FellowMichal Mruczkiewicz
Project NameWaves in Exotic Spin Textures
Host organisationInstitute of Electrical Engineering
Duration of the project08.01.2016 - 31.12.2018

Abstract
The research project is focused on the theoretical investigation of collective excitations in various magnetic architectures and magnetic field configurations (spin textures). In particular, the dynamical properties of magnetic vortices and skyrmions will be studied in disks, nontrivial geometries and ultrathin films. Possible mechanisms of excitations (e.g., current or external field) will be considered and the possibility of experimental verification of numerically studied structures will be evaluated. An important part of the work will be dedicated to the study of collective skyrmion modes, a topic of research that is completely unexplored yet. On the basis of the obtained results a logic device will be proposed.

Project Summary with Interim Results

The project is focused on the theoretical investigation of magnetization dynamics in nano-objects, such as ultrathin cobalt disks. The motivation is to understand the excitation of spin waves in media with nontrivial magnetic configurations, (e.g., topological solitons: vortices, skyrmions, see Fig. 1) and to use that knowledge to effectively control them. Proposition for implementation of the investigated effects for logic device is a final aim of the project. The following activities were undertaken during the first year of the project realization towards the ultimate goal.

Fig. 1 a) Vortex, b) Block-like skyrmion and c) Neel-like skyrmion magnetic states.

Fig. 1 a) Vortex, b) Block-like skyrmion and c) Néel-like skyrmion magnetic states./Obr. 1 a) magnetický vír, b) Bloch-ov a c) Néel-ov skyrmión.

A model of spin wave propagation in complex geometries with Dzyaloshinskii-Moriya interaction was implemented in frequency domain. This allowed to perform numerical analysis, characterize spin wave properties in such materials and to propose an alternative method for experimental estimation of Dzyaloshinskii-Moriya interaction strength. The method uses the effect of ferromagnetic resonance and the presence of asymmetry in spin waves dispersion in patterned structures.1


Fig. 2 A wave-like character of magnetization dynamics in array of Co nanodisks. The picture shows a snapshot at time t1 of magnetization difference (z-component) between time t1 and t0./Obr. 2 Vlnový charakter dynamiky magnetizácie v poľi kobaltových nanodiskov. Obrázok zobrazuje rozdiel magnetizácie (z-zložku) zaznamenanú v čase t1 a t2.

A theoretical study on nontrivial magnetic configuration that can arise in CoFeB alloy nanodisk was undertaken. Dynamics of found states, e.g, vortex, Bloch-like and Neel-like skyrmions were investigated (see Fig. 1 a, b and c respectively for the picture of this static configurations) was investigated in details. It has been shown that classification on low frequency soliton translation modes and spin wave excitations over the soliton background can be made for skyrmions (analogically to classification already made and widely employed for vortex dynamics excitations).2

Next, a collective dynamics of skyrmions has been investigated in array of cobalt nanodisks. A possibility of wave-like skyrmion excitation was demonstrated, see Fig. 2. The determined dispersion relation of coupled skyrmions shows a periodic dependence on the wave vector, a characteristic feature of the band structure in magnonic crystals. With analysis of the spatial distribution of magnetization amplitude in the magnonic bands, identification of the excited modes as breathing and clockwise gyrotropic dynamic skyrmions was possible.3