The spin Seebeck effect in magnetic insulating oxides

Abstract

The spin Seebeck effect (SSE), the generation of a spin current from a thermal gradient, is a novel effect which involves the interaction between charge, spin and heat. Insulating magnetic materials, like yttrium iron garnet (YIG, Y₃Fe₅O₁₂) and nickel ferrite (NFO, NiFe₂O₄), are ideal for the study of this new effect due to avoiding other magnetic effects. Thin films of Y₃Fe₅O₁₂, Ce₀.₇₅Y₂.₂₅Fe₅O₁₂ and NiFe₂O₄ have been grown and optimized on different substrates (MgAl₂O₄, MgGa₂O₄, CoGa₂O₄) using the pulsed laser deposition (PLD) technique, and their crystal structures were investigated using X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). For the magnetocrystalline anisotropy in the thin films, vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) measurements are done. We further did spin Seebeck effect measurements on optimized samples. First, for thin films of Ce₀.₇₅Y₂.₂₅Fe₅O₁₂, homogeneous substitution of Ce in YIG results in the enhancement of the signal in magneto-optic Kerr effect (MOKE) without forming CeO₂ when at lower O₂ atmosphere. The spin Seebeck effect measurements on Ce:YIG films show similar trends and comparable results with pure YIG films suggesting potential applications for thermoelectric generation. Second, an increase in the spin Seebeck voltage is observed with decreasing lattice mismatch between NFO thin films and substrates, which also correlates well with the decrease in the Gilbert damping parameter from FMR measurements. Furthermore, we have developed a vector measurement of the spin Seebeck effect in epitaxial NiFe₂O₄ thin films, which were grown by pulsed laser deposition on (011)- or (001)-oriented MgGa₂O₄ and CoGa₂O₄ substrates with varying lattice mismatches. This new method for SSE measurement shows the existence of a magnetic strain anisotropy in NiFe₂O₄ thin films significantly impacts the shape and magnitude of the SSE voltage hysteresis loops, which demonstrates that voltage signals from bidirectional SSE measurements can be utilized as a new vectorial magnetometry technique to reveal the complete magnetization reversal process.