Reduced spin-Hall effects from magnetic proximity

Harnessing spin-orbit coupling for the manipulation of spins and magnetization via electric charge currents is the key objective of spin-orbitronics. Towards this end ferromagnetic materials are combined with nonmagnetic materials with strong spin-orbit coupling (typically involving heavy elements). However, many of the nominally nonmagnetic materials are highly susceptible to magnetic proximity effects, and the role of induced moments for spin transport has been controversial. Here we demonstrate that for Pt and Pd increased induced magnetic moments are correlated with strongly reduced spin-Hall conductivities. This observation finds an intuitive explanation in the development of a spin splitting of the chemical potential and the energy dependence of the intrinsic spin-Hall effect determined by first-principles calculations. This work provides simple guidance towards the optimization of spin current efficiencies for devices based on spin-orbit coupling phenomena.

Figure 1

X-ray reflectivity data for Py/Pt(Pd) and Py/Cu/Pt(Pd) samples and the corresponding fittings for determining the roughness values.

Figure 2

(a) Schematic illustration of the spin pumping and ISHE experiments showing the respective polarity (rf input, external field, H, and dc voltage contacts, $V_{+}/V_{-})$, and the external field direction, $\alpha$ = ${40}^{\mathrm{o}}$ with respect to the central signal line. Due to magnetic proximity effect, the spin-Hall conductivities (SHC) are reduced near the interface, since they are functions of induced magnetic moments that decay away from the interface within a very thin thickness range; these effects are observed by ISHE measurements only for very thin layers of Pt and Pd on top of Py. AMR-ISHE spectra measured at 4 GHz for (b) Py/Pt and (c) Py/Pd at selective temperatures.

Figure 3

DC voltage spectra measured at 4 GHz for Py/Cu/Pt and Py/Cu/Pd reference samples at selective temperatures.

Figure 4

Temperature dependence of ${\mathit{W}}_{\mathrm{ISHE}}$ at 4, 5, and 6 GHz for (a) Py/Pt, (b) Py/Cu/Pt, (c) Py/Pd, and (d) Py/Cu/Pd bilayers.

Figure 5

Temperature dependence of (a) normalized magnetization, M(T)/M${}_{300}\mathrm{K}$ (with respect to 300 K) of Py/Pt and Py and their differences $\left(\Delta \right)$, measured by a SQUID magnetometry with 2k Oe in-plane applied field. (b) AMR and ISHE voltage amplitudes at 4 GHz, (c) spin mixing conductance, (d) device total resistance and Pt resistivity, and (e) calculated spin-Hall angle and spin-Hall conductivity for Pt. Right panels (f)–(j) are corresponding results for Pd.

Figure 6

Panels (a) and (b): Chemical potential dependence of the intrinsic SHE conductivity in Pt and Pd. The dashed lines indicate the spin-Hall conductivities ${\sigma }_{\mathrm{spin}}\left(E_{\downarrow }\right)$ of the majority spins and ${\sigma }_{\mathrm{spin}}\left(E_{\uparrow }\right)$ of the minority spins in a spin-polarized system with an induced moment of 1 ${\mu }_B$. Panels (c) and (d): Estimated dependence of the intrinsic SHE conductivity on the proximity-induced spin magnetic moment.