Spin-to-Charge Conversion in Bi Films and Bi/Ag Bilayers

Recently, there have been reports on the superior prowess of bismuth (Bi) in spin-to-charge conversion, including large spin Hall angle and long spin diffusion length, qualities that are highly favorable for exploring pure spin current phenomena and devices [1]. Bi/Ag bilayers have also been featured prominently in the inverse Rashba-Edelstein effect (IREE), a related effect offering strong spin-to-charge conversion at interfaces [2,3]. One notes that in most cases the spin injection has been accomplished by spin pumping from Py, a process well known to harbor a variety of parasitic effects due to metallic Py that complicate the voltage measurements of the inverse spin Hall effect (ISHE).

In this work [4], we use thermal injection from a ferromagnetic insulator YIG into Bi films and Bi/Ag bilayers, demonstrated to be without the parasitic effects. The Bi layers of various thicknesses have been made by both MBE and sputtering at 77 K with similar results. The contribution of the pure spin current can be unequivocally determined by the insertion of a MgO(3 nm) layer, which blocks the spin current and eliminates the ISHE voltage. In sharp contrast to previous results, we have observed much shorter spin diffusion length in Bi of about 1 nm and negligibly small spin Hall angle. We have also observed no evidence of the IREE in the Bi/Ag bilayers. We note that Bi is a well-known thermoelectric material with low carrier concentration and very large Nernst voltage.

[1] D. Hou et al., Appl. Phys. Lett. 101, 042403 (2012).
[2] J. C. Rojas Sanchez et al., Nat. Comm. 4, 2944 (2013).
[3] Y. Ando et al., J. Phys. Soc. Jpn 86, 011001 (2017).
[4] D. Yue et al., Phys. Rev. Lett. 121, 037201 (2018).