Spin-orbit torque and Nernst effects in Bi-Sb/ferromagnet heterostructures

Topological insulators have gained considerable interested recently due to their potential as spin current generators for room temperature memory applications. In this work we show harmonic Hall measurements of spin-orbit torque (SOT) in MBE grown Bi-Sb/Co bilayers [1]. We find that signals in our samples are dominated by the ordinary Nernst effect (ONE). This thermal effect can be a spurious signal in harmonic Hall measurements and has not been taken into account in previous experiments.

We are proposing two control experiments to separate the ONE in Bi-Sb from SOT. First, we compare harmonic measurements in Bi-Sb/Co and Bi-Sb/Al. While SOT is absent in the sample without magnet, we still see large signals in the second harmonic Hall voltage due to the ONE. Second, we investigate the scaling of V_2ω with external magnetic field B_ext. While it is expected that SOT effects are suppressed at large magnetic fields, we find that the second harmonic voltage in Bi-Sb/Co scales linearly with the external field, as expected from the ONE.
These results provide an important contribution to the understanding of the ultra-large SHA that was recently reported in TI/ferromagnets.

[1] N. Roschewsky et al., arXiv:1810.05674