Temperature dependence of spin-orbit torques in W/CoFeB bilayers

Abstract

We report on the temperature and layer thickness variation of spin-orbit torques in perpendicularly magnetized W/CoFeB bilayers. Harmonic Hall voltage measurements reveal dissimilar temperature evolutions of longitudinal and transverse effective magnetic field components. The transverse effective field changes sign at 250 K for a 2 nm thick W buffer layer, indicating a much stronger contribution from interface spin-orbit interactions compared to, for example, Ta. Transmission electron microscopy measurements reveal that considerable interface mixing between W and CoFeB is primarily responsible for this effect.

The influence of an ultra-high resistivity Ta underlayer on perpendicular magnetic anisotropy in Ta/Pt/Co/Pt heterostructures

“…For example, in tungsten/magnetic metal lms, the high resistivity of b-W can signicantly increase the effective magnetic anisotropic eld (H an ), thus leading to a decrease in the critical switching current density and a signicant increase in the spin Hall angle. [19][20][21][22][23][24] The same results were also reported for high resistivity b-Ta based multilayered lm structures. [25][26][27][28] Moreover, the insertion of a non-magnetic layer between the heavy metal/magnetic metal layers will also affect the spin-orbit coupling (SOC) of the lm interface, and an appropriate insertion layer can effectively improve the PMA and reduce the spin Hall current density.…”

Influence of intermixing at the Ta/CoFeB interface on spin Hall angle in Ta/CoFeB/MgO heterostructures

“…This approximation is justified in some cases, but it is generally invalid for amorphous structures. For example, in CoFeB/Ta and CoFeB/W 12 , X-ray and neutron reflectometry 13 indicate strong intermixing between the two layers, resulting in a relatively wide interface region. When considering the interface contribution to the total SOT, spin transport across the interfaces and possible mechanisms of spin relaxation need to be taken into account.…”

“…High resistance phases of heavy metals are desirable as the spin Hall angle is enhanced. For example, tungsten above a certain thickness shows a phase transition from the high resistivity β -W phase to the low resistivity α -W phase, which results in a decrease of the spin Hall angle 12 , 15 , 16 . A detailed assessment of structural effects on SOTs is complicated by a relatively broad distribution of experimental data from different laboratories and the use of different definitions.…”

Low Gilbert Damping Constant in Perpendicularly Magnetized W/CoFeB/MgO Films with High Thermal Stability

“…At T ann = 400 °C, the dead-layer formation leads to a larger damping presumably due to an increase in scattering sites (diffused W atoms) that contribute to spin-flip events, as described by the Elliot-Yafet relaxation mechanisms 18 . Additionally, W atoms can increase the spin-orbit coupling and thus the damping as the inter-diffusion increases with T ann 47 . The observation that our W-seeded samples still sustain excellent PMA properties at T ann = 400 °C confirms their enhanced thermal stability, compared with Ta/CoFeB/MgO stacks which fail at T ann = 350 °C or higher.…”