. 98, 132501 (2011).

[2] P. Ho, G. C. Han, K. H. He, G. M. Chow, and J. S. Chen, Appl. Phys. Lett. 99, 252503

(2011).

[3] P. F. Carcia, A. D. Meinhalt, and A. Suna, Appl. Phys. Lett. 47, 178 (1985).

[4] P. Ho, G. C. Han, G. M. Chow, and J. S. Chen, Appl. Phys. Lett. 98, 252503 (2011).

[5] P. Ho, G. C. Han, K. H. He, G. M. Chow, and J. S. Chen, J. Appl. Phys. 111, 083909

(2012).

(Figure Next Page)


18

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Figure. Minor hysteresis loops of the L10-FePt (4 nm)/MgO (2.5 nm)/[Co (0.3 nm)/Pd (0.8

nm)]30 MTJ recorded under the influence of the different magnetization states of the top fixed

[Co/Pd]30 layer, created through an applied field of (a) -1 (b) -5 and (c) -20 kOe. The dotted line

indicates the center of the minor hysteresis loop; the arrow indicates the direction of the shift

of the minor hysteresis loop. Insets indicate schematically the influence of the top fixed

[Co/Pd]30 layer on the reversal of the bottom free L10-FePt. (d) Interlayer coupling field Hint

within the MTJ with respect to the applied field. Hint is the difference between the coercivity in

the first and second quadrants of the minor hysteresis loop.


19

Half-Metallic Antiferromagnets: A New Class of Materials for Spintronic Devices

M. E. Jamer, B. A. Assaf, and D. Heiman

Department of Physics, Northeastern University, Boston, MA, USA

Half-metallic antiferromagnets (HMAF) are a theorized class of materials that would be

beneficial for applications in processors for quantum computers and nonvolatile RAM

memory devices. HMAF materials can generate a spin-polarized current without

generating a disrupting magnetic field. These materials are expected to exhibit

antiferromagnetism at room temperature, which makes them well positioned for

practical devices. In this project, V3Al and Mn3Al were synthesized to investigate their

possible HMAF properties. The preliminary data has shown the V3Al displays

antiferromagnetic behavior below TN~570 K (Neel temperature). The electrical transport

measurements show that the compound’s carriers are holes and displays metallic

behavior at room temperature. Current progress on these compounds and other

possible HMAF compounds will be presented, including X-ray magnetic linear dichroism

and circular dichroism results from Brookhaven National Laboratory. The long term

outcome of this project would be to find possible materials exhibiting HMAF properties

for future electronic memory devices.


20

Large-area Periodic Magnetic Microstructures for Controlling Magnetic Micro-particles

Minae Ouk, and G.S.D. Beach


Superparamagnetic microbeads (SBs) are widely used to capture biological entities in a fluid

environment. Chip-based magnetic actuation provides a means to transport SBs in lab-on-a-chip

technologies. This is usually accomplished using the stray field from patterned magnetic

microstructures [1], or domain walls in magnetic nanowires [2]. However, lithographic

patterning over a large area is costly and impractical using conventional techniques such as

electron beam lithography. Here we use a simple floating-transfer technique [3] for large-area

self-assembly of polystyrene microspheres on a Si wafer to produce lithographic masks texturing

a substrate. Hexagonal patterns are used as lift-off and etching masks to create magnetic dot

and anti-dot arrays in Co thin films, with a size and spacing that can be tuned via sphere diameter

and RIE etch time. Using a rotating magnetic fields, we show that these magnetically-patterned

substrates can transport SBs across large distances on the wafer surface, opening the possibility

to augment or replace microfluidic actuation for long distance transport. Supported by the MIT

Deshpande Center.

References:

[1] B. Yellen, et al., Lab Chip, 7, 1681 (2007)

[2] E. Rapoport and G. S. D. Beach, APL 100, 082401 (2012)

[3] X. Ye and L. Qi, Nano Today 6, 608 (2011)


21

Understanding L10-phase Formation in the FeNi System through the Study of FePd(Ni)

Compounds

A. M. Montes-Arango1, N. Bordeaux1, K. Barmak2, L. H. Lewis1

1Department of Chemical Engineering, Northeastern University, Boston, MA 2Department of Applied Physics and Applied Mathematics, Columbia University, New

