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EVIEW

Copyright copy 2012 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 12 1006ndash1023 2012

Interface Characterization of Epitaxial FeMgOFe

Magnetic Tunnel Junctions

S G Wang12lowast R C C Ward2 T Hesjedal2 X-G Zhang3 C Wang4A Kohn5 Q L Ma1 Jia Zhang1 H F Liu1 and X F Han1

1State Key Laboratory of Magnetism Beijing National Laboratory for Condensed Matter Physics

Institute of Physics Chinese Academy of Sciences Beijing 100190 China2Clarendon Laboratory Department of Physics University of Oxford Oxford OX1 3PU UK

3Center for Nanophase Materials Sciences and Computer Science and Mathematics Division

Oak Ridge National Laboratory Oak Ridge Tennessee 37831-6493 USA4Department of Materials University of Oxford Oxford OX1 3PH UK

5Department of Materials Engineering Ilse Katz Institute for Nanoscale Science and Technology

Ben-Gurion University of the Negev Beer-Sheva 84105 Israel

Following predictions by first-principles theory of a huge tunnel magnetoresistance (TMR) effect inepitaxial FeMgOFe magnetic tunnel junctions (MTJs) measured magnetoresistance (MR) ratios ofabout 200 at room temperature (RT) have been reported in MgO-based epitaxial MTJs Recentlya MR ratio of about 600 has been reported at RT in MgO-based MTJs prepared by magnetronsputtering using amorphous CoFeB as the ferromagnetic electrode These MTJs show great poten-tial for application in spintronic devices Fully epitaxial MTJs are excellent model systems thatenhance our understanding of the spin-dependent tunneling process as the interface is well definedand can be fully characterized Both theoretical calculations and experimental results clearly indi-cate that the interfacial structure plays a crucial role in the coherent tunneling across a single crystalMgO barrier especially in epitaxial MgO-based MTJs grown by molecular beam epitaxy (MBE) Sur-face X-ray diffraction Auger electron spectroscopy X-ray absorption spectra and X-ray magneticcircular dichroism techniques have been reported previously for interface characterization Howeverno consistent viewpoint has been reached on the interfacial structures (such as FeO layer formationat the bottom FeMgO interface) and it is still an open issue In this article our recent studies onthe interface characterization of MgO-based epitaxial MTJs by X-ray photoelectron spectroscopyhigh resolution transmission electron microscopy and spin-dependent tunneling spectroscopy willbe presented

Keywords Magnetic Tunnel Junctions Magnesium Oxide Interface Characterization First-Principles Theory Spin Dependent Tunneling

CONTENTS

1 Introduction 1006

11 MgO-Based Magnetic Tunnel Junctions 1006

12 Coherent Tunneling Across MgO Barrier 1009

13 Interfacial Structures in MgO-Based MTJs 1012

2 Interface Characterization 1012

21 Theoretical Models and Experimental Results 1012

22 High Resolution Transmission Electron Microscopy 1014

23 X-ray Photoelectron Spectroscopy 1016

24 Spin Dependent Tunneling Spectroscopy 1018

3 Conclusions 1022

Acknowledgments 1022

References and Notes 1022

lowastAuthor to whom correspondence should be addressed

1 INTRODUCTION

11 MgO-Based Magnetic Tunnel Junctions

The magnetic tunnel junction (MTJ) is a key element

of next generation spintronic devices12 such as read

heads in hard disk drives magnetic random access mem-

ory (MRAM) and magnetic sensors A MTJ consist-

ing of two ferromagnetic (FM) electrodes separated by

a thin insulating barrier exhibits a tunnel magnetoresis-

tance (TMR) effect originating from different electrical

resistances in the antiparallel (AP) and parallel (P) con-

figurations of the two FM layers according to the direc-

tion of an external magnetic field The TMR effect was

first studied by Julliere3 in FeGeCo system which showed

1006 J Nanosci Nanotechnol 2012 Vol 12 No 2 1533-48802012121006018 doi101166jnn20124257


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Wang et al Interface Characterization of Epitaxial FeMgOFe Magnetic Tunnel Junctions

S G Wang got his bachelor degree in physics from Anhui University and received his

PhD degree from the Institute of Solid State Physics Chinese Academy of Sciences in

2001 He worked as a research fellow in the National University of Singapore from August

2001 to July 2003 Then he moved to the Max-Planck-Institute of Microstructure Physics

Halle (Saale) as a post-doctor From February 2005 to December 2007 he worked in the

Clarendon Laboratory Department of Physics at the University of Oxford He joined the

State Key Laboratory of Magnetism Institute of Physics Chinese Academy of Sciences

at the end of 2007 and now is an associate professor of condensed matter physics His

research interests include the physics materials and device of spintronics

R C C Ward is a Senior Research Fellow in the Clarendon Laboratory Department

of Physics of the University of Oxford He received his DPhil from Oxford in 1981

His research interests include the growth and characterisation of epitaxial metal films and

superlattices for magnetic research fundamental processes of growth with atomic layer

control by molecular beam epitaxy (MBE) and UHV sputtering and the growth of bulk

single crystals from melts and solutions for optical and magnetic research Most recently

his work has focused on the fields of epitaxial spintronic devices superlattices containing

rare earth elements and epitaxial films of uranium metal and compounds

T Hesjedal is a Lecturer in Materials Design for Condensed Matter Physics in the Claren-

don Laboratory at the University of Oxford and the Rutherford-Appleton Laboratory He

received his PhD from the Humboldt University in Berlin (Germany) in 1997 Dr Hesjedal

came to Oxford from the University of Waterloo and Stanford University in late 2010 where

he was Associate Professor since 2005 He focuses on the synthesis and exploration of novel

quantum materials and materials for spintronics applications

X-G Zhang received his bachelor degree in physics from Peking University in 1983 and

PhD in physics from Northwestern University USA in 1989 He worked as a postdoctoral

fellow at Lawrence Berkley Laboratory from 1990 to 1991 a postdoctoral scholar at the

University of Kentucky from 1991 to 1995 Since 1995 he is a staff scientist at Oak Ridge

National Laboratory Currently he holds the position of Senior Staff Scientist at the Center

for Nanophase Materials Sciences Oak Ridge National Laboratory His research interests

include physics and materials science on the nanoscale in particular electron transport in

spintronics and molecular electronics

C Wang graduated from the Department of Mechanical Engineering Tsinghua University in

2000 and 2003 as bachelor and master After one year as process engineer in Semiconductor

Manufacturing International Corporation Shanghai China he pursued his PhD degree

in Department of Materials University of Oxford UK In 2008 he started to work as a

hardware engineer in Applied Materials Inc California USA Recently his role switches to

be responsible for technology collaborations with China universities and research institutes

in equipment applications for renewable energy

J Nanosci Nanotechnol 12 1006ndash1023 2012 1007


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Interface Characterization of Epitaxial FeMgOFe Magnetic Tunnel Junctions Wang et al

A Kohn is a senior lecturer at the Department of Materials Engineering Ben-Gurion Uni-

versity of the Negev and a member of the Ilse Katz Institute for Nanoscale Science and

Technology He obtained his PhD in Materials Engineering from the Technion - Israel

Institute of Technology Between 2004 and 2010 Dr Kohn was a post-doctoral research

assistant and then a Royal Academy of Engineering Research Fellow at the Department of

Materials University of Oxford where he was a member of the Electron Microscopy and

Microanalysis Group His research focuses on understanding how the structure of magnetic

and electronic materials determines the magneto-transport properties of information stor-

age devices To achieve the aims of his research Dr Kohn and his group apply analytical

methodologies in transmission electron microscopy as well as Lorentz TEM for magnetic

imaging In recent years his interests are MgO-based magnetic tunnel junctions Exchange-

bias amorphous ferromagnetic thin films and the development of phase-reconstruction methodologies in Lorentz TEM

Q L Ma graduated from School of Physics Lanzhou University in 2006 and received his

PhD degree from the Institute of Physics Chinese Academy of Sciences in 2011 Currently

he is working in the WPI-AIMR at Tohoku University as a post-doctor His research fields

include spintronic materials physics and devices

Jia Zhang graduated from Lanzhou University in 2007 and obtained his BS degree in theo-

retic physics From 2007 he is a PhD student in the Institute of Physics Chinese Academy

of Sciences His current research focuses on the spin dependent transport phenomenon in

magnetic tunneling junctions (MTJs) by using the first-principles calculations His work

tries to search the high spin-polarized magnetic electrode tunneling barrier materials for

achieving high tunneling magnetic resistance in MTJs and to design new MTJ structure for

example MgO-MTJs with metallic insertion layers to tailor its transport properties

H F Liu graduated from School of Physics Shandong University and obtained a bachelorrsquos

degree in 2008 He is the PhD student in the State Key Laboratory of Magnetism Institute

of Physics Chinese Academy of Sciences His main research fields focus on the transport

properties of MgO-based magnetic tunnel junctions and spin transfer torquee in MgO-based

MTJs

X F Han got his bachelor degree from Lanzhou University and received his PhD degree

from Jilin University in 1993 From 1998 to 2002 he worked at the Center of Brazilian Phys-

ical Research (Brazil) Tohoku University (Japan) University of New Orleans (USA) and

Trinity College Dublin (Ireland) respectively He obtained financial support of the Hundred

Outstanding Young Researchers Projects from Chinese Academy of Science (CAS) in 2000

the Outstanding Young Researcher Foundation from Natural Science Foundation of China

(NSFC) in 2003 and the Outstanding Innovation Team Foundation together with his partners

from NSFC both in 2007 and 2010 He is the head of Group M02 at the Institute of Physics

Chinese Academy of Sciences His research interest includes the physics materials devices

of spintronics (more details are available from group homepage httpwwwm02groupcom)

