From spinresolved photoemission to spindependent electron transmission

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Electronic Magnetic Properties of Oxide-Based
Tunnel Junctions

• Arthur Ernst, Mohammed Bouhassoune, Jürgen Henk,
  Patrick Bruno

Theory Department Max-Planck Institute of
Microstructure Physics Halle (Saale), Germany
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Motivation
Aim Electron spin electronics
spintronics Devices Spin valves, magnetic tunnel
transistors, MRAMs,

• Model systems Planar tunnel junctions
• Recent results
• Co/vacuum/Co - Origin of the zero-bias anomaly
• Ni/vacuum/Ni - Resonant tunneling
• Fe/MgO/Fe - Effect of the interface structure

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Outline

• Spin-dependent ballistic transport
• Theoretical aspects
• Effect of the interface structure Fe/MgO/Fe
• Electronic magnetic structure
• Band-gap engineering ZnxMg1-xO
• Electronic correlations Fe/FeO/MgO
• Summary

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Outline

• Spin-dependent ballistic transport
• Theoretical aspects
• Effect of the interface structure Fe/MgO/Fe
• Electronic magnetic structure
• Band-gap engineering ZnxMg1-xO
• Electronic correlations Fe/FeO/MgO
• Summary

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Spin-dependent ballistic transport
Magneto-electronics of planar tunnel junctions
Parallel configuration (P, "")
Anti-parallel configuration (AP, " )
Insulator S
Magnetic electrodes L R
Tunnel magneto-resistance
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Spin-dependent ballistic transport II
Spin polarization in the leads L R

• Jullières model
• Spacer properties completely ignored
• TMR related to the lead spin polarizations

• Slonczewskis model
• Free electrons in the leads
• Spacer rectangular step barrier
• Effective spin polarizations

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Spin-dependent ballistic transport III

• Landauer-Büttiker theory for planar
• tunnel junctions
• Conductance
• Transmission
• Averaged conductance
• Tunnel magneto-resistance
• MacLaren-Butler formulation in layer-KKR
• J.M. MacLaren et al., Phys. Rev. B 59 (1999) 5470

Scattering channels Bloch states
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Spin-dependent transport IV Multiple-scattering
Theory

• Numerical realization layer-KKR method
• Green function method Dyson equation
• Free electrons ) atom ) layer ) stack of layers )
  solid
• Flexible geometry (semi-infinite systems, films,
  adatoms, interfaces)
• Numerically efficient
• Computer program package omni2k for electron
  spectroscopies (JH et al.) spin-polarized
  relativistic computations (Dirac equation)
• Ab initio calculations Arthur Ernsts KKR code

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Outline

• Spin-dependent ballistic transport
• Theoretical aspects
• Effect of the interface structure Fe/MgO/Fe
• Electronic magnetic structure
• Band-gap engineering ZnxMg1-xO
• Electronic correlations Fe/FeO/MgO
• Summary

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Fe/MgO/Fe Interface Geometry
Bulk Fe bcc
a 2.81 Å
a 2.81 Å
2.1 mismatch
Bulk MgO fcc, NaCl
a 2.87 Å
a 4.05 Å
Standard model Ideal cut paste structure
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Fe/MgO/Fe Interface Geometry II
X-ray diffraction (H.L. Meyerheim et al.)
Fe
New Formation of a partially occupied FeO
interface layer
FeO interface layer
Confirmed by total-energy calculations (A.
Ernst) Gain of 0.73 eV/atom
O
Mg
Effect of the geometry on the electronic
magnetic structure (full occupation)?
Fe
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Fe/MgO/Fe Charge Magnetic Profiles
2 MgO layers P configuration
Magnetic moments
Charge distribution
Increased moment
Cut paste geometry
Spin down depletion
Oscillatory moments
FeO-layer geometry
Oscillatory behavior
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Fe/MgO/Fe - Transmission
Wavevector-resolved conductance
transmission Additional scattering at the FeO
layer ? change of the transmission
Cut paste geometry
FeO-layer geometry
P configuration
Resonant tunneling
AP configuration
Fe/(MgO)4/Fe
Fe/FeO/(MgO)2/FeO/Fe
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Fe/MgO/Fe Conductance Tunnel
Magneto-resistance
Conductance TMR vs. spacer thickness
Increasing TMR
P
G(cut paste) gt G(FeO)
AP
Exponential decay Slope determined by MgO band gap
Moderate effect of the interface
structure Band-gap engineering? Localized
electrons?
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Outline