York, NY

The development of next-generation permanent magnets for energy conversion

requires realization of novel materials with lower cost and reduced environmental

impact. To move beyond rare-earth-based magnetic materials, research interest has

shifted to compounds comprised of earth-abundant elements. With high uniaxial

magnetocrystalline anisotropy and large saturation magnetization, ferrous compounds

with the tetragonal chemically-ordered L10 structure are promising for permanent

magnet applications. Among these, L10-FePt and FePd have attracted considerable

attention, but their high cost limits their use to thin film applications. In contrast, the

low cost and good availability of the constituent elements make L10-FeNi ideal for bulk

permanent magnet construction. However, L10 phase formation in this system is highly

kinetically limited and thus it is only found naturally in meteorites that have cooled over

billions of years. To date, bulk laboratory synthesis has not been achieved, with very

limited amounts produced using irradiation techniques.

To gain insight into the development of chemical ordering in the FeNi system, Ni

additions to the model FePd compound were studied. Fe50Pd50-xNix (x=0-7at%) alloys

annealed for 100h at 500℃ were analyzed by x-ray diffraction, vibrating sample

magnetometry and differential scanning calorimetry. The amount of L10-phase formed

decreased with increasing Ni additions, diminishing substantially the magnetic

anisotropy. A reduction in the order-disorder transition temperature and the associated

enthalpy in Ni-containing alloys leads to believe that Ni produces unfavorable

characteristics for the formation of an L10 phase. These results highlight the need to

utilize nonconventional processing techniques that enhance diffusion to attain the L10-

phase in FeNi.

(This work is supported by NSF CMMI Division Grant # 1129433)


22

Nanoscale-driven Crystal Growth of Hexaferrite Architecture for Magnetoelectrically

Tunable Microwave Semiconductor Integrated Devices

Bolin Hu1, Zhaohui Chen1, Zhijuan Su1, Andrew Daigle1, Parisa Andalib1, Xian Wang2,

Jason Wolf 3, Michael E. McHenry3, and Yajie Chen1, , Vincent G. Harris1

1 Center for Microwave Magnetic Materials and Integrated Circuits, and Department of

Electrical and Computer Engineering, Northeastern University, Boston, MA, USA 2 School of Optical and Electronic Information, Huazhong University of Science and

Technology, Wuhan, Hubei, China 3Materials Science and Engineering, Carnegie Mellon University, Pittsburgh,

Pennsylvania, USA

Thick barium hexaferrite films, i.e. Ba2Co2Fe12O22 (Co2Y), were epitaxially grown on c-axis

oriented GaN/Al2O3 substrates by a low temperature process in which the growth

temperature was substantially reduced by the use of Co2Y nanoparticles thus precluding

the need for flux. X-ray diffraction showed (00l) crystallographic texture while pole

figure analyses confirmed epitaxial growth. Saturation magnetization, 4πMs, was

measured for as-grown films to be 2.5 ± 0.1 kG with an out of plane magnetic anisotropy

field, , 32 kOe (the easy magnetic polarization aligns in the film plane). The

ferromagnetic resonance (FMR) spectrum measured at 9.53 GHz had an FMR line width

H, of 280 Oe with an in-plane 6-fold symmetric magnetic anisotropy field

, of 55 Oe.

These properties demonstrate an innovative, scalable and cost effective pathway to

growing thick high quality ferrite films that enable the integration of ferrite microwave

passive devices with active semiconductor circuit elements for potential ME application.


23

Sub-100 nm Magnetic Wires with Low Edge Roughness

S. Siddiqui1, J. Currivan2,3, S. Ahn4, G. Beach, M. Baldo1 and C. Ross4

1Electrical Engineering and Computer Science, Massachusetts Institute of Technology, USA 2Physics, Massachusetts Institute of Technology, Cambridge, USA

3Physics, Harvard University, Cambridge, USA 4Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, USA

Patterning of thin films into <100 nm wide structures is essential for device scaling, and

low edge roughness is required for reproducibility of the magnetic switching

characteristics, since edge roughness in the nanostructures can act as domain wall traps.