1008 J Nanosci Nanotechnol 12 1006ndash1023 2012


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a magnetoresistance (MR) ratio of 14 at low tempera-

ture However more intensive attention was attracted to

MTJs after the discovery of junctions with amorphous alu-

minium oxide (AlO) as the barrier which showed a high

MR ratio of 18 at room temperature (RT)45 Although

the MR ratio in AlO-based MTJs has since been increased

to 81 it is still lower than that needed in many spintronic

devices6

Following predictions78 by first-principles theory of a

giant TMR effect in single-crystal FeMgOFe MTJs in

2001 two MgO-based systems with a TMR ratio of about

200 at RT were reported in 2004 CoFeMgOCoFe

MTJs prepared by magnetron sputtering and epitaxial

FeMgOFe MTJs grown by molecular beam epitaxy

(MBE) respectively910 The discovery of single crystal

MgO-based junctions with giant MR ratio has galvanized

the worldwide interest of researchers in MgO-based MTJs

and MgO-based spin transfer torque devices11 The MgO

barrier has a single-crystal structure instead of the amor-

phous structure of AlO barrier A single-crystal MgO bar-

rier exhibits a spin filtering effect due to the conservation

of wave-function symmetry The conductance in the P con-

figuration dominated by the majority 1 states is high

because 1 states decay relatively slowly through the bar-

rier and can transfer into similar symmetry states in the

second FM electrode By contrast the AP conductance is

low due to the absence of receptor states in the second

electrode (symmetry blocking) leading to a giant MR ratio

in MgO-based MTJs

Currently the study of MgO-based MTJs can be gen-

erally classified into three main groups according to the

materials technology involved The first group is based

on CoFeB electrodes and grown by magnetron sputtering

This is the system used for commercial production of

MTJs Post-growth thermal annealing is the critical part of

the process to obtain high MR ratios The as-grown CoFeB

layers are amorphous but remarkably the MgO barrier

is crystalline with strong (001) structure12 During the

annealing process the CoFeB layers become B-depleted

and crystallize by solid state epitaxy on each side of

the MgO barrier The B atoms rejected from crystallized

CoFeB layers are found to be dissolved in upper amor-

phous Ta layers and segregated in the bottom crystalline Ta

layer13 This results in a pseudo-epitaxial CoFeMgOCoFe

junction in which coherent tunneling leads to very high

TMR ratios Up to now the reported record of MR

ratios of 604 at RT and 1144 at 5 K were observed

in sputtered CoFeBMgOCoFeB structures14 The second

group involves using the high-polarization Heusler-alloys

as electrodes grown by sputtering with MgO as the bar-

rier or alternatively one Heusler electrode is used with

CoFe as the other electrode15ndash17 The TMR ratio reaches

330 at RT for electrodes of a Heusler-alloy film and

CoFe film separated by MgO barrier17 Again the origin

of highest TMR ratio in Heusler samples is thought to

be coherent tunneling when a single-crystal structure is

produced18 The third group is the fully epitaxial junc-

tions usually grown by MBE method such as FeMgOFe

and CoMgOFe which were the first MgO-based struc-

tures to achieve huge TMR ratios1019ndash22 Although MBE-

grown structures will probably not be used in devices they

remain an excellent model system to compare theoretical

calculations with experimental results and to enhance our

understanding of the coherent tunneling across MgO barri-

ers The results of such investigations can be applied gen-

erally to the other two groups where coherent tunneling is

of great importance

It is important to note the research into new and alter-

native barriers to MgO For example a MTJ with a

single-crystal NaCl barrier which has an identical crystal

structure and also a markedly similar band structure to that

of MgO has been reported23 Another example are MTJs

based on single crystal MgAl2O4 barrier with the spinel

structure where Heusler alloy electrodes were used1824

A MgndashBndashO barrier was also investigated where a CoFeB

layer was used for one of the FM electrodes25 The cur-

rent TMR ratio in MTJs with these novel barriers is still

much lower than that in MgO-based MTJs but may be

developed further in the future

In this topical article an overview of coherent tunneling

in epitaxial MgO-based is presented in the second part of

the Introduction The third part of the Introduction empha-

sizes the importance of the interface structure between

the electrode and barrier to the efficiency of the tunnel-

ing process in MgO-based MTJs The characterization of

this interface forms most of this paper including theo-

retical models and experimental results on MBE-grown

samples using transmission electron microscopy (TEM)

X-ray photoelectron spectroscopy (XPS) and spin depen-

dent tunneling spectroscopy technique Finally a summary

of the present status of interfacial characterization in epi-

taxial MgO-based MTJs is presented

12 Coherent Tunneling Across MgO Barrier

A phenomenological model proposed by Julliere (called

Jullierersquos model) has been widely applied to explain the

experimental data obtained both in FeGeCo junctions and

in AlO-based junctions where the spin polarization of the

FM electrodes can be achieved by fitting TMR ratio with

the model The model is based on two assumptions one

is that the spin of electrons is conserved during tunnel-

ing process and the other is that tunneling of up-spin and

down-spin electrons is independent (two-current model)

The key parameter is spin polarization which is assumed

as the total electronic density of states (DOS) of the FM

layers at the Fermi energy The value of spin polarization

can also be obtained by analysis of superconducting tun-

neling spectroscopy where the junction consists of one FM

electrode and one superconducting electrode separated by

a thin barrier However the values obtained from fitting



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TMR ratio with Jullierersquos model and from superconducting

tunneling spectroscopy measurement show a large varia-

tion strongly depending on the FM electrodes The large

variation and the sample dependence of spin polarization

indicates a shortcoming for Jullerersquos model The detailed

discussion about Jullierersquos model and its great success in

AlO-based junctions could be found from other topical

review papers26ndash28

In order to get a better explanation for the experimen-

tal data the Jullierersquos model was further developed by

Slonczewski29 who introduced an effective spin polariza-

tion However the Jullierersquos model and Sloncwewskirsquos

model can not be used to explain the experimental data

in MgO-based junctions where the barrier is single crys-

tal structure The validity of the Jullierersquos model of spin

dependent tunneling was carefully checked before30 For

example a flat temperature dependence of the resistance in

the P configuration (RP) with thick MgO barrier and even

an obvious increase of RP with increasing temperature in

MTJs with thinner MgO layer was observed (shown in

Fig 1)31 Another example a flat bias (low voltage) depen-

dence of dynamic conductance in the P configuration of

epitaxial FeMgOFe junctions shown in Figure 2 can not

be explained by Jullierersquos model2131 One of characteristic

features of high quality MgO barrier is that RP is almost

independent of temperature such as 3 nm thick MgO bar-

rier shown in Figure 1(a) This temperature dependence

has also been observed in fully epitaxial FeMgAl2O4Fe

MTJs recently24 where the majority 1 states of Fe elec-

trodes play a critical role for tunneling

Fig 1 RA as a function of temperature in the P and AP (solid and

open signs) states for various tMgO in FeMgOFe junctions Reprinted

with permission from [31] Q L Ma et al Appl Phys Lett 95 052506(2009) copy 2009 American Institute of Physics

Fig 2 Dynamic conductance (dIdV) as a function of bias voltage in

the P and AP configurations at 300 K and 10 K in epitaxial FeMgOFe

junctions Reprinted with permission from [21] S G Wang et al PhysRev B 78 180411R (2008) copy 2008 American Physical Society

First-principles calculations done in 2001 for magnetic

tunnel junctions with single crystal MgO layer as barrier

predicted a huge MR ratio of over 1000 In 2004 two

typical systems with the MR ratio of about 200 at RT

were reported in sputtered CoFeMgOCoFe MTJs and in

epitaxial FeMgOFe MTJs respectively Before that a lot

of experimental work was carried out about growth of

MgO on Fe and vice versa3233 and about a full structure

of FeMgOFe as well3435 but with very low MR ratio due

to pinholes and structure disorders Later on a MR ratio as

high as 60 was achieved in epitaxial FeMgOFeCo junc-

tions grown by sputtering and laser ablation on GaAs(001)

substrate36 and a MR ratio of 67 at RT and 100 at

80 K was obtained in fully epitaxial FeMgOFe junctions

grown by MBE37

The large MR ratio in MgO based MTJs arises from

a symmetry filtering effect in the MgO barrier layer For

epitaxial MTJs this can be explained in terms of the differ-

ent decay rates of the wave functions with different lateral

symmetry2738 Within a simple barrier represented by a

constant potential VB the decay wave vector is given by

2 = 2m

2VBminusEminus 2

x2+ 2

y2

(1)

where E is the electron energy is its wave function and

xy plane is parallel to the film The last term is positive

and increases with the number of nodes of x y Thusfor epitaxial films because the wave function symmetry

parallel to the film is preserved across the interfaces the

decay rate is positively correlated to the lateral symmetry

of the wave function in the electrodes In real materials the

decay rate is determined by the complex band structure of

the barrier material Figure 3 shows the complex bands of

bulk MgO at kx = ky = 0 near the Fermi energy (E = 0 eV

in the figure) within the band gap of MgO There are four

complex bands shown in the figure labeled as 1 22prime

and 5 These are the states with the square symmetry in



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ndash4

ndash3

ndash2

ndash1

0

1

2

3

4

0 02 04 06 08 1 12 14

Ene

rgy

(eV

)

Im k (1Angstrom)

Δ Δ Δ Δ1 5 2rsquo 2

Fig 3 Complex band structure of MgO near the Fermi energy

(E = 0 eV)

the xy plane The 1 state has the smallest Imkz thus it

decays the slowest within the barrier layer leading to the

largest tunneling probability

Figure 4 shows the tunneling density of states (TDOS)

for k = 0 for Fe(100)8MgOFe(100) This is defined as

the electron density due to a single incident Bloch wave

from the left electrode The TDOS for the majority spin

channel is shown in the upper left panel and for the mino-

rity channel in the upper right panel and for the AP con-

figuration of the two electrodes in the lower panels In the

(a) (b)

(c) (d)

Fig 4 Tunneling DOS for k = 0 in Fe(100)8MgOFe(100) for majority (a) minority (b) and AP alignment of two electrodes (c d) Additional

Fe layers are included in lower panels to show the TDOS variation in Fe Each TDOS curve is labelled by the symmetry of the incident Bloch state

in the left Fe electrode Reprinted with permission from [7] W H Butler et al Phys Rev B 63 054416 (2001) copy 2001 American Physical Society

two lower panels additional Fe layers are included to show

the TDOS variation into the right electrode Each TDOS

curve is labelled by the symmetry of the incident Bloch

wave in the left electrode For the P configuration shown

in the upper panels for the tunneling DOS of two spin

channels only the majority channel has the slow decaying

1 state leading to a higher conductance than those of the

minority channel and either of the spin channels for the

AP configuration

For the AP configuration (shown in the lower panels)

electrons of 1 symmetry from the majority spin of left

electrode readily enter the MgO barrier where they decay

slowly However when they enter the right electrode these

states cannot propagate because there are no available

minority spin 1 propagating states at the Fermi energy

Therefore they continue to decay into the right electrode

leading to a total reflection of the 1 Bloch state Because

the symmetry filtering effect of the MgO barrier strongly

favors the transmission of electrons with the 1 symme-

try the conductance due to the 1 state is many orders

of magnitude greater than the other symmetry states On

the other hand for Fe Co and FeCo bcc(100) electrodes

there are no 1 states at the Fermi energy in the minority

spin In other words as far as the 1 state is concerned

these electrodes are half-metallic These two factors com-

bine leading to a giant TMR ratio



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The tunneling across a crystalline MgO barrier is

