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Topological insulators have emerged as an important material class for efficient spin-charge interconversion. Most topological insulators considered to date are binary or ternary compounds, with the exception of $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$. Here we report a comprehensive characterization of the growth, magnetotransport properties, and current-induced spin-orbit torques of $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ and $\ensuremath{\beta}\ensuremath{-}\mathrm{Sn}$-based ferromagnetic heterostructures. We show that $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ grown with a Bi surfactant on CdTe(001) promotes large spin-orbit torques in a ferromagnetic FeCo layer at room temperature, comparable to Pt, whereas $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ grown without Bi surfactant and the nontopological phase, $\ensuremath{\beta}\ensuremath{-}\mathrm{Sn}$, induce lower torques. The dampinglike and fieldlike spin-orbit torque efficiency in $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ with Bi are 0.12 and 0.18, respectively. Further, we show that $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ grown with and without Bi presents a spin-Hall-like magnetoresistance comparable to that found in heavy metal/ferromagnet bilayers. Our work demonstrates direct and efficient charge-to-spin conversion in $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ ferromagnetic heterostructures, showing that $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ is a promising material for current-induced magnetization control in spintronic devices.

We characterize the spin Hall magnetoresistance (SMR), spin Seebeck effect (SSE), and dampinglike spin-orbit torque (SOT) in thulium iron garnet/platinum bilayers with perpendicular magnetic anisotropy by using harmonic Hall effect measurements. By consecutive annealing steps followed by measurements on a single device, we reveal that the spin-dependent effects gradually decrease in amplitude as the annealing temperature increases. We attribute this behavior primarily to the changes in the spin-mixing conductance, which sensitively depends on the interface quality. However, further analysis demonstrates that although the SSE scales closely with the SMR, the dampinglike SOT shows a significantly different trend upon annealing, contrary to theoretical expectations. By comparing the dampinglike SOT with the field-induced Hall effect, we found evidence that scattering from Fe impurities in the Pt at the interface might be responsible for the distinct annealing temperature dependence of the dampinglike SOT.

We present harmonic transverse voltage measurements of current-induced thermoelectric and spin-orbit torque (SOT) effects in ferromagnet/normal metal bilayers, in which thermal gradients produced by Joule heating and SOT coexist and give rise to ac transverse signals with comparable symmetry and magnitude. Based on the symmetry and field-dependence of the transverse resistance, we develop a consistent method to separate thermoelectric and SOT measurements. By addressing first ferromagnet/light metal bilayers with negligible spin-orbit coupling, we show that in-plane current injection induces a vertical thermal gradient whose sign and magnitude are determined by the resistivity difference and stacking order of the magnetic and nonmagnetic layers. We then study ferromagnet/heavy metal bilayers with strong spin-orbit coupling, showing that second harmonic thermoelectric contributions to the transverse voltage may lead to a significant overestimation of the antidamping SOT. We find that thermoelectric effects are very strong in Ta(6nm)/Co(2.5nm) and negligible in Pt(6nm)/Co(2.5nm) bilayers. After including these effects in the analysis of the transverse voltage, we find that the antidamping SOTs in these bilayers, after normalization to the magnetization volume, are comparable to those found in thinner Co layers with perpendicular magnetization, whereas the field-like SOTs are about an order of magnitude smaller., Comment: 26 pages, 6 figures, 1 table