We have patterned sub-100 nm ferromagnetic wires with very low edge roughness

using a removable bilayer poly(methyl methacrylate) (PMMA) and hydrogen

silsesquioxane (HSQ) resist mask. All patterning was done on silicon substrates with a

native oxide. 10 nm of polycrystalline Co60Fe20B20 was deposited using UHV DC

magnetron sputter deposition. 2% PMMA in Anisole and then 2% HSQ in methyl isobutyl

ketone were spun on the CoFeB. The HSQ was exposed at 125 kV electron energy. After

development, an O2 reactive ion etch (RIE) was used to remove the PMMA except under

the HSQ, resulting in a bilayer removable mask. The RIE power and time determined the

wire width. The CoFeB was ion milled using Ar ion etching at base pressure 1×10–6 Torr

with 10 mA beam current. After etching the pattern, the PMMA/HSQ mask was

removed by NMP along with sonication. SEM imaging gives a low average edge

roughness, less than 4% of the wire width. Examples of patterned films with both in

plane and perpendicular anisotropy will be given.


24

Thermomagnetic Behavior of L10 FeNi (Tetrataenite) from Meteorites

N. Bordeaux1, A. Mubarok2, E. Poirier3, K. Barmak4, J. I. Goldstein2, F. Pinkerton3 and L. H. Lewis1

1Department of Chemical Engineering, Northeastern University, Boston, USA

2 University of Massachusetts, Amherst, USA 3GM R&D Center, Warren, USA

4Columbia University, New York, USA

Chemically-ordered ferrous L10-structured compounds with tetragonal symmetry have

high magnetocrystalline anisotropy and high magnetization suitable for application as

rare-earth-free permanent magnet materials. L10-structured FeNi (tetrataenite), which

is a metastable phase in the Fe-Ni system, is especially attractive due to the availability

and low cost of its constituents; however the magnetic character and kinetic parameters

of the order-disorder phase transformation of tetrataenite have not been well-studied.

In this work, tetrataenite extracted from a meteorite was utilized as a natural source of

the chemically-ordered phase (43 at% Ni composition). Characterization of the chemical

disordering transformation was studied using magnetometry and differential scanning

calorimetry (DSC) in the temperature range 25-700 °C.

Meteoritic tetrataenite features an apparent Curie temperature TC at ~534 °C

consistent with DSC results that show an endothermic peak with an onset temperature

of ~534 ºC and a transformation enthalpy of 3.8 kJ/mol corresponding to the L10→A1

chemical order-disorder phase transformation. After heating, the coercivity value

measured at 5 K decreases from 1075 to 3 Oe, and the magnetization at 5 T increases by

14%. The coupled magnetic-structural phase transformation temperature is well above

the reported order/disorder temperature of 320 ºC signifying that the disordering

process is kinetically limited [1,2]. The DSC results will be discussed in the context of 1-D

interface-controlled and 1-D diffusion-controlled phase growth models [3].

Establishment of the disordered phase growth mechanism and transformation enthalpy

provides guidance for planned laboratory synthesis of the L10 FeNi phase.

(This work is supported by ARPA-E REACT Grant # 0472-1537)

Reference:

[1] J. Paulevé, D. Dautreppe, J. Laugier, and L. Néel, J. Phys. Radium 23, 841-843 (1962).

[2] K.B. Reuter, D.B. Williams, and J.I. Goldstein, Metall. Trans. 20A, 711-718 (1989).

[3] C. Michaelsen, K. Barmak, and T.P. Weihs, J. Phys. D: Appl. Phys. 30, 3167-2186

(1997).


25

Formation and Current Effects on 360° Domain Walls in Magnetic Nanowires

L. Tryputen, J. Zhang, J. A. Currivan, F. Liu, D. Bono, C. A. Ross

Department of Materials Science and Engineering, MIT, Cambridge, MA, USA

([email protected], [email protected], [email protected], [email protected], [email protected],

[email protected])

The dynamic behavior of 360° domain walls (360DWs) is of intense interest as it differs

significantly from the behavior of the 180° domain walls (180DWs) currently used in

several proposed memory devices. A study of the effects of nanosecond current pulses

and magnetic fields on 360DWs in curved NiFe nanostructures is presented. The

360DWs are first formed in a wire attached to a circular injection pad by applying a

saturating magnetic field perpendicular to the wire to form a 180DW, followed by a

smaller reverse field to inject a second 180DW of opposite sense, which combines with

the first 180DW to produce a 360DW [1]. Higher order walls such as 540DWs can be

generated with additional field cycling. The formation and equilibrium structure of

360DWs in the wire was verified by MFM measurements. An array of wire/pad

structures was made and after field cycling and MFM, electrical contacts were made to

selected wire/pad structures enabling anisotropic magnetoresistance to be used to

detect 360DWs. A coplanar waveguide was used to inject current pulses with ns

duration. Micromagnetic simulations [2] predict that current pulses will either translate

a 360DW or lead to its destruction, with the annihilation threshold varying with applied

field. However, fields alone do not translate 360DWs, but instead compress or

dissociate them. The comparison between experimental results of current pulsing and

the micromagnetic predictions are discussed. This work could provide insight into the

behavior of 360DWs in racetrack devices and the possibility of new magneto-electronic

applications using 360DWs.