called the coherent tunneling in contrast to the incoherent

tunneling in AlO-based MTJs Due to the amorphous

structure of the AlO tunneling barrier there is no crystallo-

graphic symmetry inside the barrier layer The Bloch states

with various symmetries in the FM electrodes can couple

with any evanescent states in the amorphous barrier lead-

ing to approximately equal tunneling probabilities for all

of the Bloch states Additional discussion about coherent

and incoherent tunneling in MgO-based MTJs could be

found elsewhere2839

13 Interfacial Structures in MgO-Based MTJs

As outlined above a lot of experimental work has

been carried out on epitaxial MgO-based junctions fol-

lowing the predictions of huge MR ratios in excess

of 1000 in 2001 by first-principles calculations The

value of MR ratio obtained experimentally in MBE-

grown devices is still significantly lower than the the-

oretical one although great progress has been achieved

during the last several years For example A MR ratio

of 410 at RT was obtained in epitaxial CoMgOCo

MTJs22 Larger MR ratios have been achieved in sputtered

CoFeBMgOCoFeB devices as high as 604 at RT and

1144 at 5 K14 The present paper addresses the origin of

the considerable discrepancy between values obtained for

MBE-grown junctions and theoretical predictions

It is well known that the interfacial structure is of great

importance in multilayers or superlattices where the scat-

tering or tunneling process at the interfaces dominates the

electron transport across the spacer (GMR effect) or the

barrier (TMR effect) In general the interface structure

can be complex due to defects such as interface rough-

ness (steps) strain lattice mismatch dislocations vacan-

cies and contamination One long-running important issue

concerns the possible oxidation of the interfacial layer of

the electrode following the results of investigations by

in-situ surface X-ray diffraction showing the evidence of

FeO formation at the FeMgO and MgOFe interfaces of

epitaxial FeMgOFe structures40ndash42

In the following sections we present the results of

experimental and theoretical investigations of the crystallo-

graphic and chemical profiles at the electrodebarrier inter-

faces of epitaxial MTJs Our own results on MBE-grown

FeMgOFe junctions are discussed within the context of

these models and experimental results reported in the

literature

2 INTERFACE CHARACTERIZATION

21 Theoretical Models and Experimental Results

The most studied interface effect has been the extend of

oxidation of the FeMgO interfaces since the first evidence

of an FeO layer was provided by Meyerheim et al using

surface X-ray diffraction40 In Figure 5 these authorsrsquo

(a)

(b)

Fig 5 (a) Top view of the MgOFe(001) interface The dashed square

indicates the Fe(001) surface unit cell Only the first MgO layer is shown

(b) Perspective view of the best fit structure model The error bar for

the MgO interlayer distances is about 015 Aring Reprinted with permission

from [40] H L Meyerheim et al Phys Rev Lett 87 076102 (2001)copy 2001 American Physical Society

model of the bottom oxidized Fe interface is shown

together with interlayer distances In 2003 first-principles

calculation of the electronic structure and MR ratio of

FeFeOMgOFe tunneling junctions with FeO layer at the

bottom FeMgO interface was done by Zhang et al with

a detailed comparison to those of pure FeMgOFe junc-

tions4344 It was shown that an atomic layer of FeO at

the FeMgO interface greatly reduces the MR ratio due

to the in-plane bonding of Fe with O which reduces the

conductance in the P configuration but has little effect

on conductance in the AP configuration Furthermore the

MR ratio decreases monotonically and exponentially with

the increasing O concentration in the FeO layer Theoret-

ical results43 of first-principles DOS calculations for the

ideal interface and the oxidized interface are shown in

Figure 6 With respect to an ideal interface shown on the

left of Figure 6 1 Bloch states in the Fe layer couple

with 1 evanescent states in the MgO barrier in the k = 0

direction This structure with an ideal interface shows a

huge MR ratio7 For an oxidized interface shown on the

right of Figure 6 where there are excess oxygen atoms

in the interfacial Fe monolayer 1 Bloch states in the



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Fig 6 The partial density of states at the Fermi energy due to the

1 state in the majority spin channel near the interface region Left panel

without the FeO layer right panel with the FeO layer Reprinted with

permission from [43] X-G Zhang et al Phys Rev B 68 092402 (2003)copy 2003 American Physical Society

Fe layer do not couple with 1 evanescent states in the

MgO barrier effectively This decoupling prevents coherent

tunneling of 1 states and greatly reduces the MR ratio

Subsequently more analytical techniques have been

applied to the interface characterization of MgO-based epi-

taxial MTJs such as Auger electron spectroscopy (AES)

X-ray absorption spectra (XAS) and X-ray magnetic cir-

cular dichroism (XMCD) Auger electron spectroscopy

was used to investigate the chemical nature of the dif-

ferent interfaces and possible segregation effects in fully

epitaxial FeMgOFe(001) oriented heterostructures fabri-

cated by combined sputtering plus laser ablation deposition

techniques45 The interfaces on each side of the MgO bar-

rier show some differences where the bottom FeMgO

interface is spatially broader and with an FeO interlayer

and the top MgOFe interface is spatially narrower with

no evidence of FeO formation XAS and XMCD tech-

niques were applied46 to study the interface of 6 mono-

layer (ML) Fe(001) in FeMgO bilayers and the results

showed evidence for a weak hybridization between Fe

and O atoms Unfortunately in these latter investigations

no MR data were shown for the full junctions which

could add further evidence for the possible formation of an

FeO monolayer However the evidence for a non-oxidized

interface was provided by the XAS and XMCD mea-

surements carried out for 1 ML and 2 ML-Fe(001) fac-

ing an MgO(001) barrier in a body-centered-cubic (bcc)

Co(001)Fe(001)MgO(001) structure47 Attempts to avoid

the FeO layer at the interface lead to the insertion of an

ultrathin (1-2 atomic layers) of Mg between the Fe elec-

trodes and the MgO barrier layer48 This has led to either

increased or reduced TMR ratio depending on the depo-

sition techniques and the particular samples Theoretical

model49 of the ultrathin Mg interlayers shows that while

adding the Mg layer generally reduces the TMR its effect

is not as severe as that of the FeO layer because the

Mg layer preserves the preferential transmission of the Fe

1 state Very clearly no consistent viewpoint has been

reached on the extent of interface oxidation and this is

still an open issue

Other effects such as interface resonant states750 have

been found to cause lower experimental MR ratios More

generally the interfacial structure is closely related to the

crystalline defects lattice mismatch interface roughness

dislocations vacancies contaminations51ndash54 The crys-

talline defects in the Fe layers including the surface

roughness are minimized by post-annealing19 which can

increase the MR ratio from around 100 for as-grown

samples to 180 for annealed samples The interfacial

contamination for example due to carbon impurities in

the MgO substrate was investigated as well5556 and the

carbon contaminations could be eliminated by growing a

MgO buffer on the MgO substrate before growth of the

bottom Fe layer The interface roughness especially of the

bottom Fe layer could also be improved by using appro-

priate growth conditions together with thermal annealing

andor by a thick MgO buffer1921 The lattice mismatch

between Fe and MgO can be tuned by doping V into the

bottom Fe layer (ie an FeV alloy film) so that the crys-

talline quality of the MgO barrier is improved leading to

an increase of the MR ratio despite the fact that the spin

polarization in the FeV alloy is lower than that of the pure

Fe layer57

Before the detailed discussion about interface charac-

terization the basic properties including the structural

analysis magnetic and transport properties in epitax-

ial MgO-based junctions will be described The core

part of epitaxial MTJs is a trilayered structure such as

FeMgOFe The top Fe layer is generally adjacent to a

hard layer such as Co layer19 or to an antiferromagnetic

(AFM) layer such as Ir022Mn078 layer2158 which exhibits

an exchange bias effect in order to achieve the P and AP

configuration between bottom and top FM electrodes by

influence of an external magnetic field

During the deposition of multilayers by MBE the struc-

ture of each layer is monitored by the in situ reflection highenergy diffraction (RHEED) technique Figure 7 shows

typical RHEED patterns recorded from FeMgOFeIrMn

multilayers on a MgO(001) substrate along the MgO[110]

azimuth It shows that the whole structure is epitaxial

including the AFM layer (IrMn) on the top Fe layer5859

Furthermore both the bottom and top Fe layers show good

crystallinity and flatness after annealing as shown by the

clear and sharp RHEED streaks

The structure of multilayers is further investigated by

X-ray diffraction (XRD) although it is hard to differen-

tiate the MgO barrier from MgO substrate due to its

thickness (such as 2 or 3 nm) Figure 8 presents the

XRD pattern for FeMgOFe trilayers which indicates

a good epitaxial growth of the entire structure as well

The epitaxial relationship between Fe and MgO layers

of Fe(001)[100]MgO(001)[110]Fe(001)[100] was con-

firmed where MgO axes rotate 45 with respect to the

equivalent Fe axes



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Fig 7 In situ RHEED pattern for FeMgOFeIrMn multilayers along

MgO[110] orientation

Finally the structure of the multilayers was exam-

ined using transmission electron microscopy (TEM)

Figure 9(a) presents a high-angle annular dark-field TEM

image (Z-contrast) of a multilayer with structure of

FeMgOFeIrMnCr (Cr as capping layer) indicating

different layers clearly The cross sectional high res-

olution TEM image across MgO barrier is shown in

Figure 9(b) The zone axis is along Fe[100] (MgO[110]

as well) The well-known epitaxial relationship of

Fe(001)[100]MgO(001)[110]Fe(001)[100] is confirmed

again together with the observation of sharp interfaces

across the MgO barrier We also found that the interface is

semicoherent due to dislocations at the interface60 which

originates from the approximately 35 lattice mismatch

between the MgO(100) and Fe(110) planes Therefore

the epitaxial structures observed by RHEED XRD and

Fig 8 XRD pattern for FeMgOFe trilayers on MgO substrate

Reprinted with permission from [59] S G Wang et al IEEE TransMagn 44 2562 (2008) copy 2008 IEEE

(a) (b)

Fig 9 (a) A high-angle annular dark-field TEM image (Z-contrast)

image of FeMgOFeIrMn structure (b) HRTEM image across MgO bar-

rier The zone axis is along Fe[100] Reprinted with permission from [59]

