Defence of doctoral thesis in the field of applied physics, M.Sc. Yifan Zhou

Title of the doctoral thesis is "Random thermal motion and voltage control of magnetic skyrmions"

Digital 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. To improve the energy efficiency, magnetic skyrmions are being intensively investigated as a promising candidate for future magnetic devices. Magnetic skyrmions are stable at room temperature with a small size and high mobility. However, the previous studies on skyrmions have focused on devices driven by electric currents. The current-driven approach still requires 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. 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 random motion in thin films without grains. Meanwhile, grains in thin magnetic films, which are omnipresent in experiments, reduce the skyrmion random motion most when the grain size and skyrmion diameter are similar. Thirdly, voltage control of the creation and annihilation of skyrmions in a GdOx/Gd/Co/Pt heterostructure is demonstrated.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, a voltage-controlled device utilizing the gyrotropic motion of skyrmions is proposed.

The results of this thesis are potentially useful for the design of energy-efficient logic devices and new computing concepts that utilize thermal fields or electric voltages to control the motion of skyrmions.

Opponent is Professor Dafiné Ravelosona, CNRS/University of Paris-Saclay, France

Custos is Professor Sebastiaan van Dijken, Aalto University School of Science, Department of Applied Physics

Contact details of the doctoral student: [email protected], +86 13917158423

The public defence will be organised via Zoom and on Campus. Link to the event

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The dissertation is publicly displayed 10 days before the defence in the publication archive Aaltodoc of Aalto University

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