Spin-orbit torque and orbital torque arising from 3d transition metals

Abstract

The Spin-Hall effect (SHE) generates a transverse spin current, which can be transferred to adjacent magnetization, thereby the magnetization dynamics is driven by the spin-orbit torque (SOT). Recent works have revealed the observation of the sizable SOT in a few light metals, which is contradicting to the fundamental of SOT requiring the strong spin-orbit coupling (SOC). This makes infer the existence of orbital Hall torque (OT), which is a result of the conversion of orbital current to spin current by SOC of the ferromagnet. To confirm that expectation, we control the spin-orbit coupling of the system in a various ways. Spin-Hall effect (SHE) generates a transverse spin current, which can be transferred to adjacent magnetization, thereby the magnetization dynamics is driven by the spin-orbit torque (SOT). Recent works have revealed the observation of the sizable SOT in a few light metals, which is contradict to the fundamental of SOT requiring the strong spin-orbit coupling (SOC). This makes infer the existence of orbital Hall torque (OT), which is a result of the conversion of orbital current to spin current by SOC of the ferromagnet. To confirm that expectation, we control the spin-orbit coupling of the system in a various way. First, we studied the gadolinium, a rare-earth material having 4f orbitals with strong SOC. We discovered the oxygens in gadolinium can be easily migrated by the small electric-field by real-time optical measurement and the resistance change. The electric control of oxygen migration can offer the tuning of magnetic properties of Gd effectively. Second, we controlled the effective SOC of ferrimagnetic CoGd by the relative composition between cobalt and gadolinium. We measured the spin-orbit torques in Pt/CoTb/Cr structures, here the Cr is selected for the generation of large OT. The switching current for magnetization reversal and the current induced effective fields are more less in the terbium-rich device than that of the cobalt-rich device. Third, we studied the additional current-induced torque from 3d transition metals (Cr, Cu, Ti), generated by inserting a thin Pt layer at the interface of the ferromagnet/3d TM. We conducted the harmonic Hall measurement and all the three materials exhibited the additional torque originated from the bulk of the material. Fourth, we additionally observed the unconventional Hall effect (UHE) in the general metal/intrinsic silicon structure. As opposite to the conventional assumption that the high resistive Si is negligible, the contribution of the interface of M/Si on the Hall resistance was sizable. In particular, since the magnetic field orientation dependence on UHE resembles the damping-like spin-orbit torque, it has to be considered carefully in the harmonic measurement. Our results offer the possibility that material platforms for SOT can be expanded to the system with weak SOC by controlling the interfacial SOC or the ferromagnetic materials.