Spin transport at the atomic scale

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Spin transport at the atomic scale
Alexandre Reily Rocha and Stefano Sanvito
Computational Spintronics Group Physics
Department, Trinity College Dublin, IRELAND
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Why nanoscale spintronics?
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Magnetoresistance Present vs. Atomic Scale
Technology
GMR Baibich et. al. PRL (1988).
New paradigm

• Self-assemble not growth/lithography

• Smaller size

• New ideas and new functionalities

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The prototype Spin Valve


I AFtot Iup Idown
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I FMtot gt I AFtot
B


I FMtot Iup Idown
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Why going to small objects?
I0
I
I
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Computational Technique
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The Model System
VR
VL
f(mL)
f(mR)
Hsn
Left Lead
Right Lead
Scattering Region
mLeFV/2
mReF -V/2
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Non-equilibrium Greens Function
Scatterers Green function
Density Matrix
Current
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Non-equilibrium Greens Function in Density
Functional Theory
Prescription for calculating the charge density
L. Sham and W. Kohn, Phys. Rev., 140, A1133
(1965)
SIESTA (Spanish Initiative for Electronic
Simulations with Thousands of Atoms)

• Localized multiple-z Pseudo-atomic orbitals
• Non-Orthogonal basis set
• Super-cells with up to 100 atoms

D. Sánchez-Portal, et. al., Int. J. Quant. Chem.
65, 453 (1997)
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In principle we solve this
?L
?R
V
But in practice .
?R
?L
V
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The Leads Selfenergies
Total Hamiltonian
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Magnetic Materials and localized orbitals
d orbitals fairly localized no coupling between
unit cells
H1 is close to singular
Unitary Transformation A

• Highlights uncoupled states in H1
• States are eliminated and couplings are
  renormalized
• Numerically Stable

We can solve for very complicated leads and the
size of the system might be drastically
downfolded!!!
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Results
Nickel Molecular Junctions
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Benzene dithiol attached to nickel
Two possible magnetic configurations
z
Nickel 100 Surface
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Transmission ( V 0 Volts)
AF
F
F
Ferromagnetic
Anti-ferromagnetic
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I-V Characteristics
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Conclusions

• Material dependent transport properties of
  magnetic materials can be probed with a powerful
  combination of Non-Equilibrium Greens functions
  and Density Functional Theory.
• I-Vs present a non-trivial behaviour.
• When d orbitals are present directional bonding
  becomes very important.
• Different orientations of the leads, position of
  S atom

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Aknowledgements

• Prof. J. Ferrer (Oviedo)
• Mr. Victor M. Garcia Suarez Oviedo)
• Prof. C.J. Lambert (Lancaster)
• Dr. Steve Bailey (Lancaster)

• Science Foundation of Ireland
• Enterprise Ireland

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