Crystalline Order Dependent Spin-Orbit Torque in Sputter Deposited Topological Insulator /Ferromagnet Heterostructures

Abstract

Topological insulators (TI) possess topologically protected surface conducting states and enormous spin-orbit coupling (SOC) [1,2]. This makes TIs ideal candidates for Spin-Orbit Torque (SOT) based Non-volatile Memory. A TI adjacent to a ferromagnetic (FM) thin film generates SOT on the magnetization in FM [2-5] because of Spin Hall Effect (SHE). SOT in TI/FM (ferromagnet) films have been reported previously with TI films epitaxially grown with MBE [2-5], which is not ideal for industrial complementary metal oxide (CMOS) processes. Although, there have been reports of SOT from sputtered TIs [6-8], effects of crystalline disorder in SOT characteristics have not been extensively studied. We performed magnetic and Spin-Torque Ferromagnetic Resonance (ST-FMR) measurements to investigate the effect of sputtering growth temperature and hence crystalline ordering on SOT characteristics of Bi2Te3 coupled with NiFe (Permalloy) and CoFeB as FM layers. The Bi2Te3/FM heterostructures revealed a steady increase in out of plane (OOP) magnetic anisotropy with the increase in crystalline order of Bi2Te3. Further, STFMR measurements revealed a nonlinear trend in damping-like spin orbit torque with increase in sputtering growth temperature. We believe that at room temperature growth, smaller grain size in Bi2Te3 causes quantum-confinement governed enhancement in SOT [6]. However, a giant enhancement in SOT characteristics is observed for highly crystalline c-axis oriented Bi2Te3 heterostructure where Bi2Te3 was grown at 250 ºC. Our results provide a clear path towards integration of crystalline TI based spintronic devices in industrial CMOS compatible processes.