References:

[1] Y. Jang, S. R. Bowden, M. Mascaro, J. Unguris, and C. A. Ross, Applied Physics Letters

100, 062407 (2012).

[2] M. D. Mascaro and C. A. Ross, Physical Review 82, 214411 (2010).


26

Transformation Kinetics in FeRh Thin Films

Melissa Loving1, F. Jimenez-Villacorta1, C.J. Kinane2, S. Langridge2,

C. H. Marrows3 and L. H. Lewis1

1Department of Chemical Engineering, Northeastern University, Boston, MA, USA 2ISIS Rutherford Appleton Laboratory, UK

3School of Physics and Astronomy, University of Leeds, Leeds, UK

Equiatomic α´-FeRh undergoes a first-order phase transformation (FOPT) from

antiferromagnetic (AF) to ferromagnetic (FM) character.1 FOPTs proceed by nucleation

and growth processes which may be monitored through measurement of a macroscopic

property of the transforming phase. Conventionally, FOPTs are monitored using

calorimetric approaches; however, calorimetry is not generally applicable to thin film

systems, due to the difficulty detecting the inherently small calorimetric signals. To

transcend this difficulty, we have employed a magnetization-based approach to gain a

comprehensive understanding of the kinetics underlying the FOPT in FeRh thin films. In

this manner, connections between the microstructure and the phase transformation

character of the FeRh films may be studied and tuned.

Epitaxial FeRh films were deposited onto (001)-MgO and annealed in-situ at 973

K to promote CsCl-type chemical order. The FOPT was examined with magnetometry

and magnetic force microscopy (MFM). Temperature- (T) and time- (t) dependent

magnetization (M) measurements were collected through the FOPT. The M(T) data

display an abrupt FOPT upon heating and thermal hysteresis upon cooling. The M(t)

measurements are sigmoid-shaped, characteristic of FOPT nucleation and growth

processes, and are best fit with the Johnson-Mehl-Avrami-Kolmogorov model for

crystallization kinetics 2-4. Temperature-dependent MFM images provide visualization of

the evolution of FM domains: the FM phase nucleates and grows upon heating, from

single domain islands emerged in an AF sea, and shrinks upon cooling. Overall, this study

provides a deeper understanding of the dynamics and transformation geometry of the

FeRh film FOPT.

(Acknowledgements: NSF: DMR-0908767 and UK-EPSRC:EP/G065640/1)

References:

[1] J. S. Kouvel and C. C. Hartelius, J. Appl. Phys. 33, 1343 (1962).

[2] M. Avrami, J. Chem. Phys. 7, 1103 (1939).

[3] M. Avrami, J. Chem. Phys. 8, 212 (1940).

[4] W. Johnson and R. Mehl, Transactions AIME 135 (1939).


27

Dzyaloshinskii-Moriya Interaction Influence on Stochastic Spin Orbit Torque Switching

S. Woo1, N. Pérez2, E. Martinez2, L. Torres2, S. Emori1 and G. S. D. Beach 1

1Massachusetts Institute of Technology, Cambridge, USA

2 Universidad de Salamanca, Salamanca, Spain

We studied the effect of the Dzyaloshinskii-Moriya interaction (DMI) in the magnetic

switching of a perpendicularly magnetized oxide / ferromagnet / heavy metal trilayer

both experimentally and through micromagnetic simulations. We report the generation

of helical magnetization stripes for a sufficiently large DMI strength in the switching

region, giving rise to intermediate states in the magnetization confirming the essential

role of the DMI on switching processes. Using both experiments and simulations, we

show the presence of helical magnetization intermediate states in current pulses

switching in Pt/CoFe/MgO, while hysteresis loops in Ta/CoFe/MgO are clean,

demonstrating the contribution of the DMI. Although the study of current-induced

magnetization dynamics in these multilayers is still in its early stages, our results also

point out the possibility of engineering complex magnetization patterns such as helices

or skyrmions which present promising perspectives for high-performance spintronics

applications.


