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

TL;DR

This study demonstrates that growing NiFe₂O₄ thin films on lattice-matched substrates significantly enhances thermally generated spin voltage, with reduced defects leading to more efficient spin injection as measured by the spin Seebeck effect.

Contribution

It provides new insights into how lattice matching and structural defect reduction improve spin injection efficiency in NiFe₂O₄/Pd interfaces.

Findings

Spin Seebeck voltage increases with decreasing lattice mismatch for thin films.

Reduced defects correlate with lower Gilbert damping and improved spin injection.

Lattice matching influences magnetic anisotropies detectable via angle-dependent measurements.

Abstract

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.