Fundamental magnetism and future technology: Mapping proximity magnetism in W and Pt layered with Rare-Earth/Transition-Metal ferrimagnets through the compensation temperature

Thin-film ferrimagnetic rare earth-transition metals (RE:TM) alloys have been widely studied due to their fundamental properties and exploited across a range of applications over several decades. Interestingly, an accurate fundamental description of the magnetism is not yet developed [1], and the physical basis for the exchange coupling between the RE and TM components is not well understood, although the seminal paper of Campbell [2] nearly 50 years ago proposed a mechanism, building from the theory of impurities in TM ferromagnets, for indirect exchange and highlighted experimental observations of doping and alloying that would evidence this mechanism. More recently, renewed research interest has emerged for future technological applications in spintronics due to the observation of bulk perpendicular magnetic anisotropy and the response of the magnetisation to magnetic field and spin-orbit torques (SOT) around the ferrimagnetic compensation point [3-6]. For SOT spin-Hall driven switching, the RE:TM ferrimagnets have been layered with a heavy metal (HM), typically platinum [e.g. 3, 4], which provides the required the additional spin-orbit interactions into the energy manifold. The propagation of spin-current from a heavy metal into a ferromagnet, that enables a spin-orbit torque (SOT), depends critically upon the details of the FM/NM interface. There is on-going debate about the role of interfacial proximity-induced magnetism (PIM) in the heavy metal in such spin transport physics, both with respect to metallic ferromagnetic/non-magnetic FM/NM systems [7, 8] and in insulating oxide ferrimagnets layered with heavy metals (FiM/NM) [9-12].