• Spin-dependent ballistic transport
• Theoretical aspects
• Effect of the interface structure Fe/MgO/Fe
• Electronic magnetic structure
• Band-gap engineering ZnxMg1-xO
• Electronic correlations Fe/FeO/MgO
• Summary

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Band-gap engineering ZnxMg1-xO
Changing the band gap of the oxide spacer by
alloying Disorder coherent potential
approximation (CPA)
Coherent potential Approximation (CPA)
Scattering path operator T V VGT
Alloy
? cpa
Mg

Zn
?
(1-x) ? A
Mg defect

Zn defect
Effective medium
x ? B
Concentration
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Band-gap engineering ZnxMg1-xO
Changing the band gap of the oxide spacer by
alloying ZnxMg1-xO in fcc structure
Density of states
Zn d-states
Band gap vs. concentration x
LDA gap of MgO 4.66 eV Experiment 7.24 eV
From Bloch spectral density fundamental gap
Saturation?! 3.57 eV Gap of ZnO ¼ 3.3 eV
Agenda MgO fcc , ZnO wurtzite structural
phase transition?!
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Outline

• Spin-dependent ballistic transport
• Theoretical aspects
• Effect of the interface structure Fe/MgO/Fe
• Electronic magnetic structure
• Band-gap engineering ZnxMg1-xO
• Electronic correlations Fe/FeO/MgO
• Summary

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Electronic correlations Self-interaction
correction

• Formation of an FeO layer at Fe/MgO interfaces
• Electrons localized at the FeO layer Stronger
  electronic correlation
• Deficiencies of the LDA
• Beyond LDA Self-interaction correction (SIC)
• Increased binding energy for localized states )
  increased localization
• SIC-configuration for Fe/FeO/MgO (criterion
  minimum total energy)
• All 5 spin-up electrons of Fe SIC-corrected
• No spin-down electron of Fe SIC-corrected
• Effect on the magnetic structure?

Bloch picture
Heitler-London picture
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Electronic correlations Self-interaction
correction II
Fe/FeO/(MgO)4/FeO/Fe Relative changes (with
respect to bulk)
LSDA
LSDA SIC
Increased magnetic moment _at_ FeO ¼ 3.5 ?B
Depletion of minority electrons
Increase of majority electrons
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Summary

• Spin-dependent electron tunneling through oxide
  spacers
• Testing ground for theoretical ab initio concepts
• Geometrical structure
• Total-energy calculations
• Tunneling
• Disorder
• Partial occupation at the interfaces
• Alloying in the spacer
• Electronic correlations
• Self-interaction correction

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Thanks

• Holger L. Meyerheim, Jürgen Kirschner
• Markus Däne, Diemo Ködderitzsch, Wolfram Hergert
• Peter Zahn, Silke Roether, Ingrid Mertig
• Arthur Ernst, Mohammed Bouhassoune, Patrick Bruno
• Contact
• E-mail henk_at_mpi-halle.de
• WWW www.mpi-halle.mpg.de/henk

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Omni2k Computer program for spectroscopies

• Purpose electron spectroscopies
• SPLEED, photoemission, transport, electronic
  structure
• History
• Feder, Ackermann, Tamura, Scheunemann, Halilov,
  Fominykh, JH
• Object oriented approach
• Objects comprise crystals, layers, atoms,
  elements, potentials, wavefunctions, beam sets,
  matrices, super matrices, vectors,
• C (1993, complete rewrite from Fortran77)
• Spin-polarized relativistic (Dirac equation)
• Scaling of spin-orbit splitting fully
  relativistic ! scalar-relativistic case (Ebert,
  Tamura)

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Omni2k Computer program for spectroscopies II

• Magnetization direction arbitrary for each site
  (local global frame)
• Full-potential within the muffin-tin spheres
• Disorder
• Averaged t-matrix approximation (ATA)
• Coherent potential approximation (CPA)
• Systems
• Bulk, surfaces, interfaces, tunnel junctions,
  defects

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Self-interaction Correction
Spin-density functional Exchange-correlation
functional Cancellation SIC
functional Additional SIC potential Localizatio
n criterion
For the true ground-state density
For the LSDA ground-state density Self-interaction
? state