28

Detection of Field and Current Effects on 360° Domain Walls by Anisotropic

Magnetoresistance Measurements

J. Zhang, L. Tryputen, J. Currivan, and C. Ross

Massachusetts Institute of Technology, Cambridge, MA, USA

360DWs are metastable structures formed by combination of two 180DWs. To

experimentally study the behavior of a 360DW is of great importance in the study of DW

dynamics as well as in DW based memory or logic devices. Modeling predicts that a

moderate magnetic field applied along the long axis of the stripe does not translate a

360DW [1], unlike 180DW. When the field is higher than a critical value, a 360DW is

annihilated in place by forming a vortex core at the stripe edge when the field is

antiparallel with the center magnetization of the 360DW. When the field is parallel with

the center magnetization of the 360DW, it is dissociated into two 180DWs. A moderate

DC will drive a 360DW to move with a constant velocity similar to that of an 180DW.

However, when the current is high enough, a 360DW will be annihilated. The presence

of 360DW can be inferred from resistance decrease due to anisotropic

magnetoresistance which can also distinguish 180DW from 360DW. 360DWs were

generated by field cycling in y direction of a narrow magnetic strip attached to an

injection pad as in Fig. 1a. The field was then applied along x. For 180DW, a resistance

jump of 0.05 Ohm is observed, corresponding to the 180DW being pushed out of the

region of the arc between the inner two contacts. For a 360DW, a larger resistance jump

of 0.07 Ohm is observed but at higher fields, corresponding to the dissociation and

annihilation of the 360DW, respectively.

Reference: [1] Mascaro, Mark D., and C. A. Ross. "AC and DC current-induced motion of

a 360° domain wall." Physical Review B 82.21 (2010): 214411.

Figure. (a) SEM of the DW generation pattern and contacts for AMR measurement. (b)

Resistance vs. field applied antiparallel to the 360DW core magnetization. (c) Resistance vs.

field applied parallel to the 360DW core magnetization.


29

Resonant Modes of Coupled Magnetic Nanodisks

Maximilian Albert1, Luzanne Fadahunsi1, Weiwei Wang1, Marc-Antonio Bisotti1,

Dmitri Chernychenko1, Marijan Beg1, Peter Metaxas2, Hans Fangohr1

1 Faculty of Engineering and the Environment, University of Southampton, United Kingdom 2 School of Physics, University of Western Australia, WA, Australia

Interacting magnetic nano-elements with tailored magnetic configurations [1] have a

wide range of applications, from magnetic logic [2] to radio-frequency and microwave

signal generation, especially if they are incorporated in spin-torque nano-oscillators

(STNOs). [3,4] We study the stability and resonant modes of metastable states in pairs

of coupled magnetic nanodisks in view of these applications. Resonant modes are

computed using an analytical eigenvalue method [5]. We investigate the dependence of

these modes on the inter-disk separation, both for double-vortex states and coupled

metastable uniform states. Frequency splitting is observed for closely spaced disks in

both cases.

References:

[1] S. Jain et al., Nanotech., 21, 285702 (2010)

[2] M. Dvornik et al., J. Appl. Phys. 109, 07B912 (2011)

[3] N. Locatelli et al., Appl. Phys. Lett., 98, 062501 (2011)

[4] A.D. Belanovsky et al., Phys. Rev. B, 85, 100409R (2012)

[5] D'Aquino et al., J. Comp. Phys., 228, 17, 6130-6149 (2009)

Oral Presentations

(Afternoon Session)

Oral Presentations (Afternoon Session)

31

The Magnetostructural Response of FeRh-based Compounds:

Tailoring with Temperature, Pressure and Magnetic Field

Radhika Barua1, Ian McDonald2, Félix Jiménez-Villacorta1,3 and L. H. Lewis1

1Department of Chemical Engineering, Northeastern University, Boston, MA 2Department of Electrical and Computer Engineering, Northeastern University, Boston, MA

3Materials Science Institute of Madrid (ICMM-CSIC), Juana Inés de la Cruz 3, 28049 Madrid, Spain

In its bulk form, Fe1-xRhx (0.47 x 0.53) possesses a B2-ordered crystal structure with

an abrupt antiferromagnetic (AFM) to ferromagnetic (FM) phase transition upon heating

to Tt ~ 350 K, accompanied by a unit cell volume increase of 1%. Strong coupling

between the magnetic spins and the lattice allow subtle variations of magnetic field,

pressure and/or composition to control and tune the transition for potential magnetic

devices such as magnetic refrigerators and sensors. Here, an arc-melted (Fe47.5Ni1.5)Rh51

alloy, serves as a test bed for understanding the simultaneous effects of temperature (2-

400 K), magnetic field (up to 5 T) and pressure (up to 10 kbar) on the magnetostructural

response of FeRh-based systems.

