SpinDependent Tunneling by First Principles

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Spin-Dependent Tunneling by First Principles

• Jürgen Henk

Theory Department Max-Planck-Institut für
Mikrostrukturphysik Halle (Saale)
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Motivation
Aim Electron spin electronics
spintronics Devices Spin valves, magnetic tunnel
transistors, MRAMs,

• Outline
• Model systems Planar tunnel junctions
• Recent results
• Fe/MgO/Fe Influence of the interface structure
• Co/vacuum/Co Zero-bias anomaly

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Planar Tunnel Junctions
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Theoretical Approaches

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

Spin polarization in the leads
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Calculational Implementation

• First-principles calculations
• Density-functional theory, local spin-density
  approximation
• Multiple scattering Spin-polarized
  scalar-relativistic KKR (A. Ernst)
• Self-consistent electronic magnetic structure

• Tunneling calculations
• Landauer-Büttiker approach for ballistic
  tunneling
• Multiple scattering MacLaren-Butler scheme
• Spin-polarized relativistic layer-KKR omni2k
  program package for electron spectroscopies (JH)
• Brillouin-zone sampling by adaptive mesh
  refinement (JH)

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Fe/MgO/Fe Interface Geometry
Bulk Fe bcc
a 2.81 Å
Bulk MgO fcc, NaCl
a 4.05 Å
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Fe/MgO/Fe Interface Geometry
X-ray diffraction H. Meyerheim et al (Expt. Dept.
I)
Fe
FeO interface layer
New Formation of an FeO interface layer
Confirmed by total-energy calculations (A.
Ernst) Gain of 0.73 eV/atom
O
Mg
Effect of the geometry on the magnetic structure?
Fe
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Fe/MgO/Fe Charge Magnetic Profiles
Charge distribution
2 MgO layers P alignment
Magnetic moments
Standard model (Butler et al.)
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Fe/MgO/Fe - Transmission
Transmission wavevector-resolved
conductance Additional scattering at the FeO
layer ?Change of the transmission
FeO interface layer
Standard model
P alignment
Fe/(MgO)4/Fe
Fe/FeO/(MgO)2/FeO/Fe
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Fe/MgO/Fe Conductance TMR
Strong influence of the interface for thin
spacers (2-4 ML MgO)
Work in progress
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Fe/MgO/Fe Summary

• Interface structure ideal vs. FeO layer
• Reduction of transmission conductance
• Change of the TMR in the pre-asymptotic regime

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Co/Vacuum/Co Zero-Bias Anomaly
Fe/Al2O3/FeCo
Maximum TMR
S. Yuasa et al, EPL 52 (2000) 344
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Co/Vacuum/Co Theoretical Description
Problem Bias voltage on a first-principles level
Results for jellium (N.D. Lang)
Linear drop
Lead potential constant
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Co/Vacuum/Co Theoretical Results
TMR constant
Theory confirms the experiment for large distances
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Co/Vacuum/Co - Summary

• Absence of the zero-bias anomaly in vacuum
  tunneling
• Model for the interface barrier (bias voltage)
• Theoretical confirmation explanation of the
  constant TMR

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