Large enhancement of the intrinsic spin Hall conductivity in W-N compounds

초록

The ultrathin tungsten (W) layer has been utilized as a spin-generating source for memory application in magnetic heterostructures. We investigate the spin Hall conductivity (SHC, sigma xz y) of W-N compounds, focusing on WN and W2N, employing first-principles calculations. We evaluate the SHC, formation energy (Delta F), and phonon frequencies for all cases. Three possible structures, including NaCl, hexagonal, and NbO type, are considered for WN. While the SHC of WN in NaCl and a hexagonal structure are substantial, the thermodynamic average of WN is -194( h/e) S/cm, much smaller than pure W. However, W2N, whose stability is confirmed by phonon calculations, shows a large SHC of sigma xz y =-966( h/e) S/cm with a spin Hall angle (theta SH) of -0.44, in good agreement with the experiment, enhanced by 18% over beta-W. Most of the spin Berry curvature, responsible for such large sigma xzy, stems from the [110] (kx = ky) plane and is analyzed from a symmetry perspective using representations of group theory.