Optimizing the Spin Hall Angle in Ultrathin Metallic Films

The spin Hall effect (SHE) is one of the key effects in modern spintronics creating pure spin currents directly in nonmagnetic materials.
The effect's strength is quantified with the so-called spin Hall angle (SHA), which is the ratio of the transverse spin conductivity to the longitudinal charge conductivity.
Reported values based on both experimental and theoretical investigations increased during the last years.
Measurements on Pt-doped Au samples yielded a SHA of about 10% introduced as giant SHE [1] while a SHA of 24% was published for thin-film Cu(Bi) alloys [2].
The large effect caused by Bi impurities in Cu was theoretically predicted for bulk samples [3].

In this work we theoretically investigate Bi-doped ultrathin noble-metal films by means of an ab initio approach using density functional theory and linearized Boltzmann equation, study various possibilities to optimize the SHE and forecast colossal SHAs slightly below 100% [4].
Furthermore, we identify systems with a strong anisotropy of the in-plane transport properties that lead to SHAs above 100%.

[1] Seki et al., Nat. Mater. 7, 125 (2008)
[2] Niimi et al., Phys. Rev. Lett. 109, 156602 (2012)
[3] Gradhand et al., Phys. Rev. Lett. 104, 186403 (2010)
[4] Herschbach et al., Phys. Rev. B 90, 180406(R) (2014)