Jari Rodewald

Off-stoichiometric Ni_xFe_3−xO₄ ultrathin films (x < 2.1) with varying Ni content x and thickness 16 (±2) nm were grown on MgO(001) by reactive molecular beam epitaxy. Synchrotron-based high-resolution X-ray diffraction measurements reveal vertical compressive strain for all films, resulting from a lateral pseudomorphic adaption of the film to the substrate lattice without any strain relaxation. Complete crystallinity with smooth interfaces and surfaces is obtained independent of the Ni content x. For x < 1 an expected successive conversion from Fe₃O₄ to NiFe₂O₄ is observed, whereas local transformation into NiO structures is observed for films with Ni content x > 1. However, angle-resolved hard X-ray photoelectron spectroscopy measurements indicate homogeneous cationic distributions without strictly separated phases independent of the Ni content, while X-ray absorption spectroscopy shows that also for x > 1, not all ${\mathrm{Fe}}^{2+}$ cations are substituted by ${\mathrm{Ni}}^{2+}$ cations. The ferrimagnetic behavior, as observed by superconducting quantum interference device magnetometry, is characterized by decreasing saturation magnetization due to the formation of antiferromagnetic NiO parts.

Here, we present the (element-specific) magnetic properties and cation ordering for ultrathin Co-rich cobalt ferrite films. Two Co-rich ${\mathrm{Co}}_x{\mathrm{Fe}}_{3-x}{\mathrm{O}}_4$ films with different stoichiometry ($x=1.1$ and $x=1.4$) have been formed by reactive solid phase epitaxy due to post-deposition annealing from epitaxial CoO/Fe${}_3$O${}_4$ bilayers deposited before on Nb-doped SrTiO${}_3$(001). The electronic structure, stoichiometry and homogeneity of the cation distribution of the resulting cobalt ferrite films were verified by angle-resolved hard X-ray photoelectron spectroscopy. From X-ray magnetic circular dichroism measurements, the occupancies of the different sublattices were determined using charge-transfer multiplet calculations. For both ferrite films, a partially inverse spinel structure is found with increased amount of ${\mathrm{Co}}^{3+}$ cations in the low-spin state on octahedral sites for the ${\mathrm{Co}}_{1.4}{\mathrm{Fe}}_{1.6}{\mathrm{O}}_4$ film. These findings concur with the results obtained by superconducting quantum interference device measurements. Further, the latter measurements revealed the presence of an additional soft magnetic phase probably due to cobalt ferrite islands emerging from the surface, as suggested by atomic force microscope measurements.

${\text{Ni}}_{\mathrm{x}}{\text{Fe}}_{3-\mathrm{x}}$ O₄ thin films with varying Ni amount (0 $\le$ x $\le$ 1.5) were deposited on MgO(001) via reactive molecular beam epitaxy. The growth process was monitored during film deposition by means of X-ray diffraction. All prepared films exhibit a well-ordered structure with complete vertical crystallinity throughout the whole film growth and flat surfaces of the final films independent of the Ni amount. An enhancement of the vertical compression in the initial growth continuously decreases up to a film thickness of 8 nm. During further growth, all films exhibit residual and constant vertical compression with lateral adaption of the final films to the substrate lattice, as observed by high energy surface X-ray diffraction experiments. Hard X-ray photoelectron spectroscopy measurements of the final films reveal increasing ${\text{Fe}}^{3+}$ :${\text{Fe}}^{2+}$ ratios for higher Ni content and point to additional NiO agglomerations within the films exceeding the stoichiometric Ni amount of x = 1.

${\mathrm{NiFe}}_2{\mathrm{O}}_4$ thin films with varying thickness were grown on ${\mathrm{SrTiO}}_3\left(001\right)$ by reactive molecular beam epitaxy. Soft and hard x-ray photoelectron spectroscopy measurements reveal a homogeneous cation distribution throughout the whole film with stoichiometric Ni:Fe ratios of 1:2 independent of the film thickness. Low energy electron diffraction and high resolution (grazing incidence) x-ray diffraction in addition to x-ray reflectivity experiments were conducted to obtain information of the film surface and bulk structure, respectively. For ultrathin films up to 7.3 nm, lateral tensile and vertical compressive strain is observed, contradicting an adaption at the interface of ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ film and substrate lattice. The applied strain is accompanied by an increased lateral defect density, which is decaying for relaxed thicker films and attributed to the growth of lateral grains. Determination of cationic site occupancies in the inverse spinel structure by analysis of site sensitive diffraction peaks reveals low tetrahedral occupancies for thin, strained ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ films, resulting in the partial presence of deficient rocksaltlike structures. These structures are assumed to be responsible for the enhanced magnetization of up to $\sim 250$% of the ${\mathrm{NiFe}}_2{\mathrm{O}}_4$ bulk magnetization as observed by superconducting quantum interference device magnetometry for ultrathin films below 7.3 nm thickness.