At zero applied pressure and magnetic field, (Fe47.5Ni1.5)Rh51, exhibits a

magnetostructural transition at Tt ~150 K. Application of an external magnetic field in

the ambient pressure state causes Tt to decrease at a rate much higher than that of the

parent FeRh compound ((dTt/dH )FeRhNi = -25 K/T; (dTt/dH )FeRh = -8 K/T). Field-induced

lowering of Tt is accompanied by an unexpected metastable retention of a fraction of

the high-temperature ferromagnetic phase below Tt and broadening of the thermal

hysteresis width (ΔTt). At Happ > 3 T, complete stabilization of the ferromagnetic phase is

noted. When pressure is applied at zero magnetic field to the Ni-modified sample, a

pronounced increase in the transition temperature ((dTt/dP) FeRhNi = 15.6 K/kbar) and a

decrease in the thermal hysteresis width (ΔTt) of the sample are noted. At high pressure,

large magnetic fields are required to completely suppress the magnetostructural

transition. At the current time, the unusual magnetostructural behavior of Ni-doped

FeRh systems at low temperatures is tentatively ascribed to the critically slow dynamics

of the phase transformation process at low temperatures.


32

Magnetothermal Multiplexing

M.G. Christiansen, R. Chen, and P.O. Anikeeva


Heat dissipation by single domain magnetic nanoparticles (SDMNPs) in the presence of

an alternating magnetic field (AMF) has long been studied for the biomedical application

of cancer hyperthermia.[1] More recently, SDMNPs in AMFs have been used to trigger

the response of individual biochemical pathways such as action potentials [2] and gene

transcription.[3] In heat mediated biological signaling, a technique to selectively heat

different types of collocated SDMNPs by changing the driving conditions of the AMF

would offer multiple signaling channels. This concept could be termed “magnetothermal

multiplexing.” Using a dynamic hysteresis model [4] that accounts for the effect of the

applied field on the anisotropy barrier, unlike typical treatments by linear response

theory (LRT),[5] we suggest how magnetothermal multiplexing could be accomplished.

We then experimentally illustrate a simple bimodal system with a set of 24nm Fe3O4

particles and 14nm MnxFe3-xO4 (x =0.04) particles. The larger particles are selectively

heated at 100 kHz and 35kA/m, whereas the smaller particles are selectively heated at

2.55MHz and 5kA/m. Multiplexing in this manner should be possible for a wide variety

of material types and field conditions, and could prove useful for numerous

applications.

Figuer. SLP multiplexing is illustrated for a 24nm Fe3O4 particle and a 14nm MnxFe3-xO4

(x=0.04) particle. For a 100kHz, 35kA/m AMF ( experimental, dynamic hysteresis simulation) the particle in the ferromagnetic regime heats more dramatically than the one in

the superparamagnetic regime. At 5kA/m and 2.55MHz ( experimental, linear response theory approximation including Brownian relaxation) the heat dissipation rates of the two particles are swapped.


33

Quantification of Strain and Charge Co-Mediated Magnetoelectric Coupling on Ultra-

Thin Permalloy/PMN-PT Interface

T. Nan1, Z. Zhou1, M. Liu2, X. Yang1, Y. Gao1, B. A. Assaf 3, H. Luo4, D. Heiman3,

B. M. Howe2, G. J. Brown2, and N. X. Sun1

1Department of Electrical and Computer Engineering, Northeastern University, Boston, MA.

2Materials & Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson, OH.

3Department of Physics, Northeastern University, Boston, MA.

4Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, China

Strain and charge co-mediated magnetoelectric coupling are expected in ultra-thin

ferromagnetic/ferroelectric multiferroic heterostructures, which could lead to

significantly enhanced magnetoelectric coupling. It is however challenging to observe

the combined strain charge mediated magnetoelectric coupling, and difficult in

quantitatively distinguish these two magnetoelectric coupling mechanisms. We

demonstrated in this work, the quantification of the coexistence of strain and surface

charge mediated magnetoelectric coupling on ultra-thin Ni0.79Fe0.21/ PMN-PT interface

by using a Ni0.79Fe0.21/Cu/PMN-PT heterostructure with only strain-mediated

magnetoelectric coupling as a control. The NiFe/PMN-PT heterostructure exhibited a

high voltage induced effective magnetic field change of 375 Oe enhanced by the surface

charge at the PMN-PT interface. Without the enhancement of the charge-mediated

magnetoelectric effect by inserting a Cu layer at the PMN-PT interface, the electric field

modification of effective magnetic field was 202 Oe. By distinguishing the

magnetoelectric coupling mechanisms, a pure surface charge modification of magnetism

shows a strong correlation to polarization of PMN-PT. A non-volatile effective magnetic

field change of 104 Oe was observed at zero electric field originates from the different

remnant polarization state of PMN-PT. The strain and charge co-mediated

magnetoelectric coupling in ultra-thin magnetic/ferroelectric heterostructures could

lead to power efficient and non-volatile magnetoelectric devices with enhanced

magnetoelectric coupling.


34

Nanoscale Magnetic Materials for Energy-Efficient Spin-Based Transistors and Logic

J. A. Currivan1,2, S. Siddiqui1, S. Dutta1, M. A. Baldo1 and C. A. Ross1

1Massachusetts Institute of Technology, Cambridge, USA, 2 Harvard University, Cambridge, USA

Energy wasted as heat dissipation is the most serious problem confronting modern

electronics. Scientists can step in and develop creative solutions to overcome this power

dissipation problem. We investigated the switching of magnetic moments in nanoscale

soft ferromagnets as a means to build logic gates and circuits. Unlike charge flowing

through a channel, spins in a material can switch collectively, thus transistors encoded

using spin have the potential to be more energy efficient than complementary metal-

oxide-semiconductor (CMOS) transistors.

The ferromagnetic logic gates we are building use current-induced domain wall

motion to write the logic state of the device and a magnetic tunnel junction to read it

out. We developed and modeled the device, and are fabricating a prototype, as shown

in Figure 1. Our modeling results showed that this device satisfies all the requirements

of beyond-CMOS logic: it has gain and concatenability; individual devices are scalable;

operating voltages are 100 mV – 10 mV; and switching energies could scale below those

of contemporary CMOS. Furthermore, the device performs as a non-volatile universal

gate with a complete set of Boolean operations. It can support its own circuits or be

integrated with CMOS. Initial fabrication uses electron-beam lithography, UHV sputter

deposition, and etching techniques.

References:

[1] J.A. Currivan, Y. Jang, M.D. Mascaro, M.A. Baldo, and C.A. Ross, IEEE Magnetic

Letters, 3 (2012).

[2] J.A. Currivan, S. Siddiqui, S. Ahn, L. Tryputen, G.S. Beach, M.A. Baldo, and C.A. Ross.

Journal of Vacuum Science & Technology B 32 (2), 021601 (2014).

[3] B. Behin-Aein, D. Datta, S. Salahuddin, S. Datta, Nature Nanotechnology 5 (2010).

Figure. Initial fabrication of device prototype. Here, a 70 nm wide, 10 nm thick NiFe arc is exchanged biased on the ends by IrMn antiferromagnetic pads (a). Using 4-point measurement techniques, we observe the domain wall motion back and forth along the wire by an abrupt change in resistance (b).


35

Fabrication of Magnetically Hard Cobalt Carbide Nanoparticles via Wet Chemical

Synthesis

Mehdi Zamanpour1, Yajie Chen1 and Vincent G. Harris1, 2

1Centre for Microwave Magnetic Materials and Integrated Circuits (CM3IC),

Northeastern University, Boston, USA 2Department of Electrical and Computer Engineering, Northeastern University, Boston,

USA

CoxC magnetic nanoparticles were successfully synthesized via a modified polyol process

without using a rare-earth catalyst during the synthesis process. The present results

show admixtures of Co2C and Co3C phases possessing magnetization values exceeding

45 emu/g and coercivity values exceeding 2.3 kOe at room temperature. Moreover,

these experiments have illuminated the important role of the reaction temperature, and

the reaction duration on the crystallographic structure and magnetic properties of CoxC,

while tetraethylene glycol was employed as a reducing agent. The role of the ratios of

Co2C and Co3C phases in the admixture on magnetic properties is discussed. The

crystallographic structure and particle size of the CoxC nanoparticles were characterized

by X-ray diffractometry and scanning electron microscopy. Vibrating sample

magnetometry was used to determine magnetic properties. Scale-up of synthesis to

more than 5 grams per batch was demonstrated with no significant degradation of

magnetic properties.


