Tailoring Magnetic and Spin–Orbit Torque Properties in CoTb Ferrimagnets: From Alloys to Multilayers

Ferrimagnetic (FiM) CoTb systems are promising for energy-efficient spintronics devices due to their strong perpendicular magnetic anisotropy and sensitivity to spin currents. This work presents a comparative study of two morphologies: an amorphous Co60Tb40 alloy and a [Co(0.4 nm)/Tb(0.27 nm)]×6 multilayer, both integrated into W/FiM/Ru heterostructures. While both systems exhibit stable perpendicular anisotropy, the alloy demonstrates superior spin–orbit torque (SOT) efficiency, generating a significantly larger effective field (Beff ≈ 10–12 mT) compared to the multilayer (Beff ≈ 2–3 mT). This leads to a lower critical switching current (Ic ≈ 35 vs. 39 mA) in the alloy, attributed to more efficient spin current absorption and stronger spin–orbit coupling in the homogeneous amorphous matrix. In contrast, the multilayer shows higher saturation magnetization, lower coercivity, and significantly enhanced thermal stability with fully reversible magnetic properties after heating cycles. The results indicate that the amorphous CoTb alloy is preferable for high-efficiency SOT memory applications, whereas the multilayer [Co/Tb]×N structure serves as a robust and tunable alternative, especially when stoichiometric alloy fabrication is challenging. This study provides clear guidelines for material selection in ferrimagnet-based spintronics, balancing switching efficiency against thermal resilience and integrability.