Abstract
When a current is passed through a non-magnetic metal with strong spin-orbit coupling, an orthogonal spin current is generated. This spin current can be used to switch the magnetization of an adjacent ferromagnetic layer or drive its magnetization into continuous precession. The interface, which is not necessarily sharp, and the crystallographic structure of the nonmagnetic metal can both affect the strength of current-induced spin-orbit torques. Here, we investigate the effects of interface intermixing and film microstructure on spin-orbit torques in perpendicularly magnetized Ta/Co40Fe40B20/MgO trilayers with different Ta layer thickness (5 nm, 10 nm, 15 nm), greater than the spin diffusion length. Effective spin-orbit torques are determined from harmonic Hall voltage measurements performed at temperatures ranging from 20 K to 300 K. We account for the temperature dependence of damping-like and field-like torques by including an additional contribution from the Ta/CoFeB interface in the spin diffusion model. Using this approach, the temperature variations of the spin Hall angle in the Ta underlayer and at the Ta/CoFeB interface are determined separately. Our results indicate an almost temperature-independent spin Hall angle of {{boldsymbol{theta }}}_{{boldsymbol{SH}}}^{{boldsymbol{N}}}approx -{bf{0.2}} in Ta and a strongly temperature-dependent {{boldsymbol{theta }}}_{{boldsymbol{SH}}}^{{boldsymbol{N}}} for the intermixed Ta/CoFeB interface.
Highlights
It is well known that spin current induced via the spin Hall effect (SHE) in a heavy metallic layer may exert a torque on the magnetic moment of an adjacent ferromagnetic layer[1]
Since available experimental data on the damping-like torque, converted to effective spin Hall angle, fluctuate between −0.0317 and −0.1120, we investigate effects caused by the interface and crystallographic structure, which may be responsible for the scattering of reported data
The θ − 2θ X-ray Diffraction (XRD) measurements on our samples do not indicate the presence of the α-Ta phase
Summary
It is well known that spin current induced via the spin Hall effect (SHE) in a heavy metallic layer may exert a torque on the magnetic moment of an adjacent ferromagnetic layer[1]. In the case of structures with PMA (considered in this paper), the harmonic Hall voltage method[22,23,24] seems to be the most appropriate because the damping-like and field-like torques are determined based on independent measurements. Since available experimental data on the damping-like torque, converted to effective spin Hall angle, fluctuate between −0.0317 and −0.1120, we investigate effects caused by the interface and crystallographic structure, which may be responsible for the scattering of reported data. Patterned structures of micrometer-dimensions are investigated with harmonic Hall voltage measurements in a wide range of temperatures in order to clearly designate the field-like and damping-like torques. Because of substantial interlayer mixing, we propose to model transport properties by considering the interface as a distinct layer with its own spin diffusion length and spin Hall angle
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