BCMaterials Fortnightly Seminar #59: Konstantin Guslienko

KONSTANTIN GUSLIENKO

(IKERBASQUE RESEARCH PROFESSOR - DEPT. OF MATERIALS PHYSICS UPV/EHU)

Magnetic Soliton Dynamics in Patterned Nanostructures: from Vortices to Skyrmions

Fundamental understanding of the spin dynamics in patterned magnetic structures is essential in the future advancements of nanomagnetism and spintronics. For mesoscopic and nanoscale particle (dot) sizes, non-uniform magnetization distributions with zero remanence (vortex states) are often observed at equilibrium. The magnetic vortex is the simplest topologically non-trivial ground state in submicron and micron-size ferromagnetic dots. Recently, new inhomogeneous magnetization nanoscale objects - magnetic skyrmions - vortex-like curling spin textures with a quantized topological number have been observed in chiral magnets possessing Dzyaloshinskii-Moriya exchange interaction. Skyrmions attracted considerable attention of researchers assuming potential applications in spintronic devices because their motion can be controlled with ultralow current density. Magnetic soliton (vortex/skyrmion) related phenomena offer insight into spin dynamics on a fundamental level, and also govern magnetization reversal at the nanoscale. In this talk, I present a review of calculations and measurements of spin excitations in the vortex ground state of soft magnetic dots. Nontrivial spin dynamic properties, such as magnetic vortex core (VC) reversal driven by oscillating magnetic fields or spin polarized currents, were observed. The dynamical origin of the VC reversal will be considered introducing a “gyrotropic” field, which is proportional to the VC velocity. The critical velocity of the VC motion (about 300 m/s for Permalloy dots) serves as a universal criterion of the VC switching. The vortex core reversal criterion is a crucial factor determining the optimal parameters of external driving forces required for reliable manipulation of the VC switching in the future devices. The interaction of high frequency azimuthal spin waves with the moving VC and their influence on VC motion will be also discussed. There is a giant frequency splitting of the spin waves having non-zero overlapping with the vortex gyrotropic mode as well as a finite vortex mass of dynamical origin. The generalized Thiele approach to the non-uniform magnetization dynamics is applied to the problem of the magnetic vortex excitations by the spin polarized current due to spin angular moment transfer effect. We calculated the main physical parameters of the spin polarized current induced magnetic vortex oscillations in layered nanopillars ferromagnet/spacer/ferromagnet, such as the range of dc current density, where the vortex steady oscillation state exists, and the oscillation frequency. The collective vortex state excitations in the dot pairs, linear chains and regular two-dimensional arrays of magnetostatically coupled dots are calculated via the multipole decomposition of the interdot interaction energy. It is shown that the excitation bandwidth can reach 30% of the gyrotropic frequency of isolated dot and the group velocity can be up to 500 m/s for the dot chains with alternative core polarizations and dense hexagonal two-dimensional vortex dot arrays.