S G Wang et al IEEE Trans Magn 44 2562 (2008) copy 2008 IEEE

HRTEM are in good agreement indicating a high quality

sample grown by MBE

The multilayers were fabricated into micro-meter size

junctions by UV-lithography together with Ar ion milling

In Figure 10 a typical RndashH loop (obtained from a patterned

junction) at RT is shown by open circles and a MndashH loop

(obtained from the unpatterned sample) by open squares

respectively It shows that the RndashH loop for patterned junc-

tions is in good agreement with the MndashH loop obtained for

the as-grown continuous sample with sharp magnetization

switches for the P and AP configurations The MR ratio is

174 at RT and increases to 318 at 10 K The results

shown from Figures 6 to 9 indicate that the fully epitaxial

FeMgOFeIrMn junction has a high quality structure with

a large MR ratio The related work described in the follow-

ing sections by the authors is based on these high quality

epitaxial samples except when specified otherwise

22 High Resolution Transmission ElectronMicroscopy

Transmission electron microscopy (TEM) is a powerful

tool to investigate the crystal structure of materials at the

Fig 10 MndashH loop in continuous FeMgOFeIrMn multilayers (right

scale) and RndashH loop for junctions (left scale) at RT Reprinted with

permission from [59] S G Wang et al IEEE Trans Magn 44 2562(2008) copy 2008 IEEE



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atomic level and especially useful for studying locally

the interface structure in multilayers Recently by using

high-resolution imaging with a small negative value of the

spherical aberration of the objective lens in an aberration-

corrected TEM atomic columns of oxygen in BaTiO3 thin

films were imaged and quantified61 However in a conven-

tional TEM (with spherical aberration) we could make use

of the high spatial resolution of HRTEM in order to exam-

ine the formation of an FendashO monolayer at the FeMgO

interface HRTEM images of samples in a cross sectional

view were obtained using a JEOL 4000EX62 This micro-

scope has a spherical aberration (Cs) point-resolution

and information limit of 1 mm 016 nm and less than

012 nm respectively The sample with a core structure

of FeMgOFe was prepared by a traditional method com-

prised of cutting polishing dimpling and ion milling In

order to find the evidence of an FeO layer it is necessary

to compare these experimental images to simulated ones

because direct interpretation is not straightforward The

TEM images are simulated using the weak phase approx-

imation namely a single scattering event Java electron

microscopy software (JEMS)63 was used to simulate the

images for various defoci and thickness values of the spec-

imens obtained in the JEOL 4000 EX microscope Super-

cell models of the interface for both sharp and oxidized

interfaces were constructed shown in Figure 11 By an

oxidized FeMgO interface we mean that an FeO layer is

inserted between the Fe and MgO layers

The zone axis of the cross section view was selected

along MgO[110] orientation (Fe[100] as well) This direc-

tion was chosen because the atomic columns imaged in

the MgO layer are either O or Mg The best match

between experimental and simulated images was obtained

(a) (b)

Fig 11 Structural input for HRTEM multislice image simulations in

the case of (a) an oxidized interface with an FeO layer at the Fe and MgO

interface and (b) a sharp FeMgO interface Reprinted with permission

from [60] C Wang et al Phys Rev B 82 024428 (2010) copy 2010

American Physical Society

for an approximately 5 nm thick sample shown in Figure

12(b center) along with two simulated images of the

FeFeOMgO shown in Figure 12(a left) and FeMgO

structures shown in Figure 12(c right) Away from the

interface the simulated FeFeOMgO and FeMgO images

appear similar because both are based on bulk Fe and MgO

interplanar distances Close to the interface the simulated

images of the two structures show subtle differences For

the abrupt FendashMgO interface the image appears sharper

(see arrow denoting the bright spot) while for the oxidized

FendashFeOndashMgO interface this region is dimmer probably

due to the existence of oxygen

We note that due to the spherical aberration the dif-

ferentiation between the two proposed models is indeed

subtle This result emphasizes the importance of using

an aberration-corrected TEM for determining the structure

of the interface Recent work conducted in our group60

clearly found a sharp interface using this approach

An additional route to check whether the interface is

oxidized is to examine the FendashO interplanar distances

For the sharp interface the FendashO distance is 02169 nm

according to first-principles calculations7 For an oxidized

interface the FendashO distance of 0235 nm was reported

following an experimental measurement40 In order to fit

the interplanar distance between the Fe and O layers at

the interface to the experimental HRTEM results dis-

tances varying from 019 to 026 nm were introduced into

the sharp FeMgO model for the multi-slice calculations

Figure 13 shows simulation results for interplanar dis-

tances of 0235 nm (a left) and 0220 nm (c right) for the

FendashO distance indicating a better fit to an FendashO interface

distance of 022 nm The arrow in (a) highlights the dis-

crepancy of the interplanar spacing between the simulated

interplanar distances of 0235 nm and the experimental

Fig 12 HRTEM images of simulated (a) Fe-FeO-MgO (c) Fe-MgO

and experimental data (b) The arrows highlight the difference between

the simulated images (Sample thickness sim5 nm Defocus 36 nm)

Reprinted with permission from [62] C Wang et al IEEE Trans Magn43 2779 (2007) copy 2007 IEEE



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Fig 13 Experimental (b) and simulated HRTEM images for FendashO dis-

tance of (a) 0235 nm (c) 0220 nm both simulated at defocus of 36 nm

and 5 nm thickness The arrow in (a) highlights the discrepancy of the

interplanar spacing between the simulated (a) and experimental image (b)

whereas the arrow in (c) highlights a good agreement Reprinted with per-

mission from [62] C Wang et al IEEE Trans Magn 43 2779 (2007)copy 2007 IEEE

image (b center) whereas the arrow in the simulated inter-

planar distances of 0220 nm highlights a good agreement

More detailed analysis of interfacial structures by HRTEM

and electron energy loss spectroscopy (EELS) is reported

in our recent publications as well as by Serin et al6064 65

The bottom interface appears smooth while the top inter-

face has considerably more atomic column steps typically

one or two atomic planes in width marked by arrows in

Figure 13

Based on the above detailed experimental HRTEM and

simulations of FeMgOFe multilayers along with our

recent TEM studies60 we conclude the bottom FeMgO

interface is sharp More exactly at least some sections of

the interface are non-oxidized with sharp interfaces The

interface roughness was characterized and compared using

high-angle annular dark-field scanning TEM60 The bottom

FeMgO interface appears smooth while the top interface

has considerably more atomic column steps typically one

or two atomic planes in width In order to further eluci-

date the issue of interfacial structures it is also helpful

to characterize the interface using additional experimen-

tal techniques such as XPS and spin dependent tunneling

spectroscopy with a focus on the evidence of FeO forma-

tion which will be discussed in the following parts

23 X-ray Photoelectron Spectroscopy

With respect to the evidence of FeO formation at

the FeMgO interfaces the measurement of X-ray

photoelectron spectroscopy (XPS) has been proven to be

a very useful tool5566 67 Generally the measurements

include two different ways One is directly to use the real

structure as same as junctions where the XPS facility

is equipped with ion milling gun enabling to peel sam-

ple layer by layer till the interfaces66 The other way is

an in situ measurement where a very thin MgO layer

(for example 2 monolayers) is deposited onto a thick Fe

layer5567 For in situ measurements typical results are

shown in Figure 14 where the evidence for weak hybridiza-

tion between Fe and MgO was provided However it is

the best way to carry out the XPS measurements on the

samples that have the same structure (cut from the same

sample or samples grown at the same conditions) show-

ing good electrical and magnetic properties together with a

high MR ratio

The resistance as a function of magnetic fields for

junction with structure of Fe(50)MgO(3)Fe(10)IrMn(10)

(thickness in nm) at RT is shown in Figure 15 where the

junction size is 6times 8 m2 For this sample the analysis

from RHEED patterns XRD curve and HRTEM images

confirmed the high quality of junctions However the

value of MR ratio is only 61 at RT indicating the imper-

fection of the junctions which can not be identified by

above mentioned techniques Then the XPS measurement

was carried out on the sample grown with same structure

and same growth conditions The multilayers were etched

by ion milling layer by layer and the Fe 2p high-resolution

XPS spectra at the FeMgO interface was collected and

presented in Figure 16 The thin solid line shows exper-

imental data and the thick solid line is the fitting curve

As well as the main peak at 7066 eV there are two small

peaks along the higher energy shoulder From the XPS

handbook peak 0 at 7066 eV is characteristic of a metallic

Fe 2p32 peak peak 1 at 7082 eV and peak 2 at 7104 eV

correspond to Fe2+ 2p32 and Fe3+ 2p32 peaks respec-

tively After MgO layer was completely removed only a

(a)

(b)

(c)

Fig 14 XPS spectra performed at RT on (a) a 50-nm-thick reference

Fe layer (b) bare Fe monolayer and (c) same sample than (b) but covered

with 2 ML of MgO Reprinted with permission from [55] M Sicot et al

J Appl Phys 99 08D301 (2001) copy 2001 American Institute of Physics



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Fig 15 RndashH loops for junction with a core structure of

Fe(50)MgO(3)Fe(10)IrMn(10) (in nm) at RT68

single peak (peak 0 here) was observed for pure Fe layer

Above results could be taken as an evidence of FeO forma-

tion at the bottom FeMgO interface The formation of the

FeO layer is attributed to Mg which acts as a catalyst to

promote the oxidation of Fe at the FeMgO interface6669

During deposition MgO single-crystal source decomposes

as elemental Mg and oxygen molecules which provides

Mg as catalyst for the FeO formation

Following these studies further optimization of the

growth parameters resulted in significant increase of the

MR ratio For example MR ratio reaches about 174 at

RT and 318 at 10 K in the junctions of Fe(25)MgO(3)

Fe(10)IrMn(10) (thickness in nm) with typical RndashH loops

at temperatures of 300 K 100 K and 10 K shown in

Figure 17 As before XPS measurements (Thermo Scien-

tific ESCALAB 250 machine) were carried out

The sample for XPS measurements has the structure of

Fe(25)MgO(5)Fe(3)Cr(5) (thickness in nm) where 5 nm

thick MgO barrier is used instead of 3 nm in real junctions

Fig 16 Fe 2p spectra near FeMgO interface for sample with structure

of Fe(50)MgO(3)Fe(10)IrMn(10) (thickness in nm) Reprinted with

permission from [66] S G Wang et al J Magn Magn Mater 3101935 (2007) copy 2007 Elsevier Science

