Evgeny Tsymbal Department of Physics and Astronomy Center for Materials Research and Analysis University of Nebraska-Lincoln Kirill Belashchenko, U. Nebraska-Lincoln Mark van Schilfgaarde, Arizona State U. Derek Stewart, Cornell U. Ivan Oleynik, U. South Florida Sitaram Jaswal, U. Nebraska-Lincoln Interface Interface - - controlled controlled tunneling spin polarization tunneling spin polarization Supported by National Science Foundation
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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
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
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
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