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

Abstract: We study spin-orbit torques (SOTs) in Ni/Pt bi-layers and multi-layers by ferromagnetic resonance (FMR) and harmonic-Hall measurements. The effect of multi-layering and crystal orientation on field-like (FL) and damping-like (DL) torque efficiencies is examined by exploiting the samples with different crystal orientations: epitaxial and poly-crystalline structures on Sapphire and SiO$_2$ substrates, respectively. We find that both DL and FL torque efficiencies are larger in multi-layer 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 poly-crystalline samples, suggesting that SOT generation is modified depending on the interfacial contribution. In addition, the spin Hall conductivity of the epitaxial multi-layer is the largest among the samples. The present results signify the importance of crystal orientation, multi-layering and interface-quality in improving the efficiency of SOTs generation combined with larger spin hall angle for developing future spintronic devices.