Spin Current Amplification by a Geometrical Ratchet Effect

A lateral spin-valve (LSV) is a structure to achieve non-local spin accumulation in future spintronic devices. Although numerous studies have been performed and have demonstrated highly efficient and reliable non-local accumulation, the use of a LSV is still hampered by the small magnitude of spin-current signals. Therefore, this study focused on the amplification of the pure spin-current signals by controlling the geometry of the non-magnetic nanowire in the LSV for the first time. A two-dimensional model was developed based on a diffusion equation and was used for a series of Cu nanowires with different shapes implemented at their centre to identify their geometrical ratchet effect (GRE) upon the spin-polarised electron transport. Asymmetric shapes, such as obtuse- and right-angle triangles, were found to induce the GRE, leading to the spin-current amplification in both time-dependent and steady states. The geometries have then been optimised for the maximum amplification. Before the experimental validation of the GRE, Py and Cu bars and conventional Py/Cu/Py LSVs were fabricated and characterised to optimise the fabrication and transport-measurement processes. The spin-current amplification was then investigated in LSVs with right-angle triangles maintaining the same base (100nm) but varying their height (0 ≤ h ≤ 60nm). The non-local signals were measured by a direct current (DC)-reversal technique. The spin-current signals were measured to be significantly amplified by a factor of more than 7 for h = 60nm as compared with the conventional LSV (h = 0 nm). These results were compared with the steady-state calculations using measured device dimensions, showing a good qualitative agreement. The measurements were also carried out with a DC setup, which revealed the junction spin polarisation (~1% in this study) allowed both up- and down-spin currents with similar amplitudes to flow. Further improvement in the junction spin polarisation should increase the GRE, leading to future device implementation.