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Equilibrium and Non-equilibrium Spin-orbit Interaction Torques: Mechanism and Temperature Dependence

7, 2015
Title:Equilibrium and non-equilibrium spin-orbit interaction Torques: Mechanism and Temperature Dependence
Reporter:Prof. Oleg N. Mryasov (Western Digital Corporation,USA)
Time: May 14, 2015 (Thursday) 2:00 p.m.
Location: Room 610, CenTral Building

 In this presentation, we consider examples of two types of spin-orbit interaction (SOC) driven effects: (i) Magnetic Anisotropy Energy (MAE) and (ii) spin orbit torque (SOT). The former effect can be described as equilibrium  property which controls stability of magnetization at nano-scale and the latter effect is an example of genuine non-equilibrium  effect which can be instrumental  in  manipulating magnetization with charge currents.  Thus, one of the fundamental challenges is to understand connection between these two effects to be able to identify energy efficient ways to manipulate  nano-magnets with magnetization stable on the long time scales. To address this fundamental challenge we develop theory capable of describing both effects on the equal footing using Green Function formalism in its more general form of Non Equilibrium Green Functions (NEGF) [1]. First,  I will show examples of understanding temperature dependence of MAE  using quantum statistical theory with effective Spin Hamiltonian  theory parameterized with ab-initio electronic structure calculations [2,3,4]  for three classes of materials. (i) M-X where M=(Fe, Co), X= (Pt, Pd), permanent magnet materials   (ii) Mn-Y where Y=(Al, Bi) and   (iii) interfaces Fe/MgO,  Fe/Ta.  Next, I discuss SOT effect driven by SOC arising both due to electric field gradient at interfaces and around impurities (disorder). On the basis of numerical and analytical results we illustrate trends in sign, strength of SOT. We show that impurity leads to a local circular charge and spin currents giving way of interpreting a number of important SOC driven transport effect such as SOT, anomalous Hall and anisotropic magneto-resistance effects。

 [1]. E. M. Lifshitz and L. P. Pitaevskii, Physical Kinetics, Vol. 10, (Pergamon, Oxford, 1981).
 [2]. O. N. Mryasov ,  U. Nowak, K. Guslienko, R.W. Chantrell  EuroPhysics Letters, 69(5), 805 (2005).
 [3]. O.N. Mryasov,  Journal of  Magnetism and Magnetic Materials, v. 272-276, p.800, (2004).  O.N. 
        Mryasov et al., JPCM 3, 7683-7690 (1991); O.N. Mryasov, et al., PRB 45, 12330 (1992).
 [4]. S.V.Faleev, O.N. Mryasov and M. van Schilfgaarde, "Effect of electron correlations on electronic transport across (001) Fe/MgO/Fe junctions", Phys. Rev. B.85, 174433 (2012)
Curriculum Vitae
Prof. Oleg N. Mryasov currently is Senior Technologist at Advanced Technology Operations of Western Digital Corporation (San Jose, CA) and also the Adjunct Professor inUA-Ph. and UMN-ECE. His prior position was an Associate Professor of Physics and Graduate Advisor of Materials Science & Engineering Tri-Campus Program at the University of Alabama, Tuscaloosa (UAT). From 2001-2009, he was Principal Research Engineer at Seagate Technology Research Center. From 1999-2001 he held joint appointment of Research Engineer-III/Technical Staff at UC Berkeley/Sandia National Laboratories (Livermore). He performed his post-doctoral research at Northwestern University (Evanson, IL) in the group of Professor Arthur Freeman (1994-1999).  He received the Ph.D. in Physics and Math from Russian Academy of Sciences in 1993, under the supervision of Prof. Alexander I. Lichtenstein and Prof. V. I. Gubanov (1989-1993). Prof. Mryasov has been awarded by Irish Science Foundation with C. T. Walton Fellowship grant and Technology achievement award from Seagate Technology LLC.
  Prof. Mryasov has pioneering and innovative contributions in the field of spin-orbit driven effects in solids including non-equilibrium torques and magnetic anisotropy energy,  CPP-GMR hetero-structures with high spin polarization Heusler alloys; transparent conductors ; constrained density functional theory (CDFT)  for magnetic excitations and interactions; beyond DFT-LDA theory of magnetic anisotropy energy (MAE); theory of magnetic tunneling junction including  beyond DFT-LDA theory of metal/insulator (Fe/MgO) interface states;  Interlayer Exchange Coupling across tunnel barrier;  charge-orbital ordering  and spin polarized current controlled magnetization states  in  Spin  Valves (SV) ;  multi-scale theory  of  thermal stability,  Heat Assisted Magnetic Recording (HAMR)  and Magnetization Process;  understanding  and utilizing  fundamental magnetic properties in multi-scale simulations. 
  PI: US Defence Research Project Agency, .Semiconductor Advanced Research Network  (STARnet http://www.src.org/program/starnet/) ,  center  C-SPIN: Spin-electronic  Materials and Interfaces (http://cspin.umn.edu/)  ($779,984, 100 %, 01/13-12/17)
  Co-PI: ARPA-E (Advanced research project Agency for Energy Research, US DOE,  http://arpa-e.energy.gov/?q=arpa-e-programs/react ) titled " Rare‐Earth‐Free Permanent Magnets for Electrical Vehicle Motors and Wind Turbine Generators: Hexagonal Symmetry Based Materials Systems Mn‐Bi and M‐type Hexaferrite "   ($1,265,589/3y.,  20 %  share, 04/14 ) 
Contact:School Office of Physics (68913163)
 Inviter:Prof. Hanchun Wu
Release date:2015-10-27