Deposition Pressure Dependence on Spin Hall Angle of W Thin Films Grown on NiFe

Abstract

Spin-to-charge conversion and vice versa due to spin-orbit coupling in ferromagnet-heavy metal heterostructure is of paramount interest for developing energy-efficient spintronic devices. Here, we have systematically investigated the effect of Ar deposition pressure ($P_{\mathrm{\text{Ar}}})$ on the tungsten (W) crystalline phase and extracted spin-dependent transport parameters. X-ray diffraction results show that 10$$nm-thick W films exhibit a structural phase transition from a mixed phase of $(\alpha +\beta )$-W to a single phase of $\beta$-W as a function of $P_{\mathrm{\text{Ar}}}$. The observed phase transition is due to a decrease in adatom’s energy and surface mobility. Interestingly, only the $(\alpha +\beta )$-W phase is found to stabilize when W sputtered on a seed Ni${}_{80}$Fe${}_{20}$ (Permalloy or Py) film. The growth of $(\alpha +\beta )$-W on the seed Py layer could be due to the strain that facilitates the mixed phase. W deposited on the Py layer is shown to be dependent on $P_{\mathrm{\text{Ar}}}$, in which the $\beta$-W relative phase fraction is relative. A ferromagnetic resonance (FMR)-based spin pumping method was employed for spin current injection. The FMR linewidth ($\mathrm{\Delta }H)$ is enhanced for Py/W compared to the bare Py layer due to the spin current transport across the interface. The spin-mixing conductance ($g_{\uparrow \downarrow })$ is found to be a function of the relative phase fraction of W. The extracted $g_{\uparrow \downarrow }$ is $4.90\times 10^{18}$$$m${}^{-2}$ for $P_{\mathrm{\text{Ar}}}=5$$$mTorr and $4.05\times 10^{18}$$$m${}^{-2}$ for $P_{\mathrm{\text{Ar}}}=10$$$mTorr. From the inverse spin Hall effect (ISHE) measurements, the effective spin Hall angle ($({\theta }_{\mathrm{\text{SH}}})$ is estimated to be $-0.17$ for $\alpha$-W rich mixed phase of $(\alpha +\beta )$-W, whereas it is $-0.10$ for $\beta$-W rich $(\alpha +\beta )$-W. Our systematic study demonstrates the relatively large effective spin Hall angle via low-longitudinal resistivity by controlling the relative phase fraction of W and helps in developing energy-efficient spintronic devices.