Micromagnetic study of skyrmion stability in confined magnetic structures with perpendicular anisotropy

Abstract

Skyrmions are emerging topological spin structures that are potentially revolutionary for future data storage and spintronics applications. The existence and stability of skyrmions in magnetic materials is usually associated to the presence of the Dzyaloshinskii-Moriya interaction (DMI) in bulk magnets or in magnetic thin films lacking inversion symmetry. While some methods have already been proposed to generate isolated skyrmions in thin films with DMI, a thorough study of the conditions under which the skyrmions will remain stable in order to be manipulated in an integrated spintronic device are still an open problem. The stability of such structures is believed to be a result of ideal combinations of perpendicular magnetic anisotropy (PMA), DMI and the interplay between geometry and magnetostatics. In the present work we show some micromagnetic results supporting previous experimental observations of magnetic skyrmions in spin-valve stacks with a wide range of DMI values. Using micromagnetic simulations of cobalt-based disks, we obtain the magnetic ground state configuration for several values of PMA, DMI and geometric parameters. Skyrmion numbers, corresponding to the topological charge, are calculated in all cases and confirm the occurrence of isolated, stable, axially symmetric skyrmions for several combinations of DMI and anisotropy constant. The stability of the skyrmions in disks is then investigated under magnetic field and spin-polarized current, in finite temperature, highlighting the limits of applicability of these spin textures in spintronic devices.

Skyrmions in coupled spin torque nano-oscillator structures

“…These are dependent only on K z and D int . This case contrasts with the case where the polarizer has magnetization m p = (0,0,1) [7,18].…”

“…Several works have shown that a skyrmion can be stabilized in geometries as, for example, disks [3,7,8], ultrathin films [9] and racetracks [10,11]. However, it is also possible to create a multiskyrmion cluster in a single nanodisk [12,13].…”

“…This leads to the question of magnetic interactions between them. There are in the literature several works about the creation and stabilization of skyrmions in isolated structures [3,7,8,18,23,24,27], but work is still lacking on the role of magnetic interaction in the process of creation of a skyrmion. The aim of this work is to study the effect of the magnetic coupling in the creation of a Néel skyrmion in a spin torque nano-oscillator system.…”

“…The material used is Cobalt with parameters [3,9,18]: saturation magnetization M s = 5.8 × 10 5 A/m, exchange stiffness A = 1.5 × 10 −11 J/m, and damping constant α = 0.3 (for faster convergence). The perpendicular uniaxial anisotropy constant (K z ) and Dzyaloshinskii-Moriya exchange constant (D int ) varied from 0.4 to 1.8 MJ/m 3 and from 3 to 4.5 mJ/m 2 , respectively [3,7,27].…”

Skyrmion dynamics in magnetic thin films

“…Their stability grants them the ability to store data in a non-volatile manner. 13,[84][85][86][87][88][89][90][91][92] 95 The mechanism relies on the formation of differential current flows resulting from the anisotropic magnetoresistance.…”