Spin-torque driven magnetization dynamics: Micromagnetic modeling

Abstract

In this paper we present an overview of recent progress made in the understanding of the spintorque induced magnetization dynamics in nanodevices using mesoscopic micromagnetic simulations. We first specify how a spin-torque term may be added to the usual LandauLifshitz-Gilbert equation of magnetization motion and detail its physical meaning. After a brief description of spin-torque driven dynamics in the macrospin approximation, we discuss the validity of this approximation for various experimentally relevant geometries. Next, we perform a detailed comparison between accurate experimental data obtained from nanopillar devices and corresponding numerical modelling. We show that, on the one hand, many qualitatively important features of the observed magnetization dynamics (e.g., non-linear frequency shift and frequency jumps with increasing current) can be satisfactory explained by sophisticated micromagnetic models, but on the other hand, understanding of these experiments is still far from being complete. We proceed with the numerical analysis of pointcontact experiments, where an even more complicated magnetization dynamics is observed. Simulations reveal that such a rich behaviour is due to the formation of several strongly nonlinear oscillation modes. In the last part of the paper we emphasize the importance of sample characterization and conclude with some important remarks concerning the relation between micromagnetic modelling and real experiments.

Propagation and control of nanoscale magnetic-droplet solitons

“…Spin torque forms the basis for tunable microwave nano-oscillators in confined nanopillar structures 3 and nanocontacts abutting an extended ferromagnet 4 . Most nanopillar dynamics can be reasonably described by single-domain modeling 5 with the notable exception of gyrotropic vortex motion 6 . In contrast, nanocontacts enable the excitation of radiating [7][8][9] and localized, coherently precessing, nonlinear wave states 8 .…”

“…In contrast, nanocontacts enable the excitation of radiating [7][8][9] and localized, coherently precessing, nonlinear wave states 8 . The analysis of solitonic waves in nanocontact systems has predominantly been limited to either the weakly nonlinear regime at threshold 10 or complex micromagnetic simulations 5,11 . We recently proposed that a spin torque driven nanocontact could act as a soliton creator in a uniaxial ferromagnet with sufficiently strong perpendiular anistropy 12 .…”

Multiscale approach to spin transport in magnetic multilayers

“…On the other hand it is becoming increasingly clear that the magnetization dynamics of real devices cannot be properly captured by simple (analytically tractable) models and that numerical simulations that treat the magnetic and transport degrees of freedom on an equal footing need to be developed. Several steps have already been taken in that direction [34][35][36][37][38] and should eventually lead to simulations of good predictive capabilities. Let us mention, as example, the recent experiments on spin torque induced ferromagnetic resonance in the nonlinear regime 39 which has been successfully compared to coupled simulations at the macrospin level for the magnetic part and onedimensional semi-classical level for spin transport.…”

Influence of material defects on current-driven vortex domain wall mobility

“…Since the introduction of this nonadiabatic term, there has been a lot of debate on the magnitude of β, with theoretically predicted values ranging from β ≈ α [4][5][6] over β = 2α [7] to β = 4α [8]. Additionally, experiments have been until now unable to converge to one value.…”