Abstract: 
Spinel ferrite thin films 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 perovskites, the investigation of high quality spinel ferrite films 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 thin films of spinel ferrite such as NiFe2O4 (NFO), grown both by both physical and chemical deposition techniques, suffer from a number of 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 and CoGa2O4 substrates, which have 0.8% and 0.2% 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 single crystals are obtained. We have compared these results with NFO film 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 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. Thickness and temperature dependence of the LSSE for NFO films grown on different substrates have been investigated.

Bio：
Arunava Gupta is Distinguished University Research Professor and MINT Professor at the University of Alabama (UA). He holds a joint appointment in UA’s College of Arts and Sciences and College of Engineering, and is associate director of UA’s Center for Materials for Information Technology (MINT Center). Gupta received his undergraduate degree from the Indian Institute of Technology, Kanpur, and Ph.D. degree in Chemical Physics from Stanford University. Prior to joining UA’s faculty in 2004, he worked as a research staff member and manager at the IBM Thomas J. Watson Research Center in New York. Gupta’s expertise is in investigating thin films and nanostructured materials for use in information technology and energy applications. He has co-authored more than 400 peer-reviewed scientific articles and holds 30 US patents. Gupta is an elected fellow of the American Physical Society, the American Association for the Advancement of Science and the Materials Research Society. He received the Humboldt Research Prize in 2010, awarded by the Alexander von Humboldt Foundation. In 2014, he was awarded the CRSI Medal, given annually by the Chemical Research Society of India.