Nonreciprocal spin-charge interconversion in topological insulator/ferromagnet heterostructures

The process of spin-charge interconversion is critical in modern spintronics. Nonetheless, experiments conducted on a wide variety of magnetic heterostructures consistently report that charge-to-spin and spin-to-charge conversion efficiencies can be vastly different, especially in the case of topological insulators (TIs). This discrepancy between the two ``reciprocal'' effects remains unexplained, hampering the development of spin devices based on spin-charge conversion. In this study, we investigate both spin-charge and charge-spin interconversion processes in TI $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{3}\text{/}\mathrm{Py}$ and Pt/Py bilayers experimentally using spin-torque ferromagnetic resonance and spin pumping techniques. We find that the measured charge-to-spin conversion efficiency $({\ensuremath{\xi}}_{\mathrm{C}\text{\ensuremath{-}}\mathrm{S}})$ in TI/Py is $\ensuremath{\sim}26$ times larger than the measured spin-to-charge conversion efficiency $({\ensuremath{\xi}}_{\mathrm{S}\text{\ensuremath{-}}\mathrm{C}})$, whereas ${\ensuremath{\xi}}_{\mathrm{C}\text{\ensuremath{-}}\mathrm{S}}$ and ${\ensuremath{\xi}}_{\mathrm{S}\text{\ensuremath{-}}\mathrm{C}}$ are comparable in the case of Pt/Py. Using a theoretical model enforcing Onsager reciprocity, we show that spin-to-charge and charge-to-spin conversions in bilayers are genuinely inequivalent, and explain our results as arising from the distinct spin current leakage that takes place during the interconversion. This work clarifies previous conflicting reports on spin-charge interconversion processes and highlights the potential of interface engineering to achieve efficient spin transport in TI-based ferromagnetic heterostructures, paving the way for highly efficient spintronic devices.