Magnetic skyrmions are particle-like topologically distinct magnetic 'twists' with a defined chirality. The existence of a non-trivial topology increases the stability of skyrmions, which is attractive for low-power unconventional computing and memory devices. We study skyrmions in magnetron-sputtered perpendicularly magnetized thin-film heterostructures. One of our aims is to control magnetic skyrmions by small voltage pulses. We have proposed concepts for voltage-driven gyrotropic skyrmion motion  and demonstrated the creation and annihilation of skyrmions by small voltages in Ta (4 nm)/GdOx (6 nm)/Gd (0.1 nm)/Co (1 nm)/Pt (4 nm) heterostructure . Currently, we are investigating voltage control of skyrmions using reversible lithium-ion migration in magneto-ionic supercapacitors.
We also investigate the temperature dependence of magnetic parameters that stabilize skyrmions and the motion of skyrmions under thermal activation. By combining bulk magnetometry and X-ray magnetic circular dichroism photoemission electron microscopy, we extracted the temperature dependence of the Dzyaloshinskii-Moriya interaction (DMI) in Ta (2 nm)/Pt (4 nm)/CoFeB (0.8 nm)/Ru (0.2 nm)/Pt (1 nm)/CoFeB (0.8 nm)/Ru (0.2 nm)/Pt (2 nm) heterostructures . Moreover, we found that the strength of DMI scales linearly with the uniaxial magnetic anisotropy and the exchange stiffness over a broad temperature range, while its variation with the saturation magnetization is approximately quadratic. We studied the thermal motion of skyrmions using magneto-optical Kerr effect microscopy and temperature-dependent micromagnetic simulations. The interaction of skyrmions with grain boundaries, which are omnipresent in magnetron-sputtered thin films, was investigated in detail . We also demonstrated the formation of a weakly pinned skyrmion liquid in a magnetic heterostructure . In this study, we inserted a Ru wedge layer at the ferromagnet/heavy metal interface to evaluate the dependence of skyrmion dynamics on the skyrmion size and density. Our experiments demonstrate that the diffusion of skyrmions is largest in dense liquids with small skyrmions.
1. Y. Zhou, R. Mansell, S. van Dijken. Driven gyrotropic skyrmion motion through steps in magnetic anisotropy. Scientific Reports 9, 6525 (2019).
2. Y. Zhou, R. Mansell, S. Valencia, F. Kronast, S. van Dijken. Temperature dependence of the Dzyaloshinskii-Moriya interaction in ultrathin films. Physical Review B 101, 054433 (2020).
3. Y. Zhou, R. Mansell, S. van Dijken. Voltage control of skyrmions: Creation, annihilation, and zero-magnetic field stabilization. Applied Physics Letters 118, 172409 (2021).
4. Y. Zhou, R. Mansell, T. Ala-Nissila, S. van Dijken. Thermal motion of skyrmion arrays in granular films. Physical Review B 104, 144417 (2021).
5. R. Mansell, Y. Zhou, K. Kohvakka, S.-C. Ying , K.R. Elder, E. Granato, T. Ala-Nissila, S. van Dijken. Weakly pinned skyrmion liquid in a magnetic heterostructure. Physical Review B 106, 054413 (2022).