Evgeny Tsymbal

Department of Physics and AstronomyCenter for Materials Research and Analysis

University of Nebraska-Lincoln

Kirill Belashchenko, U. Nebraska-LincolnMark van Schilfgaarde, Arizona State U.Derek Stewart, Cornell U.Ivan Oleynik, U. South FloridaSitaram Jaswal, U. Nebraska-Lincoln

InterfaceInterface--controlled controlled tunneling spin polarizationtunneling spin polarization

Supported by National Science Foundation

OutlineOutline

IntroductionTunneling conductance in a simple tight-binding modelSpin-polarized tunneling from clean and oxidized Co surfaces through vacuumSpin polarization in Co/Al2O3/Co tunnel junctionsInterface resonant states in Fe/MgO/Fe tunnel junctions

TunnelingTunneling magnetoresistance (TMR)magnetoresistance (TMR)

Ferromagnet

FerromagnetInsulator

↑↓ ↑↑

↑↑

R - RTMR =

R

Experimental values at room temperature:FM|Al2O3|FM MTJs: TMR~70%Fe|MgO|Fe MTJs: TMR~200%

Magnetic tunnel junction

Symmetry argumentsSymmetry arguments

-8

-4

0

4

majority minority

∆1

∆1

EF

∆ H[001]En

ergy

(eV)

Γ[000]

Ferromagnet

Complex band structure: E=E(k||,kz), where kz=q+iκ, ψ ∝ e-κz

State of smallest κ dominates in conductance States of different symmetry tunnel with a different decay lengthOften this state belongs to the identity representation

InsulatorFe [001]

DeficienciesDeficiencies

Symmetry arguments

limited only by special k-points

assume sufficiently thick barrier

do not take into account electronic and atomic structure of ferromagnet/insulator interfaces

1D tight1D tight--binding modelbinding model

2 22 24 d

i rbeG(E) V (E)e (E)

hκπ

ρ ρ−=

Vb

ViV0E0

E

i r

Eb

Ei

Conductance (high barrier):

Interface DOS ρi(E)

Conductance depends strongly on interface bonding

Ei = 0.6

-2 0 2 E

Vi

2

1

0

3D tight3D tight--binding modelbinding model

In-plane dispersion: 2 x y|| ||E (k ) cos(k ) cos(k )⎡ ⎤= − +⎢ ⎥⎣ ⎦

k||-resolved interface DOS

Interface potential and bonding control the conductance

iV 1.5=

iE 0.6=

iE 0=

=iV 1

iE 0.6=

iE 0=

SpinSpin--dependent tunneling from dependent tunneling from clean and oxidized Co surfacesclean and oxidized Co surfaces

Electrodes: Co (111) and Al (111)Co surface: clean or oxidized (absorbed O monolayer)Tunneling barrier: vacuum

Al

Co0

Relevance to spin-polarized STMRelevance to Tedrow-Meservey experiments

Calculation methodsCalculation methods

{ }= Γ Γ2

R AR L

e ˆ ˆˆ ˆG Tr G Gh

Electronic structure: • Tight-binding linear muffin-tin orbital method• Fully self-consistent potentials Conductance: • Layered Green’s function approach• Transmission matrix formalism

Atomic structure: • Plane-wave pseudopotential method

L RC

Fermi surface of bulk Fermi surface of bulk fccfcc Co Co along [111] directionalong [111] direction

Majority spin Minority spin

Holes of different diameter near the Γ point

KK||||--resolved transmission resolved transmission from clean Co surface to Alfrom clean Co surface to Al

Majority spin Minority spin

Negative spin polarization (-60%)Majority spins dominate at large barrier thickness Crossover only at ~10nm

KK||||--resolved transmission resolved transmission from oxidized Co surface to Alfrom oxidized Co surface to Al

Majority spin Minority spin

Minority conductance is filtered out by O layer Spin polarization is close to +100%

Density of states Density of states for oxidized Co surfacefor oxidized Co surface

2

4

DO

S (e

V-1at

om-1)

Energy (eV)

O-surface

2

0

2

4Co-surface

4

2

0

-8 -4 0 4

2

4Co-subsurface

2

0

2

4Co-bulk

-8 -4 0 44

2

0

KK||||--resolved DOS resolved DOS for oxidized Co surfacefor oxidized Co surface

Bulk Co Surface Co Surface O

Minority spin

Surface Co-O antibonding state creates an additional tunneling barrier in minority-spin channel

Bulk states strongly mix with Co-O antibonding states and extend to the O layer

KK||||--resolved DOS resolved DOS for oxidized Co surfacefor oxidized Co surface

Bulk Co Surface Co Surface O

Majority spin

SpinSpin--dependent tunneling dependent tunneling in Co/Alin Co/Al22OO33/Co tunnel junctions/Co tunnel junctions

Model 1: O-terminated Al2O3

Model 2: Adds O(II) atoms adsorbed at the interfaces

Structure is relaxed

Strong bonding between Co(II) and O(II) atoms

Model 1 Model 2Co (111)

Co (111)

Al2O3 (0001)

Transmission functionTransmission function

Model 1

Majority

Model 2

Spin polarization

negative

positive

Minority

CoCo--O bonding at the interfaceO bonding at the interface

Exchange-split antibonding Co(II)-O(II) states

E (eV)

DOS

EF

KK||||--resolved majority DOSresolved majority DOS

Co-O antibonding state controls spin polarization

MgO

Fe

Fe

MgOAgFe

FeAg

Interface resonance Interface resonance in Fe/in Fe/MgOMgO/Fe junctions/Fe junctions

1

10-2

10-4

10-6

2.5·10-3 4.0·10-3

1.5·10-3 2.2·10-4

Majority Minority

Minority interface resonant state is filtered out by Ag

K.D.Belashchenko R1.00159

ConclusionsConclusions

Tunneling spin polarization in magnetic tunnel junctions is controlled primarily by the interface bonding and structure A monolayer of oxygen on Co (111) surface reverses the spin polarization from negative to positive due to the Co-O bonding. This phenomenon can be detected by spin-polarized STMOxygen absorbed at the Co surface in Co/Al2O3/Co tunnel junctions controls positive spin polarization. Oxidation of a ferromagnet at the interface might be important for obtaining large magnetoresistanceInterface resonant states control spin polarization in Fe/MgO/Fe junctions at small barrier thickness. This states can be filtered out by a thin Ag layer