Anomalous Current-Induced Spin-Orbit Torques in Ferrimagnets

Spin-orbit torques (SOTs) have been recently studied for its potential applications in memory and logic devices, mostly with ferromagnetic thin films. As the cell size shrinks, these ferromagnetic thin films are thermally unstable due to their sub nano-meter thicknesses. We study SOT-induced magnetization switching and effective fields in rare earth-transition metal (RE-TM) ferrimagnet alloy films and multilayers. We show that devices from the structure of heavy metal/ferrimagnet present a huge switching efficiency and maximum spin-orbit effective fields. It is observed that the switching efficiency and SOT effective fields increase by one order of magnitude as the ferrimagnet approaches compensation. This anomalous increase of the switching efficiency and SOT effective fields is attributed to the negative exchange interaction field due to the antiferromagnetic ordering between the Co and Gd sub-lattices. The negative exchange interaction makes the ferrimagnet thermally stable near compensation by increasing its anisotropy and also provides the exchange coupling torque that assists in switching, effectively increasing the overall switching efficiency of ferrimagnetic SOT devices. We also show that the SOT in the Co/Tb ferrimagnet system increases with the ferrimagnet thickness. The SOT effective fields and the switching efficiency in Co/Tb are about one order magnitude larger than that of ferromagnets. Our results trigger a ferrimagnet as a core building block in spintronics.