Investigation of Spinel Ferrite Thin Films for Spintronics Applications

Spinel ferrite thin films have garnered special attention due to their usefulness in recent technological advances. They hold a significant promise for various device applications, including microwave devices, memory chips, transformer cores, antenna rods, and millimeter-wave integrated circuitry. These materials are attractive due to their extremely high specific resistance, high saturation magnetization, high Curie temperature, and exceptional flexibility in tailoring magnetic properties. In this dissertation, we attempt to address a most critical issue impeding the successful integration of spinel ferrites in ferrite-based microwave devices, spin-Seebeck effect (SSE)-based thermoelectric devices, and magnetostrictive device applications by presenting a successful fabrication and characterization of high-quality (Ni, Co, Fe)−Fe₂O₄ thin films with minimal defects, grown epitaxially on single crystalline lattice-matched substrates using the pulsed laser deposition method. Typically, spinel ferrite thin films such as NiFe₂O₄ (NFO), CoFe₂O₄ (CFO), and Fe₃O₄ (FFO), deposited by both physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques suffer from several structural defects resulting in degraded magnetic properties. These defects are primarily the formation of antiphase boundaries and misfit dislocations that result in low saturation magnetization and high magnetic saturation field. We show that by combining isostructural substrates with lower lattice-mismatch, as low as 0.06%, and optimum deposition parameters, one can successfully eliminate essentially all the film defects to obtain characteristics comparable to bulk single-crystal. We utilized spinel structured MgAl₂O₄, MgGa₂O₄, and ZnGa₂O₄ substrates for film growth with varying lattice mismatches with NFO, FFO, and CFO crystals. The deposited films are structurally mostly defect-free and have a smooth surface morphology with less than 200 pm root mean square (RMS) roughness. A 400 nm NFO film on ZnGa₂O₄ substrate shows uniaxial perpendicular anisotropy (Hu⊥ = 0.1 (±0.1) kOe), Gilbert damping parameter equal to αₑff = 9 × 10−⁴ (±7×10−⁵), and very low strain-induced anisotropy (Hσ = 0.4 (±0.1) kOe). It has also been demonstrated that similar improvements are attainable from other members of the spinel ferrite family with minimal substrate-induced film strain, such as a sharper Verwey transition in FFO films and a lower magnetoelastic uniaxial anisotropy in CFO films.