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NiFe$_2$O$_4$ thin films with varying thickness were grown on SrTiO$_3$(001)by reactive molecular beam epitaxy. Soft and hard x-ray photoelectronspectroscopy measurements reveal a homogeneous cation distribution throughoutthe whole film with stoichiometric Ni:Fe ratios of 1:2 independent of the filmthickness. Low energy electron diffraction and high resolution (grazingincidence) x-ray diffraction in addition to x-ray reflectivity experiments wereconducted to obtain information of the film surface and bulk structure,respectively. For ultrathin films up to 7.3 nm, lateral tensile and verticalcompressive strain is observed, contradicting an adaption at the interface ofNiFe$_2$O$_4$ film and substrate lattice. The applied strain is accompanied byan increased lateral defect density, which is decaying for relaxed thickerfilms and attributed to the growth of lateral grains. Determination of cationicsite occupancies in the inverse spinel structure by analysis of site sensitivediffraction peaks reveals low tetrahedral occupancies for thin, strainedNiFe$_2$O$_4$ films, resulting in partial presence of deficient rock salt likestructures. These structures are assumed to be responsible for the enhancedmagnetization of up to $\sim$250\% of the NiFe$_2$O$_4$ bulk magnetization asobserved by superconducting quantum interference device magnetometry forultrathin films below 7.3 nm thickness.

Enhanced magnetization of ultrathin NiFe$_2$O$_4$ films on SrTiO$_3$(001) related to cation disorder and anomalous strain