Random thermal motion and voltage control of magnetic skyrmions

Defense is held on 22.9.2021 12:00 – 16:00 (Zoom), https://aalto.zoom.us/j/4249624310Digital data permeates all aspects of daily life, and magnetic materials provide the dominant medium of the digital data storage. However, current magnetic storage devices suffer from high energy consumption. In order to improve the energy efficiency, magnetic skyrmions are being intensively investigated as a promising candidate for future magnetic devices. Magnetic skyrmions show advantage of being stable at room temperature with a small size and high mobility. Previous research on skyrmions has focused on devices driven by electric currents, which still require large energy consumption per bit due to the Joule heating. In this thesis, two alternatives to manipulate skyrmions in thin magnetic films are explored: by thermal fields and by applying voltages. In both cases, the heating by electric currents is mitigated. The results of this thesis, based on magnetic thin films hosting skyrmions, cover four aspects: the methodology of extracting magnetic parameters at different temperatures, the random thermal motion of skyrmions in granular films, the voltage control of the creation and annihilation of skyrmions, and skyrmion dynamics guided by anisotropy steps. In the first topic, the correlations between magnetic parameters that determine the behavior of skyrmions are investigated. By performing experiments as a function of temperature and utilizing different models, linear correlations between the strength of the Dzyaloshinskii-Moriya interaction, the perpendicular magnetic anisotropy, and the exchange stiffness are demonstrated. Combined with a model of skyrmion stability, the results imply the skyrmions should be thermally stable over a wide range of temperature. Secondly, using micromagnetic simulations, it is shown that interactions between skyrmions in dense skyrmion arrays increase the skyrmion diffusion coefficient in thin films without grains. Meanwhile, grains in thin magnetic films, which are omnipresent in experiments, reduce the skyrmion diffusion coefficient most when the grain size and skyrmion diameter are similar. For larger grains, random thermal motion leads to the clustering of skyrmions in grains with lower magnetic anisotropy. Thirdly, voltage control of the creation and annihilation of skyrmions in a GdOx/Gd/Co/Pt heterostructure is demonstrated. Micromagnetic simulations are used to explore how changes in the magnetic anisotropy and Dzyaloshinskii-Moriya interaction affect the skyrmion density. The voltage effects are shown to occur on a few-second time scale, which suggests voltage-induced changes in the orbital filling at the Co/GdOx interface as the most likely origin. Finally, by exploiting voltage control of magnetic anisotropy, the motion of skyrmions at magnetic anisotropy steps is simulated. Based on these results, a voltage-controlled device utilizing the gyrotropic motion of skyrmions is proposed