Fig 17 RndashH loops at 300 100 and 10 K in MTJs with structure of

Fe(25)MgO(3)Fe(10)IrMn(10) (thickness in nm) Reprinted with per-

mission from [21] S G Wang et al Phys Rev B 78 180411R (2008)copy 2008 American Physical Society

in order to distinguish the XPS signal of bottom Fe layer

from that of top Fe layer In order to locate the interfaces

more exactly the etching rate is reduced to 05 nmetching

(etching time is fixed for 120 seconds) After each etch-

ing the XPS measurement is carried out including the

whole range energy survey (such as from 0 eV to 1350 eV

to check all elements in our systems) Fe 2p spectra Mg

1s spectra O 1s spectra Mg 2p spectra C 1s spectra

and Cr 2p spectra With etching the peak from Mg 1s

appears indicating the location of the top FeMgO inter-

face (TI) meanwhile the intensity of Fe 2p from top Fe

layer shows a little decrease With etching step by step the

intensity of Fe 2p from top Fe layer decreases Then the

intensity of Fe 2p shows a great enhancement indicating

location of the bottom MgOFe interface The typical XPS

spectra of Fe 2p Mg 1s and O 1s are shown in Figures 18

19 and 20 respectively The spectra at top interface bot-

tom interface and in the MgO barrier are shown by blue

red and black lines respectively The spectra in the bot-

tom thick Fe layer and in the top Fe layer are also shown

for reference

Fig 18 Fe 2p high resolution XPS spectra for sample with structure of

Fe(25)MgO(5)Fe(3)Cr(5) (thickness in nm)68



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The focus is given on Fe 2p high resolution spectra

first shown in Figure 18 The Fe 2p spectra in the bot-

tom Fe layer (its top layers is removed completely by ion

milling) shown by the pink line as a reference in Figure 18

is a typical curve with two peaks located at about 7070 eV

and 7200 eV for Fe 2p32 and Fe 2p12 respectively

The spectrum with sharp peaks and high intensity is a

characteristic of metallic Fe No additional peaks were

observed at higher energy of main Fe 2p32 peak which

were clearly shown in Figure 16 for samples with rela-

tively low MR ratio (60) at RT The Fe 2p spectra at

the top interface and even in the MgO barrier is similar to

pure Fe curve without any additional peaks or any peak

shifts However the Fe 2p spectra at the bottom FeMgO

interface shown by the red line presents an energy shift

of about 10 eV although the whole curve looks simi-

lar with ones from the top interface and from the MgO

barrier suggesting a strong FendashO bonding at the bottom

interface Based on the comparison of Fe 2p spectra from

samples with low and high MR ratio shown in Figure 16

and 18 respectively it is reasonable to conclude that much

less FeO formation exists at the bottom interface in sam-

ples with higher MR ratio Unfortunately little informa-

tion about top MgOFe interface could be obtained from

current XPS results

The Mg 1s spectra are presented in Figure 19 With

etching the peak located at around 13035 eV appears

indicating the exact location of top MgOFe interface

Then the intensity of main peak of Mg 1s shows a dramatic

increase in the MgO barrier without any shift of peak posi-

tion However the peak position shifts to higher energy at

the bottom FeMgO interface where the intensity of main

peak of Fe 2p increases sharply shown in Figure 18 The

binding energy shift of Mg 1s at bottom interface should

be due to the strong bonding between Fe and O as dis-

cussed above for Fe 2p spectra It is worthy to point out

that the intensity scale for Mg 1s at TI and BI is different

shown on the right and left respectively The O 1s spec-

tra are shown in Figure 20 With respect to O 1s spectra

the main peak locates at the typical binding energy at top

Fig 19 Mg 1s high resolution XPS spectra for sample with structure

of Fe(25)MgO(5)Fe(3)Cr(5) (thickness in nm)68

Fig 20 O 1s high resolution XPS spectra for sample with structure of


MgOFe interface but shows a shift at bottom FeMgO

interfaces The same mechanism discussed above for Fe

2p and Mg 1s spectra can be applied here

In this part a detailed discussion for the interface char-

acterization by using XPS technique is given The XPS

measurement was carried out in the same samples or

samples with same growth conditions which could be

compared directly For the samples with low MR ratio

(such as 60 at RT) two small peaks were observed in

the Fe 2p spectra showing an evidence of FeO formation

at the bottom FeMgO interface For the samples with high

MR ratio (such as 170 at RT) the binding energy shift in

the Fe 2p Mg 1s and O 1s spectra showing a strong bond-

ing between Fe and O Based on the comparison of Fe 2p

spectra from samples with low and high MR ratio shown

in Figure 16 and 18 respectively we conclude that the bot-

tom interface is less oxidized in samples with higher MR

ratios Unfortunately no conclusive information regarding

the top MgOFe interface was achieved from these XPS

results Therefore at this stage XPS measurements and

HRTEM images have not characterized conclusively the

structure of the top MgOFe interface

24 Spin Dependent Tunneling Spectroscopy

For MTJs the measurement of dynamic conductance

(dIdV ) and inelastic electron tunnel spectrum (IETS-

d2IdV 2) has been proven to be a powerful tool to study

the spin dependent tunneling70ndash78 Using this method the

influence of the density of states (DOS) and inelastic scat-

tering process on conductance can be clarified by mea-

suring first and second derivative conductance (dIdV d2IdV 2) In tunneling spectroscopy measured dIdVcurves of common tunnel junctions are rather featureless

Peaks in d2IdV 2 are used to identify inelastic processes

in tunneling usually called inelastic electron tunneling

spectroscopy (IETS) Very recently analytic expression for

contributions to the IETS from surface magnon scattering

and magnetic impurity scattering has been published79 It

is shown that surface magnon scattering alone does not



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lead to peaks in the IETS The peaks at small bias often

observed in the IETS of magnetic junctions are due to

magnetic impurity scattering in agreement with the tradi-

tional model for zero-bias anomaly

Exception for very few works2078 most dIdV mea-

surements for both P and AP configurations are feature-

less However the IET spectra (d2IdV 2) for P and AP

configurations show a different behavior There are usu-

ally multiple peaks in the d2IdV 2 of the P configuration

which are generally identified with the electronndashmagnon

and electronndashphonon scattering processes For the AP con-

figuration the dominant features are the peaks in d2IdV 2

due to the logarithmic singularity of the zero bias anomaly

which is attributed to the electronndashmagnon scattering80

Due to the lack of features in the measured dynamic con-

ductance spin-dependent tunneling spectroscopy has not

been able to provide much information on interfacial struc-

tures of MTJs despite the realization that such measure-

ments should be sensitive to the changes at the interfaces81

Recently the dynamic conductance of the P configura-

tion at 10 K in high quality epitaxial FeMgOFe MTJs was

presented with some features where the junction showed

a MR ratio of about 330 at 10 K20 The peaks could

be seen in the top panel of Figure 21 but no explanation

was offered in that work However this observation of fea-

tures in dIdV curve shows a good chance to compare the

experimental data with first-principles calculation

Another detailed investigation of dynamic conductance

and inelastic electron tunnel spectrum has been carried

out very recently on the epitaxial FeMgOFe MTJs

together with theoretical calculations78 Firstly the evi-

dence for high quality epitaxial MTJs was provided from

the MR ratio and the MgO thickness dependence of prod-

uct of resistance and junction area (RA) Figure 22(a)

(a) (b)

Fig 21 (a) dIdV in P (top panel) and AP (bottom panel) magne-

tization states at 300 K (open circles) and 10 K (full circles) Top

panel inset TMR curves at 300 K (open circles) and 4 K (full cir-

cles) (b) Bulk band structure diagram of bcc Fe Reprinted with permis-

sion from [20] R Guerrero et al Appl Phys Lett 91 132504 (2007)copy 2007 American Institute of Physics

(a)

(b)

Fig 22 (a) Normalized MR ratio at 42 K in junctions with tMgO = 15

21 and 30 nm respectively (b) RA at 42 K versus MgO thickness

Reprinted with permission from [31] Q L Ma et al Appl Phys Lett95 052506 (2009) copy 2009 American Institute of Physics

shows the normalized MR ratio (RRPminus1times100) as a

function of magnetic field at 42 K for junctions with struc-

ture of Fe(25)MgO(tMgO)Fe(10)IrMn(10) (in nm) with

tMgO = 30 21 and 15 nm where R is the resistance

at different magnetic fields The MR ratio of these junc-

tions at low temperatures is 318 218 and 164 for

MgO thicknesses of 30 21 and 15 nm respectively

Figure 22(b) shows the product of resistance and area (RA)as a function of MgO thickness at 42 K The exponential

increase in RA as a function of tMgO is typical of high qual-

ity tunnel junctions and the barrier height of 060 eV is

obtained from the slope of log (RA) versus MgO thickness

The results shown in Figure 22 together with results from

RHEED XRD HRTEM and XPS measurements presented

above indicate that the sample is of high epitaxial qual-

ity and therefore used for the measurement of dynamic

conductance and inelastic electron tunneling spectroscopy

In Figure 23 the dynamic conductance (dIdV ) is

shown in panels (a) and (b) and IET spectrum (d2IdV 2minusV ) is shown in panels (c) and (d) for the P and AP

configurations respectively Here the positive bias means

the dc current flowing from bottom to top electrode or elec-

trons injected from the top electrode A strong asymmetry

in the dIdV curve between positive and negative biases is

observed shown in Figures 23(a) and (b) even though the

epitaxial FeMgOFe structure is stoichiometrically sym-

metric This asymmetry is further evident by a broad shoul-

der around 02 V in Figure 23(b) which is only observed in

the positive bias Such asymmetry has been reported pre-

viously by some groups2082 Possible explanations include

the interface dislocations82 different electronic structures



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(a) (b)

(c) (d)

Fig 23 dIdV ((a) and (b)) and IETS ((c) and (d)) for the P and AP configurations at 42 K for the junction with tMgO = 21 nm Reprinted with

permission from [78] G X Du et al Phys Rev B 81 064438 (2010) copy 2010 American Physical Society

of the top and bottom electrodes at FeMgO interfaces19

and formation of FendashO layer at the interfaces4043 66

Another important characteristic of dynamic conduc-

tance is that the dIdV curve for the P configuration in

Figure 23(a) shows some unambiguous features The IET

spectrum presented in Figure 23(c) shows several peaks as

well but all of which can be obtained from the derivative

of the peaks in Figure 23(a) Here it is necessary to point

out that the IET spectrum shown in Figure 23(c) is mea-

sured by lock-in method not numerically obtained from

dIdV data shown in Figure 23(a) different from the work

reported before73

To understand the origin of these peaks dynamic con-

ductance and IET spectra were measured for three samples

with different barrier thicknesses (30 21 and 15 nm

shown in Fig 22) and as functions of the tempera-

ture and applied magnetic fields No difference in the

peak positions between three samples were observed The

sample independence and the barrier thickness indepen-

dence exclude both the defect scattering inside the bar-

rier and the interference effect of tunneling states in the

MgO barrier710 82 as possible origins of these peaks In-

plane magnetic fields up to 10 T (not shown here) were

applied during the measurement for the P configuration

(AP configuration is possible only at low magnetic fields)

The peaks do not show any changes under the magnetic

fields With respect to temperature variation all peak posi-

tions remain unchanged below 77 K Above 77 K the

peaks become unobservable due to thermal smearing The

absence of any magnetic field dependence and temperature

dependence of the peak positions also make magnon and

phonon scattering unlikely origins

In the previous works743 an ideally symmetric

FeMgOFe structure and an asymmetric FeFeOMgOFe

structure where a single atomic FeO layer is assumed

at the bottom interface were calculated by the first-

principles theory Recently a third asymmetric structure

in FeMgOFe junction called FeMgOvacancyFe struc-

ture with vacancies on the oxygen sublattice of the top

MgO layer is also calculated in order to compare the cal-

culation with the experimental measurements The details

about the theoretical calculations can be found in another

paper78 The oxygen vacancies might form due to the

decomposing process of MgO into O atoms that form

O2 molecules in ultra high vacuum chamber and the con-

tinuous pumping out of the chamber during MgO growth

by evaporation10 The vacancy concentration should not

(a)

(b)

