Effect of multilayering and crystal orientation on spin-orbit torque efficiency in Ni/Pt layer stacking

Abstract

We study spin-orbit torques (SOTs) in Ni/Pt bilayers and multilayers by ferromagnetic resonance (FMR) and harmonic-Hall measurements. The effect of multilayering and crystal orientation on fieldlike (FL) and dampinglike (DL) torque efficiencies is examined by exploiting the samples with different crystal orientations: epitaxial and polycrystalline structures on sapphire and SiO2 substrates, respectively. We find that both DL and FL torque efficiencies are larger in multilayer samples and there is no complete cancellation of torque efficiencies that is generally expected for ideal symmetric stacking structures. The results of SOT-FMR indicate that the epitaxial samples show higher efficiency for SOT generation compared to the polycrystalline samples, suggesting that SOT generation is modified depending on the interfacial contribution. In addition, the spin Hall conductivity of the epitaxial multilayer is the largest among the samples. The present results signify the importance of crystal orientation, multilayering, and interface quality in improving the efficiency of SOT generation combined with larger spin Hall angle for developing future spintronic devices.