Enhancement in Thermally Generated Spin Voltage at Pd/NiFe\(_2\)O\(_4\) Interfaces by the Growth on Lattice-Matched Substrates

Efficient spin injection from epitaxial ferrimagnetic NiFe\(_2\)O\(_4\) thin films into a Pd layer is demonstrated via spin Seebeck effect measurements in the longitudinal geometry. The NiFe\(_2\)O\(_4\) films (60 nm to 1 \(\mu\)m) are grown by pulsed laser deposition on isostructural spinel MgAl\(_2\)O\(_4\), MgGa\(_2\)O\(_4\), and CoGa\(_2\)O\(_4\) substrates with lattice mismatch varying between 3.2% and 0.2%. For the thinner films (\(\leq\) 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.