Enhancing the spin Seebeck effect by controlling interface condition in Pt/polycrystalline nickel ferrite slabs

The spin Seebeck effect (SSE) is an emergent thermoelectric phenomenon, which enables a thermal-to-electrical energy conversion via the thermal injection of spin currents from a ferromagnet (FM) into an attached paramagnetic metal (PM). Recent studies have revealed that the SSE is very sensitive to the PM/FM interface condition, suggesting a potential way to enhance the SSE by controlling the interface condition. However, most of the previous studies are limited to conventional Pt/bulk single-crystal or thin-film YIG systems, lacking consideration for mesoscale surface defects such as pores and grain grooves, which frequently exist in more prevalent bulk polycrystalline magnets. Here, we investigate the effect of interface condition on the longitudinal SSE (LSSE) in a Pt/polycrystalline NiFe₂O₄ (NFO) slab system. Different interface conditions are induced by treating the surface of NFO slabs with varying combinations of polishing force (F_p) and post-annealing temperature (T_a) before the Pt deposition. The resultant LSSE signals show strong correlations with different interface parameters. In particular, we find that mesoscale surface defects (cracks, pores, and grain grooves) and the surface roughness play a crucial role in determining the magnitude of LSSE signals and demonstrate that those parameters can be deliberately controlled by properly choosing F_p and T_a. We report one sample with a spin Seebeck coefficient of 0.58 μV/K, which is significantly larger than that of bulk polycrystalline magnets reported thus far.