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 CoFe ₂O₄, MgFe₂O₄, or NiFe ₂O₄ 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 MgFe ₂O₄ which leads to correlated growth of the MgFe ₂O₄ pillars directly above the CoFe₂O ₄ pillars and provides a path for interdiffusion. The magnetic hysteresis of the nanocomposites is related to the pillar structure.