Coercivity control of thin films with perpendicular magnetic anisotropy by surface acoustic waves

Abstract

Encoding information in thin films with perpendicular magnetic anisotropy (PMA) holds promise for the next generation of data storage and logic operation applications [1]. Thin films with a large coercivity provide high stability of the encoded information but cause a high energy cost of writing data [2]. Therefore, reducing coercivity while writing data is a practical method to decrease the energy cost. Here, we experimentally demonstrated the control of coercivity in a PMA film using surface acoustic waves (SAWs), which can introduce oscillating strain waves in the thin films over millimetric distances. Two interdigitated transducers (IDTs, Fig.1a), whose centre frequency is 93.35 MHz (Fig. 1b), were patterned on opposite ends of a 2-mm-wide stripe of Ta(5.0 nm)/Pt(2.5 nm)/Co(1.1 nm)/Ir(1.5 nm)/Ta(5.0 nm) film. The film was dc magnetron sputtered onto a lithium niobate (LiNbO3) substrate, which supports the SAW propagation. A wide-field Kerr microscope was used to measure the hysteresis loops of the film with SAW both on and off at room temperature. Results showed that the coercivity of the thin films without SAW is 4.93±0.06 mT (Fig. 2a). Owing to the magnetoelastic coupling effect [3,4], the standing SAWs locally reduce the anisotropy and thus the thin film coercivity, which can be controlled by the applied SAW power. Coercivity reduction, which is enhanced by increasing the SAW power, can be up to about 18% for an input SAW power of 28 dBm (4.04±0.09 mT, Fig. 2b). Coercivity reduction only occurs at the centre frequency (Fig. 2c), which rules out the effect of a rising temperature that a SAW potentially can cause. This study indicates the possibility of remote and energy-efficient control of magnetization switching.  Fig.1 (a) Diagram of experimental setting-up. Interdigitated transducers (IDTs) were patterned by the side of the Ta(5.0)/Pt(2.5)/Co(1.1)/Ir(1.5)/Ta(5.0) film (thickness in nm). Hysteresis loops were taken by a Kerr microscope. (b) S-parameters of the IDTs (centre frequency of 93.35 MHz).  Fig. 2 (a) Hysteresis loops and (b) coercivity of the thin film under various surface acoustic waves (SAW) power (23 to 28 dBm) at 93.35 MHz. (c) Coercivity of the thin films under various frequency at 28 dBm.