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.