Fig 24 Calculated parallel conductance for junctions with (a) sym-

metric FeMgOFe asymmetric FeMgO05vacancyFe and FeMgO

1vacancyFe and (b) asymmetric FeFeOMgOFe Reprinted with per-

mission from [78] G X Du et al Phys Rev B 81 064438 (2010)copy 2010 American Physical Society



IP 129714347Sun 15 Apr 2012 143724

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not exceed the measurement resolution of 1 A previous

first-principles study83 found that oxygen vacancies can

greatly reduce the MR ratio and produce a resonant tun-

neling at a high bias of 1 V Three different contents

vacancy (0 05 and 1) was calculated using the

coherent potential approximation84 Calculations with less

vacancies were not carried out because the extra peaks in

the spectrum would diminish so much that it looks very

similar to that of pure FeMgOFe structure

The calculated results for three types of junctions

symmetric FeMgOFe asymmetric FeMgOvacancyFe

(05 and 1 vacancy) and FeFeOMgOFe are shown in

Figure 24 For each calculation the transmission probabil-

ity is integrated over 8256 k-points in the irreducible two-

dimensional Brillouin zone The total conductance includ-

ing both majority and minority spin is plotted For ideal

FeMgOFe structure there is one main peak at zero bias

and two symmetric shoulders at about plusmn027 V shown

in Figure 24(a) For FeMgOvacancyFe in addition to

the features similar to ideal FeMgOFe structure there

is also a strong peak at about +004 V and a shoul-

der at about +017 V The peak positions do not shift

for different vacancy content but the intensity increases

with increasing vacancy content For FeFeOMgOFe

one strong peak appears in the negative bias at about

minus011 V shown in Figure 24(b) The antiparallel con-

ductance was also calculated (not shown here) with-

out major peaks in good agreement with experimental

measurement shown in Figure 23(b) For the symmet-

ric FevacancyMgOvacancyFe with 05 vacancy (not

shown here) both the positive and the negative bias sides

of the spectrum are essentially the same as the positive

bias part of the asymmetric FeMgOvacancyFe spectrum

(a)

(b)

Fig 25 Parallel dIdV for two junctions with tMgO = 21 nm (open

circles) and 30 nm (open squares) compared to calculations (solid

curves) (a) positive bias and (b) negative bias Reprinted with permission

from [78] G X Du et al Phys Rev B 81 064438 (2010) copy 2010


In Figure 25 the calculated GEF V is compared with

the measured dIdV for the P configuration for two junc-

tions with tMgO = 21 and 30 nm shown by open circles

and open squares respectively To facilitate the com-

parison we plot the positive and negative biases sepa-

rately in panels (a) and (b) A fourth-order polynomial

background is removed from the experimental data to

accentuate the peaks Each calculated spectrum is also

shifted by a constant in order to be plotted in the same

range as the experimental data The asymmetric struc-

ture of FeMgOvacancyFe with 1 vacancy was used in

Figure 25 The solid lines in Figure 25 from top to bot-

tom are calculations for FeMgOFe FeMgOvacancyFe

and FeFeOMgOFe structure respectively For the posi-

tive bias in Figure 25(a) where the electrons are injected

from the top electrode an experimental peak at 0026 V

and a shoulder at 013 V are matched well with the

calculated spectrum for the FeMgOvacancyFe structure

For the negative bias in Figure 25(b) where the electrons

are injected from the bottom electrode there are a peak

at minus011 V and a shoulder at around minus024 V in the

experimental spectrum The former matches with the cal-

culated peak for the FeFeOMgOFe structure while the

latter matches with the shoulder for the ideal FeMgOFe

structure It is reasonable to conclude that for the posi-

tive bias voltage the main features in dIdV are due to

the FeMgOvacancyFe structure and that for the negative

bias voltage the main features in dIdV are due to the

FeMgOFe and FeFeOMgOFe structures

There have been theoretical suggestions together with

experimental support that an FeO layer is formed at the

interface between the bottom electrode and the MgO

layer4043 66 and that the interface between the MgO layer

and the top electrode has dislocations and vacancies1085

The results from spin-dependent tunneling spectroscopy

here present direct evidence of the existence of a mix-

ture of FeMgO and FeFeOMgO at the bottom interface

and the presence of oxygen vacancies at the top inter-

face Dynamic conductance measurement also suggests

that only the interface on the side of the electrons injecting

electrode determines the spin dependent tunneling spec-

trum This is consistent with the presence of diffusive scat-

tering inside the MgO barrier layer The previous estimate

of the scattering length inside MgO barrier is about 2 nm86

It is necessary to emphasize that the top interfacial struc-

ture could be studied by the dynamic conductance together

with theoretical calculations Further refinement of both

experiment and theory may allow spin-dependent tunnel-

ing spectroscopy to determine interface structures quanti-

tatively For example the difference in the relative heights

of the two measured peaks for two samples in Figure 25(b)

might indicate different amount of FeO at the bottom inter-

face in two samples which was further proved by the MTJ

samples with MgO barrier grown at 200 C (not shown

here) compared to the sample grown at RT used for this

study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS

In this topical review article the interface characteriza-

tion in epitaxial FeMgOFe magnetic tunnel junctions

is the main focus together with a brief introduction

on recent development of MgO-based MTJs In epitax-

ial MTJs the interfacial structures play a critical role

in the spin dependent tunneling such as the MR ratio

which has been a debating issue during past several

years The techniques applied to investigate the interfaces

include the surface X-ray diffraction Auger electron spec-

troscopy X-ray absorption spectra and X-ray magnetic

circular dichroism high resolution and analytical transmis-

sion electron microscopy and spin dependent tunneling

spectroscopy Different techniques and measurements are

sensitive to various properties For examples the measure-

ment of XMCD shows the different Fe chemical states and

XPS technique is sensitive to the environmental bonding

of specified elements Fortunately the different methods

could be used together probably from various groups to

elucidate the mechanism behind Based on the review of

results from many groups andor from different experimen-

tal methods (sometime with support from theoretical calcu-

lations) it is reasonable to conclude that there is bonding

between Fe and O at the bottom FeMgO interface and

this bonding could be decreased under the optimization of

growth conditions The bonding has a close relationship to

the MR ratio in the junctions With respect to the bottom

FeMgO interface most parts are sharp and non-oxidized

These parts contribute to the tunneling process leading to

a high MR ratio More exactly the bottom FeMgO inter-

face should be a mixture of sharp and clean structures and

some oxidizedbonding parts The top MgOFe interface

includes the dislocations and oxygen vacancies

It is worthy to point out that only the interface on the

side of electrons injecting electrodes plays a dominant role

on the coherent tunneling process based on the analy-

sis of dynamic conductance together with first-principles

calculation In short the interfacial structure is of great

importance to the spin dependent tunneling in epitax-

ial MgO-based junctions probably in CoFeBMgOCoFeB

junctions but is a complicated issue as well We hope this

topical review article could shed some light on this issue

Acknowledgments This work was supported by the

National Basic Research Program of China (MOST No

2009CB929203 and 2010CB934400) Chinese National

Natural Science Foundation (NSFC No 50972163

50721001 and 10934009) the U K Engineering and

Physical Research Council (EPSRC) and the Royal

Academy of Engineering Portion of the research was con-

ducted at the CNMS of ORNL operated by UT-Battelle

for Office of User Facilities Basic Energy Sciences US

Department of Energy The authors thank Keith Belcher

from Oxford University for his technical expertise in MBE

growth and Bin Cheng from Beijing University of Chem-

ical Technology for the XPS measurement

References and Notes

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Rev Lett 74 3273 (1995)5 T Miyazaki and N Tezuka J Magn Magn Mater 139 L231

(1995)6 H X Wei Q H Qin M Ma R Sharif and X F Han J Appl

Phys 101 09B501 (2007)7 W H Butler X-G Zhang T C Schulthess and J M MacLaren

Phys Rev B 63 054416 (2001)8 J Mathon and A Umerski Phys Rev B 63 220403R (2001)9 S S P Parkin C Kaiser A Panchula P M Rice B Hughes

M Samant and S H Yang Nat Mater 3 862 (2004)10 S Yuasa T Nagahama A Fukushima Y Suzuki and K Ando

Nat Mater 3 868 (2004)11 S Ikeda K Miura H Yamamoto K Mizunuma H D Gan

M Endo S Kanai J Hayakawa F Matsukura and H Ohno

Nat Mater 9 721 (2010)12 D D Dyayaprawira K Tsunekawa M Nagai H Maehara

S Yamagata N Watanabe S Yuasa Y Suzuki and K Ando ApplPhys Lett 86 092502 (2005)

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H Ohno J Appl Phys 106 023920 (2009)14 S Ikeda J Hayakawa Y Ashizawa Y M Lee K Miura

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and A Schuhl Phys Rev Lett 93 106602 (2004)20 R Guerrero D Herranz F G Alieva F Greullet C Tiusan

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Phys Lett 89 042505 (2006)23 M Nakazumi D Yoshioka H Yanagihara E Kita and T Koyano

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W H Wang S Kasai S Mitani K Inomata and K Hono ApplPhys Lett 96 212505 (2010)

25 J C Read J J Cha W F Egelhoff Jr H W Tsueng P Y Huang

Y Li D A Muller and R A Buhrman Appl Phys Lett 94 112504(2009)

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56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

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31 Q L Ma S G Wang J Zhang Y Wang R C C Ward C Wang

A Kohn X-G Zhang and X F Han Appl Phys Lett 95 052506(2009)

32 M Dynna J L Vassent A Marty and B Gilles J Appl Phys80 2650 (1996)

33 J L Vassent M Dynna A Marty B Gilles and G Patrat J ApplPhys 80 5727 (1996)

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35 W Wulfhekel M Klaua D Ullmann F Zavaliche J Kirschner

R Urban T Monchesky and B Heinrich Appl Phys Lett 78 509(2001)

36 M Bowen V Cros F Petroff A Fert C M Boubeta J L Costa-

Kraumler J V Anguita A Cebollada F Briones J M de Teresa

L Morelloacuten M R Ibarra F Guumlell F Peiroacute and A Cornet ApplPhys Lett 79 1655 (2001)

37 J Faure-Vincent C Tiusan E Jouguelet F Canet M Sajieddine

C Bellouard E Popova M Hehn F Montaigne and A Schuhl

Appl Phys Lett 82 4507 (2003)38 W H Butler X-G Zhang T C Schulthess and J M MacLaren

Phys Rev B 63 092402 (2001)39 T Nagahama and J S Moodera Journal of Magnetics 11 170

(2006)40 H L Meyerheim R Popescu J Kirschner N Jedrecy M Sauvage-

Simkin B Heinrich and R Pinchaux Phys Rev Lett 87 076102(2001)

