Compositionally Modulated Magnetic Epitaxial Spinel/Perovskite Nanocomposite Thin Films

There is great interest in self-assembled oxide vertical nanocomposite films consisting of epitaxial spinel pillars in a single crystal perovskite matrix, due to their tunable electronic, magnetic, and multiferroic properties. Varying the composition or geometry of the pillars in the out-of-plane direction has not been previously reported but can provide new routes to tailoring their properties in three dimensions. In this work, ferrimagnetic epitaxial CoFe2O4, MgFe2O4, or NiFe2O4 spinel nanopillars with an out-of-plane modulation in their composition and shape are grown in a BiFeO3 matrix on a (001) SrTiO3 substrate using pulsed laser deposition. Changing the pillar composition during growth produces a homogeneous pillar composition due to cation interdiffusion, but this can be suppressed using a sufficiently thick blocking layer of BiFeO3 to produce bi-pillar films containing for example a layer of magnetically hard CoFe2O4 pillars and a layer of magnetically soft MgFe2O4 pillars, which form in different locations. A thinner blocking layer enables contact between the top of the CoFe2O4 and the bottom of the MgFe2O4 which leads to correlated growth of the MgFe2O4 pillars directly above the CoFe2O4 pillars and provides a path for interdiffusion. The magnetic hysteresis of the nanocomposites is related to the pillar structure.