Interaction of surface acoustic waves and magnetic thin films

Surface acoustic waves (SAWs) have emerged as innovative and energy-efficient means to manipulate domain walls (DWs) and skyrmions in thin films with perpendicular magnetic anisotropy (PMA) owing to the magnetoelastic coupling effect. This thesis focuses on the complex interplay between SAWs and magnetic thin films with PMA through both experimental and micromagnetic studies. The effects of the standing SAWs on the magnetisation dynamics in a Ta(5.0 nm)/Pt(2.5 nm)/Co(1.1 nm)/Ir(1.5 nm)/Ta(5.0 nm) thin film with PMA were first investigated. SAWs with frequency of 93.35 MHz significantly reduced the coercivity of the thin film by 21% and enhanced the magnetisation reversal speed by 11-fold. Standing SAWs introduce a dynamic energy landscape with a unique spatial distribution, forming striped domain patterns in the thin film. The use of radio-frequency signals for generating SAWs inevitably causes a heating effect in the device. This heating effect was carefully examined in situ in a SAW device featuring a Ta(5.0 nm)/Pt(2.5 nm)/Co(0.9 nm)/Ta(5.0 nm) thin film with PMA using an on-chip platinum thermometer. It was shown that the temperature increased by 10 K in the presence of SAWs at centre freqneucy of 48 MHz. The DW velocity was significantly enhanced in the presence of the standing SAWs by a factor of 4 compared to that with temperature change alone owing to the magnetoelastic coupling effect. To understand the SAW-enhanced DW motion, comprehensive micromagnetic simulations were performed on thin films in the presence of travelling SAWs with frequencies from 50 to 200 MHz. The findings highlighted that SAW-induced vertical Bloch lines within DWs can simultaneously boost DW depinning and dissipate energy at the DW via spin rotation. The SAW-induced strain gradient can be exploited to control skyrmion motion in a current-free manner. Micromagnetic simulations revealed that the use of orthogonal SAWs, combining horizontal travelling and vertical standing waves, offered a promising approach to direct skyrmion motion along desired trajectories, avoiding undesirable Hall-like motion. In conclusion, this thesis offers a significant contribution to the understanding of how SAWs influence magnetisation dynamics.