41 H L Meyerheim R Popescu N Jedrecy M Vedpathak

M Sauvage-Simkin R Pinchaux B Heinrich and J Kirschner

Phys Rev B 65 144433 (2002)42 C Tusche H L Meyerheim N Jedrecy G Renaud A Ernst

J Henk P Bruno and J Kirschner Phys Rev Lett 95 176101(2005)

43 X-G Zhang W H Butler and A Bandyopadhyay Phys Rev B68 092402 (2003)

44 C Zhang X-G Zhang P S Krstic H-P Cheng W H Butler and

J M MacLaren Phys Rev B 69 134406 (2004)45 F J Palomares C Munuera C M Boubeta and A Cebollada

J Appl Phys 97 036104 (2005)46 M Sicot M Andrieu P Turban Y Fagot-Revurat H Cercellier

A Tagliaferri C de Nadai N B Brookes F Bertran and

F Fortuna Phys Rev B 68 184406 (2003)47 K Miyokawa S Saito T Katayama T Saito T Kamino

K Hanashima Y Suzuki K Mamiya T Koide and S Yuasa

Jpn J Appl Phys 44 L9 (2005)48 M Muumlller F Matthes and C M Schneider Europhys Lett

80 17007 (2007)49 Y Wang J Zhang X-G Zhang H-P Cheng and X F Han Phys

Rev B 82 054405 (2010)50 K D Belashchenko J Velev and E Y Tsymbal Phys Rev B

72 140404R (2005)51 G H Yu C L Chai F W Zhu J M Xiao and W Y Lai Appl

Phys Lett 78 1706 (2001)52 T Nozaki N Tezuka and K Inomata Phys Rev Lett 96 027208

(2006)53 L Wang S G Wang S Rizwan Q H Qin and X F Han Appl

Phys Lett 95 152512 (2009)54 L Ding J Teng X C Wang C Feng Y Jiang G H Yu S G

Wang and R C C Ward Appl Phys Lett 96 052515 (2010)55 M Sicot S Andrieu C Tiusan F Montaigne and F Bertran

J Appl Phys 99 08D301 (2001)56 C Tiusan M Sicot J Faure-Vincent M Hehn C Bellouard

F Montaigne S Andrieu and A Shuhl J Phys Condens Matter18 941 (2006)

57 F Bonell S Andrieu F Bertran P Lefegravevre A T Ibrahimi

E Snoeck C Tiusan and F Montaigne IEEE Trans Magn45 3467 (2009)

58 S G Wang A Kohn C Wang A K Petford-Long S Lee R Fan

J P Goff L J Singh Z H Barber and R C C Ward J Phys DAppl Phys 42 225001 (2009)

59 S G Wang R C C Ward G X Du X F Han C Wang and

A Kohn IEEE Trans Magn 44 2562 (2008)60 C Wang A Kohn S G Wang L Y Chang S-Y Choi A I

Kirkland A K Petford-Long and R C C Ward Phys Rev B82 024428 (2010)

61 C L Jia and K Urban Science 303 2001 (2004)62 C Wang S G Wang A Kohn R C C Ward and A K Petford-

Long IEEE Trans Magn 43 2779 (2007)63 P A Stadelmann Ultramicroscopy 21 131 (1987)64 C R Seabourne A J Scott G Vaughan R Brydson S G Wang

R C C Ward C Wang A Kohn B Mendis and A K Petford-

Long Ultramicroscopy 110 1059 (2010)65 V Serin S Andrieu R Serra F Bonell C Tiusan L Calmels

M Varela S J Pennycook E Snoeck M Walls and C Colliex

Phys Rev B 79 1444131 (2009)66 S G Wang G Han G H Yu Y Jiang C Wang A Kohn and

R C C Ward J Magn Magn Mater 310 1935 (2007)67 F Bonell S Andrieu A M Bataille C Tiusan and G Lengaigne

Phys Rev B 79 224405 (2009)68 S G Wang Q L Ma H F Liu G H Yu B Cheng R C C Ward

and X F Han (unpublished)

69 H Oh S B Lee H G Min and J-S Kim Appl Phys Lett 82 361(2003)

70 Y Ando J Murai H Kubota and T Miyazaki J Appl Phys87 5209 (2000)

71 X F Han J Murai Y Ando H Kubota and T Miyazaki ApplPhys Lett 78 2533 (2001)

72 M Mizuguchi Y Hamada R Matsumoto S Nishioka H Maehara

K Tsunekawa D D Djayaprawira N Watanabe T Nagahama

A Fukushima H Kubata S Yuasa M Shiraishi and Y Suzuki

J Appl Phys 99 08T309 (2006)73 G X Miao K B Chetry A Gupta W H Bulter K Tsunekawa

D Djayaprawira and G Xiao J Appl Phys 99 08T305 (2006)74 K Ono T Daibou S-J Ahn Y Sakuraba Y Miyakoshi M Oogane

Y Ando and T Miyazaki J Appl Phys 99 08A905 (2006)75 S Nishioka R Matsumoto H Tomita T Nozaki Y Suzuki H Itoh

and S Yuasa Appl Phys Lett 93 122511 (2008)76 D Bang T Nozaki D D Djayaprawira M Shiraishi Y Suzuki

A Fukushima H Kubata T Nagahama S Yuasa H Maehara

K Tsunekawa Y Nagamine N Watanabe and H Itoh J ApplPhys 105 07C924 (2009)

77 V Drewello M Schaumlfers G Reiss and A Thomas Phys Rev B79 174417 (2009)

78 G X Du S G Wang Q L Ma Y Wang R C C Ward

X-G Zhang C Wang A Kohn and X F Han Phys Rev B81 064438 (2010)

79 H X Wei Q H Qin Q L Ma X-G Zhang and X F Han PhysRev B 82 134436 (2010)

80 S Zhang P M Levy A C Marley and S S P Parkin Phys RevLett 79 3744 (1997)

81 P LeClair J T Kohlhepp H J M Swagten and W J M de Jonge

Phys Rev Lett 86 1066 (2001)82 Y Ando T Miyakoshi M Oogane T Miyazaki H Kubota

K Ando and S Yuasa Appl Phys Lett 87 142502 (2005)83 J P Velvev K D Belashchenko S S Jaswal and E Y Tsymbal

Appl Phys Lett 90 072502 (2007)84 J S Faulkner and G M Stocks Phys Rev B 21 3222 (1980)85 J Ozeki H Itoh and J Inoue J Magn Magn Mater 310 E644

(2007)86 X-G Zhang Y Wang and X F Han Phys Rev B 77 144431

(2008)

Received 1 November 2010 RevisedAccepted 6 January 2011



IP 129714347Sun 15 Apr 2012 143724

REVIEW


S G Wang got his bachelor degree in physics from Anhui University and received his

PhD degree from the Institute of Solid State Physics Chinese Academy of Sciences in

2001 He worked as a research fellow in the National University of Singapore from August

2001 to July 2003 Then he moved to the Max-Planck-Institute of Microstructure Physics

Halle (Saale) as a post-doctor From February 2005 to December 2007 he worked in the

Clarendon Laboratory Department of Physics at the University of Oxford He joined the

State Key Laboratory of Magnetism Institute of Physics Chinese Academy of Sciences

at the end of 2007 and now is an associate professor of condensed matter physics His

research interests include the physics materials and device of spintronics

R C C Ward is a Senior Research Fellow in the Clarendon Laboratory Department

of Physics of the University of Oxford He received his DPhil from Oxford in 1981

His research interests include the growth and characterisation of epitaxial metal films and

superlattices for magnetic research fundamental processes of growth with atomic layer

control by molecular beam epitaxy (MBE) and UHV sputtering and the growth of bulk

single crystals from melts and solutions for optical and magnetic research Most recently

his work has focused on the fields of epitaxial spintronic devices superlattices containing

rare earth elements and epitaxial films of uranium metal and compounds

T Hesjedal is a Lecturer in Materials Design for Condensed Matter Physics in the Claren-

don Laboratory at the University of Oxford and the Rutherford-Appleton Laboratory He

received his PhD from the Humboldt University in Berlin (Germany) in 1997 Dr Hesjedal

came to Oxford from the University of Waterloo and Stanford University in late 2010 where

he was Associate Professor since 2005 He focuses on the synthesis and exploration of novel

quantum materials and materials for spintronics applications

X-G Zhang received his bachelor degree in physics from Peking University in 1983 and

PhD in physics from Northwestern University USA in 1989 He worked as a postdoctoral

fellow at Lawrence Berkley Laboratory from 1990 to 1991 a postdoctoral scholar at the

University of Kentucky from 1991 to 1995 Since 1995 he is a staff scientist at Oak Ridge

National Laboratory Currently he holds the position of Senior Staff Scientist at the Center

for Nanophase Materials Sciences Oak Ridge National Laboratory His research interests

include physics and materials science on the nanoscale in particular electron transport in

spintronics and molecular electronics

C Wang graduated from the Department of Mechanical Engineering Tsinghua University in

2000 and 2003 as bachelor and master After one year as process engineer in Semiconductor

Manufacturing International Corporation Shanghai China he pursued his PhD degree

in Department of Materials University of Oxford UK In 2008 he started to work as a

hardware engineer in Applied Materials Inc California USA Recently his role switches to

be responsible for technology collaborations with China universities and research institutes

in equipment applications for renewable energy



IP 129714347Sun 15 Apr 2012 143724R

EVIEW





























MTJs













IP 129714347Sun 15 Apr 2012 143724

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devices6





















in MgO-based MTJs






































of great importance













































IP 129714347Sun 15 Apr 2012 143724R

EVIEW




















































grown by MBE37







2 = 2m

2VBminusEminus 2

x2+ 2

y2

(1)















IP 129714347Sun 15 Apr 2012 143724

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ndash4

ndash3

ndash2

ndash1

0

1

2

3

4

0 02 04 06 08 1 12 14

Ene

rgy

(eV

)

Im k (1Angstrom)



(E = 0 eV)











(a) (b)

(c) (d)












AP configuration




















IP 129714347Sun 15 Apr 2012 143724R

EVIEW












found elsewhere2839



































literature







(a)

(b)






























IP 129714347Sun 15 Apr 2012 143724

REVIEW













































still an open issue























Fe layer57






























equivalent Fe axes



IP 129714347Sun 15 Apr 2012 143724R

EVIEW






















(a) (b)






sample grown by MBE
























IP 129714347Sun 15 Apr 2012 143724

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(a) (b)














































IP 129714347Sun 15 Apr 2012 143724R

EVIEW
















Figure 13


































high MR ratio






















(a)

(b)

(c)







IP 129714347Sun 15 Apr 2012 143724

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for reference





IP 129714347Sun 15 Apr 2012 143724R

EVIEW


























current XPS results


























































IP 129714347Sun 15 Apr 2012 143724

REVIEW




































(a) (b)






(a)

(b)



































IP 129714347Sun 15 Apr 2012 143724R

EVIEW


(a) (b)

