Ferrimagnetic amorphous rare-earth-transition-metal alloy thin films for spintronics: a perspective

Rare-earth:transition metal (RE:TM) ferrimagnetic alloy thin-films have complex, compositionally tunable magnetization behaviour that offers significant benefits as a magnetic layer for nanomagnetic and spintronic applications. These properties include ferrimagnetic compensation effects that enable compositional and temperature dependent control of the net magnetization and angular momentum for applications. Importantly, under suitable conditions these thin-films host perpendicular magnetic anisotropy (PMA), which is commonly required for spintronic applications. However, in contrast to the interface-driven PMA in ultrathin-film ferromagnetic systems, PMA in RE:TM alloys is a bulk effect in these amorphous thin-films. This perspective summarizes keys observations of magnetic behaviour in the RE:TM alloys, highlighting the sensitivity of magnetic compensation points to composition, temperature, and thickness, the effects of natural and controlled compositional variations and discusses details of the origins of PMA in these systems. The aim being to provide insights that underpin and guide the selection and development of RE:TM materials for future applications. There is a focus on Tb- and Gd-based alloys, as these are most relevant for spintronics, where analysis indicates different sources of anisotropy are dominant in these systems that also affects the composition dependence. Natural compositional and structural variations within nominally uniform thin-films have been observed and these may be an issue for future device applications. In contrast, controlled compositional variations through the film thickness can be introduced by design, enabling variations from in-plane to out-of-plane magnetization and Dzyaloshinskii–Moriya interactions through a film that enable field-free spin–orbit-torque switching.