Cation Orderings of Strained CoFe₂O₄ in Self-Assembled Heterostructures

Complex oxide heteroepitaxy has attracted lots of interests due to their unique physical properties through couplings of spin, charge, orbital and lattice orderings between two neighboring materials. In order to reveal the strain effects caused by heterointerfaces on the spinel ferrite CoFe₂O₄ (CFO), we studied the crystal symmetry of the epitaxial CFO film and the CoFe₂O₄-SrRuO₃ (CFO-SRO) vertically aligned heterostructure. XRD patterns show a strain-relaxed state in the CFO film, while the interfacial strain results in distorted CFO structures in CFO-SRO film. In addition, the angle-resolved polarized Raman spectroscopy indicates there are different short-range octahedral-site orderings between the CFO film and the heterostructure. Based on Raman polarization selection rules and first principle predictions, the relaxed CFO (c/a=1) is confirmed with the Imma symmetry, while the strained CFO (c/a>1) in heterostructure film is with the P4122 symmetry. This study indicates the interfacial strain plays an important role to manipulate the cation distribution and magnetic properties in spinel ferrite.