(c) (d)














reported before73




































(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

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(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)




IP 129714347Sun 15 Apr 2012 143724R

EVIEW





























MTJs













IP 129714347Sun 15 Apr 2012 143724

REVIEW









devices6





















in MgO-based MTJs






































of great importance













































IP 129714347Sun 15 Apr 2012 143724R

EVIEW




















































grown by MBE37







2 = 2m

2VBminusEminus 2

x2+ 2

y2

(1)















IP 129714347Sun 15 Apr 2012 143724

REVIEW


ndash4

ndash3

ndash2

ndash1

0

1

2

3

4

0 02 04 06 08 1 12 14

Ene

rgy

(eV

)

Im k (1Angstrom)



(E = 0 eV)











(a) (b)

(c) (d)












AP configuration




















IP 129714347Sun 15 Apr 2012 143724R

EVIEW












found elsewhere2839



































literature







(a)

(b)






























IP 129714347Sun 15 Apr 2012 143724

REVIEW













































still an open issue























Fe layer57






























equivalent Fe axes



IP 129714347Sun 15 Apr 2012 143724R

EVIEW






















(a) (b)






sample grown by MBE
























IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a) (b)














































IP 129714347Sun 15 Apr 2012 143724R

EVIEW
















Figure 13


































high MR ratio






















(a)

(b)

(c)







IP 129714347Sun 15 Apr 2012 143724

REVIEW

















































for reference





IP 129714347Sun 15 Apr 2012 143724R

EVIEW


























current XPS results


























































IP 129714347Sun 15 Apr 2012 143724

REVIEW




































(a) (b)






(a)

(b)



































IP 129714347Sun 15 Apr 2012 143724R

EVIEW


(a) (b)

(c) (d)














reported before73




































(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)




IP 129714347Sun 15 Apr 2012 143724

REVIEW









devices6





















in MgO-based MTJs






































of great importance













































IP 129714347Sun 15 Apr 2012 143724R

EVIEW




















































grown by MBE37







2 = 2m

2VBminusEminus 2

x2+ 2

y2

(1)















IP 129714347Sun 15 Apr 2012 143724

REVIEW


ndash4

ndash3

ndash2

ndash1

0

1

2

3

4

0 02 04 06 08 1 12 14

Ene

rgy

(eV

)

Im k (1Angstrom)



(E = 0 eV)











(a) (b)

(c) (d)












AP configuration




















IP 129714347Sun 15 Apr 2012 143724R

EVIEW












found elsewhere2839



































literature







(a)

(b)






























IP 129714347Sun 15 Apr 2012 143724

REVIEW













































still an open issue























Fe layer57






























equivalent Fe axes



IP 129714347Sun 15 Apr 2012 143724R

EVIEW






















(a) (b)






sample grown by MBE
























IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a) (b)














































IP 129714347Sun 15 Apr 2012 143724R

EVIEW
















Figure 13


































high MR ratio






















(a)

(b)

(c)







IP 129714347Sun 15 Apr 2012 143724

REVIEW

















































for reference





IP 129714347Sun 15 Apr 2012 143724R

EVIEW


























current XPS results


























































IP 129714347Sun 15 Apr 2012 143724

REVIEW




































(a) (b)






(a)

(b)



































IP 129714347Sun 15 Apr 2012 143724R

EVIEW


(a) (b)

(c) (d)














reported before73




































(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)




IP 129714347Sun 15 Apr 2012 143724R

EVIEW




















































grown by MBE37







2 = 2m

2VBminusEminus 2

x2+ 2

y2

(1)















IP 129714347Sun 15 Apr 2012 143724

REVIEW


ndash4

ndash3

ndash2

ndash1

0

1

2

3

4

0 02 04 06 08 1 12 14

Ene

rgy

(eV

)

Im k (1Angstrom)



(E = 0 eV)











(a) (b)

(c) (d)












AP configuration




















IP 129714347Sun 15 Apr 2012 143724R

EVIEW












found elsewhere2839



































literature







(a)

(b)






























IP 129714347Sun 15 Apr 2012 143724

REVIEW













































still an open issue























Fe layer57






























equivalent Fe axes



IP 129714347Sun 15 Apr 2012 143724R

EVIEW






















(a) (b)






sample grown by MBE
























IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a) (b)














































IP 129714347Sun 15 Apr 2012 143724R

EVIEW
















Figure 13


































high MR ratio






















(a)

(b)

(c)







IP 129714347Sun 15 Apr 2012 143724

REVIEW

















































for reference





IP 129714347Sun 15 Apr 2012 143724R

EVIEW


























current XPS results


























































IP 129714347Sun 15 Apr 2012 143724

REVIEW




































(a) (b)






(a)

(b)



































IP 129714347Sun 15 Apr 2012 143724R

EVIEW


(a) (b)

(c) (d)














reported before73




































(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)




IP 129714347Sun 15 Apr 2012 143724

REVIEW


ndash4

ndash3

ndash2

ndash1

0

1

2

3

4

0 02 04 06 08 1 12 14

Ene

rgy

(eV

)

Im k (1Angstrom)



(E = 0 eV)











(a) (b)

(c) (d)












AP configuration




















IP 129714347Sun 15 Apr 2012 143724R

EVIEW












found elsewhere2839



































literature







(a)

(b)






























IP 129714347Sun 15 Apr 2012 143724

REVIEW













































still an open issue























Fe layer57






























equivalent Fe axes



IP 129714347Sun 15 Apr 2012 143724R

EVIEW






















(a) (b)






sample grown by MBE
























IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a) (b)














































IP 129714347Sun 15 Apr 2012 143724R

EVIEW
















Figure 13


































high MR ratio






















(a)

(b)

(c)







IP 129714347Sun 15 Apr 2012 143724

REVIEW

















































for reference





IP 129714347Sun 15 Apr 2012 143724R

EVIEW


























current XPS results


























































IP 129714347Sun 15 Apr 2012 143724

REVIEW




































(a) (b)






(a)

(b)



































IP 129714347Sun 15 Apr 2012 143724R

EVIEW


(a) (b)

(c) (d)














reported before73




































(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

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(2008)




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found elsewhere2839



































literature







(a)

(b)






























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still an open issue























Fe layer57






























equivalent Fe axes



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(a) (b)






sample grown by MBE
























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(a) (b)














































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Figure 13


































high MR ratio






















(a)

(b)

(c)







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for reference





IP 129714347Sun 15 Apr 2012 143724R

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current XPS results


























































IP 129714347Sun 15 Apr 2012 143724

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(a) (b)






(a)

(b)



































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(a) (b)

(c) (d)














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(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

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(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

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3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

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(2008)




IP 129714347Sun 15 Apr 2012 143724

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still an open issue























Fe layer57






























equivalent Fe axes



IP 129714347Sun 15 Apr 2012 143724R

EVIEW






















(a) (b)






sample grown by MBE
























IP 129714347Sun 15 Apr 2012 143724

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(a) (b)














































IP 129714347Sun 15 Apr 2012 143724R

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Figure 13


































high MR ratio






















(a)

(b)

(c)







IP 129714347Sun 15 Apr 2012 143724

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for reference





IP 129714347Sun 15 Apr 2012 143724R

EVIEW


























current XPS results


























































IP 129714347Sun 15 Apr 2012 143724

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(a) (b)






(a)

(b)



































IP 129714347Sun 15 Apr 2012 143724R

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(a) (b)

(c) (d)














reported before73




































(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

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(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)




IP 129714347Sun 15 Apr 2012 143724R

EVIEW






















(a) (b)






sample grown by MBE
























IP 129714347Sun 15 Apr 2012 143724

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(a) (b)














































IP 129714347Sun 15 Apr 2012 143724R

EVIEW
















Figure 13


































high MR ratio






















(a)

(b)

(c)







IP 129714347Sun 15 Apr 2012 143724

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for reference





IP 129714347Sun 15 Apr 2012 143724R

EVIEW


























current XPS results


























































IP 129714347Sun 15 Apr 2012 143724

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(a) (b)






(a)

(b)



































IP 129714347Sun 15 Apr 2012 143724R

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(a) (b)

(c) (d)














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(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

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(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

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(2008)




IP 129714347Sun 15 Apr 2012 143724

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(a) (b)














































IP 129714347Sun 15 Apr 2012 143724R

EVIEW
















Figure 13


































high MR ratio






















(a)

(b)

(c)







IP 129714347Sun 15 Apr 2012 143724

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for reference





IP 129714347Sun 15 Apr 2012 143724R

EVIEW


























current XPS results


























































IP 129714347Sun 15 Apr 2012 143724

REVIEW




































(a) (b)






(a)

(b)



































IP 129714347Sun 15 Apr 2012 143724R

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(a) (b)

(c) (d)














reported before73




































(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)




IP 129714347Sun 15 Apr 2012 143724R

EVIEW
















Figure 13


































high MR ratio






















(a)

(b)

(c)







IP 129714347Sun 15 Apr 2012 143724

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for reference





IP 129714347Sun 15 Apr 2012 143724R

EVIEW


























current XPS results


























































IP 129714347Sun 15 Apr 2012 143724

REVIEW




































(a) (b)






(a)

(b)



































IP 129714347Sun 15 Apr 2012 143724R

EVIEW


(a) (b)

(c) (d)














reported before73




































(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)




IP 129714347Sun 15 Apr 2012 143724

REVIEW

















































for reference





IP 129714347Sun 15 Apr 2012 143724R

EVIEW


























current XPS results


























































IP 129714347Sun 15 Apr 2012 143724

REVIEW




































(a) (b)






(a)

(b)



































IP 129714347Sun 15 Apr 2012 143724R

EVIEW


(a) (b)

(c) (d)














reported before73




































(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)




IP 129714347Sun 15 Apr 2012 143724R

EVIEW


























current XPS results


























































IP 129714347Sun 15 Apr 2012 143724

REVIEW




































(a) (b)






(a)

(b)



































IP 129714347Sun 15 Apr 2012 143724R

EVIEW


(a) (b)

(c) (d)














reported before73




































(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)




IP 129714347Sun 15 Apr 2012 143724

REVIEW




































(a) (b)






(a)

(b)



































IP 129714347Sun 15 Apr 2012 143724R

EVIEW


(a) (b)

(c) (d)














reported before73




































(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)




IP 129714347Sun 15 Apr 2012 143724R

EVIEW


(a) (b)

(c) (d)














reported before73




































(a)

(b)







IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)




IP 129714347Sun 15 Apr 2012 143724

REVIEW



































(a)

(b)





























































study



IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)




IP 129714347Sun 15 Apr 2012 143724R

EVIEW


3 CONCLUSIONS


























































































56 11827 (1997)



IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)




IP 129714347Sun 15 Apr 2012 143724

REVIEW













































































(2008)



interface characterization of epitaxial fe/mgo/fe magnetic … · 2018-11-16 · 1state key...

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