36

Integration of Self-Assembled Nanocomposite on Silicon Substrate

Dong Hun Kim, Nicolas M. Aimon, X. Sun, and C. A. Ross

Massachusetts Institute of Technology, Cambridge, MA, USA

Self-assembled nanocomposite thin films such as BaTiO3-CoFe2O4, BiFeO3-CoFe2O4 (BFO-

CFO), and BiFeO3-NiFe2O4, in which a ferrimagnetic spinel phase grows epitaxially as

pillars within an immiscible ferroelectric perovskite phase, have been studied intensively

as new multiferroic materials.[1-2] Vertical epitaxial nanocomposites have been

exclusively grown on single crystal oxide substrates which limits their utility in

microelectronic devices. Integration of nanocomposites on a Si platform would provide

a path towards large scale and low cost devices such as multiferroic memory and logic

device. We have found previously that Sr(Ti1-xFex)O3 (STF) films can be grown epitaxially

on CeO2/YSZ-buffered (001) Si.[3-5] In that work, films grown in vacuum with x = 0.1 ~ 0.5

exhibited room-temperature magnetism and a strong out-of-plane anisotropy of

magnetoelastic origin, but STF deposited in oxygen did not show significant room-

temperature magnetism, attributed to the lower lattice strain and oxygen vacancy

concentration. When nanocomposite films grow on STF layer STF not only guides the

BFO-CFO epitaxial growth but also contributes to the magnetic properties of the film

stack. Cubic or retangular factted CFO pillars in BFO matrix grew epitaxially (Fig. 1 (a))

but abnormal CFO pillar growth orientation was observed on rough STF layer (Fig. 1 (b)).

We have also epitaxially grown BFO-CFO nanocomposites on MBE grown (001) SrTiO3

film coated Si substrate. BFO-CFO nanocomposites on both STF and STO layer showed

strong out-of-plane anisotropy due to the combination of shape anisotropy and

magnetoelastic anisotropy. Removal of the BFO matrix relaxed the strain and lowered

the anisotropy.

References: [1] H. Zheng et al., Science, 303, 661 (2004); [2] S. C. Liao et al., ACS Nano, 5, 4118

(2011); [3] D. H. Kim et al., Phys. Rev. B, 84, 014416 (2011); [4] D. H. Kim et al., J. Appl. Phys.,

111, 07A918 (2012); [5] D. H. Kim et al., J. Phys.: Condens. Matter, 25, 026002 (2013).

Figure. Top view SEM image of nanocomposite on (a) smooth and (b) bumpy STF35/CeO2/YSZ/Si. Insets are top view SEM image of STF layer. The circled CFO pillars in Figure (b) are 45˚ rotated cube on-cube epitaxial growth on STF35. Scale bars correspond to 100 nm and scale of inset is the same.

List of Presenters

37

Presenters Email Address

Aimon, Nicolas M. [email protected]

Assaf, Badih A. [email protected]

Barua, Radhika [email protected]

Bauer, Uwe [email protected]

Bordeaux, Nina [email protected]

Chang, Cui-Zu [email protected]

Chen, Ritchie [email protected]

Chen, Yajie [email protected]

Christiansen, Michael G. [email protected]

Currivan, Jean Anne [email protected]

Emori, Satoru [email protected]

Ho, Pin [email protected]

Hosseinpour, Pegah [email protected]

Hu, Bolin [email protected]

Jamer, Michelle E. [email protected]

Katmis, Ferhat [email protected]

Kim, Dong Hun [email protected]

Lejune, Brian [email protected]

Liu, Frank [email protected]

Loving, Melissa [email protected]

McDonald, Ian [email protected]

Montes-Arango, Anna Maria [email protected]

Nan, Tianxiang [email protected]

Ouk, Minae [email protected]

Siddiqui, Saima [email protected]

Tryputen, Larysa [email protected]

Woo, Seonghoon [email protected]

Zamanpour, Mehdi [email protected]

Zhang, Jinshuo [email protected]

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