Theories and modeling

Context

At the moment, there is no theoretician permanent staff member at SPINTEC. However, besides the micromagnetic simulations, SPINTEC is carrying out theoretical studies in the field of spin-dependent transport phenomena and exchange bias in collaboration with other laboratories. In particular, a long lasting collaboration exists with Pr A.Vedyaev’s team at Lomonosov University in Moscow. These studies are closely connected to the on-going experimental activity. Some examples of recent theoretical studies are described below.

On-going studies at SPINTEC

Magnetic atomistic simulation of exchange bias and interlayer coupling between two ferromagnetic layers through an antiferromagnetic spacer layer : The purpose was to understand the experimental observation of 90° magnetic coupling between NiFe and Co layers in sandwiches of the form NiFe3nm/NiO 6nm/Co 2nm. Numerical simulations were performed using an Heisenberg model of interacting spins to understand the magnetic behavior of the system at the atomic scale (Coll. F.Lançon, L.Billard CEA/DRFMC/SP2M/Lsim). These simulations are particularly useful to obtain information on the spin configurations in the antiferromagnetic layer since no experimental technique allows to visualize them with the necessary resolution. An examples of such spin configurations are shown in Fig.1. These simulations are now pursued to understand the effect of reduced lateral dimensions on exchange bias in magnetic nanostructures.

Orange Peel coupling mechanism in multiayers with out-of plane anisotropy : In parallel to our experimental studies involving (Pt/Co) type of multilayers with perpendicular anisotropy, we have modeled the magnetostatic interaction between successive layers in presence of a correlated roughness (Orange Peel mechanism). While it was known that the Orange Peel coupling always favors parallel alignment in multilayers with in plane magnetization, we discovered that it can favor either parallel or antiparallel alignment with out-of-plane anisotropy depending on the relative amplitude of the exchange stiffness constant and interfacial magnetic anisotropy (Fig.2).

Giant magnetoresistance perpendicular to plane in magnetic multilayers : This theoretical activity is connected to our experimental work on magnetoresistive heads for computer disk drives. The general purpose is to develop a thorough understanding of the giant magnetoresistance phenomena and develop numerical tools to facilitate the optimization of magnetoresistive multilayers. These theories are based either on a semiclassical approach or on Kubo formalism of quantum linear response.

Spin transfer in magnetic multilayers and tunnel junctions : This work aims at the understanding of our experimental results on spin transfer effects both in MRAM and magnetoresistive heads with current perpendicular to plane. Two aspects are addressed :

1. Including Slonczewski’s spin torque term in the Landau Lifshitz Gilbert equation, dynamic micromagnetic simulations are carried out to understand the very peculiar magnetic dynamic behavior induced by this additional term.
2. Based on the Keldish technique, spin transfer effects through low resistance tunnel junctions are studied taking into account inelastic scattering effects.

Tunnel magnetoresistance in magnetic tunnel junctions : Various aspects of tunnel magnetoresistance were investigated using Kubo formalism:
  Role of the thickness of the electrodes on the tunnel magnetoresistance.
  Influence of inserting a non magnetic layer between the magnetic electrode and the barrier.
  Impact of impurites in the barrier on the magnetoresistance amplitude and its dependence with temperature.
  sd model of tunnel magnetoresistance, understanding of the bias dependence.
  Magnetoresistance of systems comprising multiple tunnel barriers, in particular giant tunnel magnetoresistance due to resonnant tunneling in triple barriers system.

Staff Involved

  B. Dieny, J. Moritz, A. Manchon, D. Gusakova, U. Ebels, D. Houssameddine (SPINTEC)
  A. Vedyaev, N. Ryzhanova, N. Strelkov (Lomonosov University, Moscou)
  C. Lacroix (CNRS/Laboratoire Louis Néel)