In this work an alternate pathway is demonstrated to form ultrathin cobalt ferrite (${\mathrm{Co}}_x{\mathrm{Fe}}_{3-x}{\mathrm{O}}_4$) films by interdiffusion of ${\mathrm{Fe}}_3{\mathrm{O}}_4$/CoO bilayers. Bilayer samples with different ${\mathrm{Fe}}_3{\mathrm{O}}_4$/CoO thickness ratios have been prepared by reactive molecular beam epitaxy on Nb-doped ${\mathrm{SrTiO}}_3$(001) substrates to obtain cobalt ferrite films of varied stoichiometry. Subsequently, oxygen-assisted postdeposition annealing experiments for consecutive temperature steps between $300{}^{\circ }\mathrm{C}$ and $600{}^{\circ }\mathrm{C}$ have been conducted monitoring the interdiffusion process by means of high-resolution x-ray reflectivity, soft and angle-resolved hard x-ray photoelectron, and x-ray absorption spectroscopy. Magnetic properties were characterized using superconducting quantum interference device magnetometry. The interdiffusion process starts from $300{}^{\circ }\mathrm{C}$ annealing temperature and is completed for temperatures above $500{}^{\circ }\mathrm{C}$. For completely interdiffused films with Co:Fe ratios larger than 0.84:2 a thin segregated CoO layer on top of the ferrite is formed. This CoO segregation is attributed to surface and interface effects. In addition, multiplet calculations of x-ray absorption spectra are performed to determine the occupancy of different sublattices. These results are correlated with the magnetic properties of the ferrite films. A stoichiometric ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ film with partial inversion has been formed exhibiting homogeneously distributed ${\mathrm{Co}}^{2+}$ and mainly ${\mathrm{Fe}}^{3+}$ valence states if the initial Co:Fe content is 1.09:2. Thus, for the formation of stoichiometric cobalt ferrite by the proposed postdeposition annealing technique an initial Co excess has to be provided as the formation of a top CoO layer is inevitable.

We present a comparative study of the morphology and structural as well as magnetic properties of crystalline Fe₃O₄/NiO bilayers grown on both MgO(001) and SrTiO₃(001) substrates by reactive molecular beam epitaxy. These structures were investigated by means of X-ray photoelectron spectroscopy, low-energy electron diffraction, X-ray reflectivity and diffraction, as well as vibrating sample magnetometry. While the lattice mismatch of NiO grown on MgO(001) was only 0.8%, it was exposed to a lateral lattice mismatch of −6.9% if grown on SrTiO₃. In the case of Fe₃O₄, the misfit strain on MgO(001) and SrTiO₃(001) amounted to 0.3% and −7.5%, respectively. To clarify the relaxation process of the bilayer system, the film thicknesses of the magnetite and nickel oxide films were varied between 5 and 20 nm. While NiO films were well ordered on both substrates, Fe₃O₄ films grown on NiO/SrTiO₃ exhibited a higher surface roughness as well as lower structural ordering compared to films grown on NiO/MgO. Further, NiO films grew pseudomorphic in the investigated thickness range on MgO substrates without any indication of relaxation, whereas on SrTiO₃ the NiO films showed strong strain relaxation. Fe₃O₄ films also exhibited strong relaxation, even for films of 5 nm thickness on both NiO/MgO and NiO/SrTiO₃. The magnetite layers on both substrates showed a fourfold magnetic in-plane anisotropy with magnetic easy axes pointing in $⟨100⟩$ directions. The coercive field was strongly enhanced for magnetite grown on NiO/SrTiO₃ due to the higher density of structural defects, compared to magnetite grown on NiO/MgO.