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Spin transport at the atomic scale Alexandre Reily Rocha and Stefano Sanvito Computational Spintronics Group Physics Department, Trinity College Dublin,

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Presentation on theme: "Spin transport at the atomic scale Alexandre Reily Rocha and Stefano Sanvito Computational Spintronics Group Physics Department, Trinity College Dublin,"— Presentation transcript:

1 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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3 u Self-assemble not growth/lithography u Smaller size u New ideas and new functionalities Magnetoresistance: Present vs. Atomic Scale Technology GMR : Baibich et. al. PRL (1988). New paradigm

4 I AF tot = I up + I down = + The prototype Spin Valve

5 I FM tot = I up + I down = + I FM tot > I AF tot

6 Why going to small objects?

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8 The Model System VRVR VLVL Scattering Region Right Lead Left Lead  L =  F +V/2  R =  F -V/2 H s [n] f(  L )f(  R )

9 Non-equilibrium Green’s Function Density Matrix Current Scatterer’s Green function

10 Non-equilibrium Green’s Function in Density Functional Theory  Localized multiple-  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) Prescription for calculating the charge density SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms) L. Sham and W. Kohn, Phys. Rev., 140, A1133 (1965)

11 In principle we solve this: But in practice …. LL RR V V LL RR

12 The Leads’ Selfenergies Total Hamiltonian Green Function Analytic form from Green equation and continuity Surface GF Use of boundary conditions

13 Magnetic Materials and localized orbitals d orbitals fairly localized: no coupling between unit cells H 1 is close to singular Unitary Transformation A  Highlights uncoupled states in H 1  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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15 Benzene dithiol attached to nickel Nickel [100] Surface z Two possible magnetic configurations

16 Transmission ( V= 0 Volts) Ferromagnetic Anti-ferromagnetic AFF F

17 I-V Characteristics

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

19 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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