Spinel ferrite thin films and heterostructures for spintronics

Spinel ferrite thin lms have numerous technological applications in areas such as telecommunications (microwave and millimeter wave devices), magneto-electric coupling devices and are also promising candidates for future spintronic devices. Unlike per- ovskites, the investigation of high quality spinel ferrite lms is quite limited, in part because of the complex crystal structure with a large unit cell consisting of many interstitial sites and that the transition metal cations can adopt various oxidation states. Usually lms of spinel ferrite such as NiFe₂O₄ (NFO), grown both by both physical and chemical deposition techniques, su er from a num- ber of structural and magnetic drawbacks, e.g. formation of antiphase boundaries and high magnetic saturation elds. 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 lms that are essentially free of antiphase boundaries and exhibit sharp magnetic hysteresis characteristics. Moreover, ferromagnetic resonance linewidths similar to those in single crystals are obtained. We have compared these results with NFO lm grown on another spinel substrate MgAl₂O₄, which has 3.1% lattice mismatch, that has antiphase boundaries and clearly exhibits degraded properties. We have also investigated spin transport properties of the lms grown on the three substrates via the longitudinal spin Seebeck e ect (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. Improvements in the magnetic and spin transport properties are also observed for other spinel ferrites, including CoFe₂O₄ and Fe₃O₄, using the lattice-matched substrates.