Atomistic modelling of spintronics

Spintronics research requires theoretical analysis and numerical modelling to understand what happens at the microscopic level. In particular, first principles atomistic modelling of quantum spin transport without using any phenomenological parameter, is desired. Such methods provide predictive power for spin transport characteristics by including atomic, chemical and material details of the nanostructure. The paper[1] by Waldron, Liu and Guo summarizes some recent advances in the development of a formalism and its associated numerical technique for analyzing non-equilibrium quantum spin transport[1,2,3]. The formalism is based on combining real space density

functional theory (DFT) with the Keldysh non-equilibrium Green’s functions (NEGF), for which a spin non-resolved version was reported several years ago[4]. The basic idea of the NEGF-DFT formalism is to calculate device Hamiltonian and electronic structure by DFT, deal with non-equilibrium quantum transport conditions by NEGF and account for open device boundary conditions by real space numerical procedures. Using NEGF-DFT one can now perform parameter-free calculations on nonlinear and non-equilibrium quantum spin transport from atomic first principles. For a specific example of Fe/MgO/Fe trilayer nanostructure well studied at equilibrium[5], we demonstrate[1,3] several important technical issues of analyzing non-equilibrium properties of this magnetic tunnel junction and compare them with corresponding experimental data. We believe that the newly developed NEGF-DFT quantum spin transport package will be useful for many quantitative calculations of spintronic systems[6,7].

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[1] “Ab initio simulation of magnetic tunnel junctions”, Derek Waldron, Lei Liu and Hong Guo, in Molecular and biological devices, a special issue of Nanotechnology 18, 424026 (2007).
[2] “Nonlinear spin-current and magnetoresistance of molecular tunnel junctions", Derek Waldron, Paul Haney, Brian Larade, Allan MacDonald and Hong Guo, Phys. Rev. Lett. 96, 166804 (2006).
[3] “First principles modeling of tunnel magnetoresistance of Fe/MgO/Fe trilayers'', Derek Waldron, Vladimir Timoshevskii, Yibin Hu, Ke Xia and Hong Guo, Phys. Rev. Lett. 97, 226802 (2006).
[4] “Ab initio modeling of quantum transport properties of molecular electronic devices'', Jeremy Taylor, Hong Guo and Jian Wang, Phys. Rev. B 63, 245407 (2001).
[5] W.H. Butler, X.-G. Zhang, T.C. Schulthess and J.M. MacLaren, Phys. Rev. B 63, 054416 (2001); J. Mathon and A. Umerski, Phys. Rev. B 63, 220403(R) (2001).
[6] “Current induced order parameter dynamics: microscopic theory applied to Co/Cu/Co spin valves", P.M. Haney, D. Waldron, R.A. Duine, A.S. Nunez, Hong Guo and A.H. MacDonald, Phys. Rev. B 76, 024404 (2007).
[7] “Ab initio giant magnetoresistance and current-induced torques in Cr/Au/Cr multilayers", P.M. Haney, D. Waldron, R.A. Duine, A.S. Nunez, Hong Guo and A.H. MacDonald, Phys. Rev. B 75, 174428 (2007).