Ultralow Damping Spinel Ferrite Films Grown on Isostructural Substrates

Spinel ferrite thin films have numerous technological applications in areas such as telecommunications, magneto-electric coupling devices and are also promising candidates for future spintronic devices. Films of spinel ferrite such as NiFe₂O₄ (NFO), grown by both physical and chemical deposition techniques, usually suffer from several structural and magnetic drawbacks, e.g., formation of antiphase boundaries and high magnetic saturation fields. We show that by using substrates having similar crystal structure and low lattice mismatch, one can avoid formation of antiphase boundaries and thereby obtain magnetic properties comparable to bulk single crystal. We used spinel MgGa₂O₄, CoGa₂O₄ and ZnGa₂O₄ substrates, which have 0.6%, 0.1% and 0.05% lattice mismatch, respectively, with NFO to grow epitaxial films that are essentially free of antiphase boundaries and exhibit sharp magnetic hysteresis characteristics. Moreover, ferromagnetic resonance linewidths similar to those in bulk single crystals are obtained with very low damping parameter. We investigated spin transport properties of the NiFe₂O₄ films grown on the three substrates via the longitudinal spin Seebeck effect (LSSE).  An increase in the spin voltage signal with reduction in lattice mismatch is observed, which is in correspondence with similar improvements in structural and magnetic properties. We further demonstrate that bidirectional field-dependent LSSE voltage curves can be utilized to reveal the complete magnetization reversal process, which offers a new vectorial magnetometry technique based on spin caloric effect.