Enhancement in Thermally Generated Spin Voltage at the Interfaces between Pd and NiFe2O4 Films Grown on Lattice-Matched Substrates

Efficient spin injection from epitaxial ferrimagnetic ${\mathrm{Ni}\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ thin films into a $\mathrm{Pd}$ layer is demonstrated via spin Seebeck effect measurements in the longitudinal geometry. The ${\mathrm{Ni}\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ films (60 nm to 1 $\ensuremath{\mu}$) are grown by pulsed-laser deposition on isostructural spinel ${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$, ${\mathrm{Mg}\mathrm{Ga}}_{2}{\mathrm{O}}_{4}$, and ${\mathrm{Co}\mathrm{Ga}}_{2}{\mathrm{O}}_{4}$ substrates with lattice mismatch varying between 3.2 and 0.2%. For the thinner films ($\ensuremath{\le}330$ nm), an increase in the spin Seebeck voltage is observed with decreasing lattice mismatch, which correlates well with a decrease in the Gilbert damping parameter as determined from ferromagnetic resonance measurements. High-resolution transmission electron microscopy studies indicate substantial decrease of antiphase boundary and interface defects that cause strain relaxation, i.e., misfit dislocations, in the films with decreasing lattice mismatch. This highlights the importance of reducing structural defects in spinel ferrites for efficient spin injection. It is further shown that angle-dependent spin Seebeck effect measurements provide a qualitative method to probe for in-plane magnetic anisotropies present in the films.