Abstract
Spinel ferrites are being considered for advanced spintronic applications. Here, we report on the magnetic properties of ultrathin (3--37 nm) epitaxial films of NiFe${}_{2}$O${}_{4}$ (NFO) on MgAl${}_{2}$O${}_{4}$ (MAO) and SrTiO${}_{3}$ (STO) single crystalline substrates. It is found that NFO films on STO display superparamagnetic response down to 50 K, whereas films grown on MAO display ferrimagnetic response up to room temperature. Microstructural information indicates that this distinct response can be attributed to the different growth mechanisms of the spinel ferrite on the isostructural MAO substrate (two-dimensional growth) and the perovskite STO (Volmer-Weber three-dimensional growth). We discuss the reasons for this distinct behavior and its relevance for the integration of ferrites in epitaxial heterostructures for tunnel devices.
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