Current-driven spin-orbit torques in ferromagnetic heterostructures

Abstract

The spin Hall effect (SHE) is the generation of spin separation by a charge current in a heavy metal (HM) with strong spin-orbit interaction. The generated pure spin current orthogonal to the charge current can exert a torque on the magnetization of an adjacent ferromagnetic layer (FM) in FM/HM bilayer. Due to the inversion symmetry breaking in such FM/HM bilayer, there exists a Rashba effect at the interface that can cause spin accumulation at the interface. The accumulated spin can also exert a torque on the magnetization of the ferromagnetic layer. The torque arising from the current-driven SHE or Rashba effect is referred to as spin-orbit torque (SOT). ☐ There are two types of SOT: field-like (FL) and damping-like (DL) SOT, both can cause FM layer magnetization reorientation or switching. SOT has attracted intensive interest recently due to its potential application in low energy memory and logic devices. DL SOT has been observed and widely investigated by researchers. However, FL SOT was rarely observed and neglected in most cases since it only can be measured in very thin FM layer because of the short spin dephasing length, usually on the order of several atomic layers. ☐ In this work, we first demonstrate the existence and observation of the effective field of FL SOT in a CoFeB/Ta bilayer by the spin transfer torque ferromagnetic resonance method. Then we quantitatively determine the FL SOT in NiFe/Pt system using the second-order planar Hall effect (PHE). It is found that FL SOT persists even with an insertion of Cu spacer layer between NiFe and Pt, which indicates that the SHE is the dominant mechanism for SOT. In order to investigate the interfacial and bulk effects of SOT in FM/HM bilayer, both the DL and FL SOTs are measured in the CoFeB/Pt system by the polar magneto-optical Kerr effect (MOKE) and second-order PHE, respectively. We found that the SHE is also the dominant mechanism in CoFeB/Pt system. However, there is also a distinct interfacial contribution, which is attributed to the Rashba effect. ☐ There are two mechanisms contributing to the SHE: intrinsic and extrinsic effect. We show that the SHE in Au-Cu alloy can be enhanced by the extrinsic scattering compared to pure Au or Cu. It is also found that the DL SOT and FL SOT depend on temperature very differently in NiFe/AuCu system. A nearly linear dependence between the spin Hall angle and longitudinal resistivity suggests that skew scattering, one of the extrinsic effect for SHE, may be the dominant mechanism in Au-Cu alloy.