In this work an alternate pathway is demonstrated to form ultrathin cobalt ferrite (Co x Fe 3−x O 4) films by interdiffusion of Fe 3 O 4 /CoO bilayers. Bilayer samples with different Fe 3 O 4 /CoO thickness ratios have been prepared by reactive molecular beam epitaxy on Nb-doped SrTiO 3 (001) substrates to obtain cobalt ferrite films of varied stoichiometry. Subsequently, oxygen-assisted postdeposition annealing experiments for consecutive temperature steps between 300 • C and 600 • 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 • C annealing temperature and is completed for temperatures above 500 • 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 CoFe 2 O 4 film with partial inversion has been formed exhibiting homogeneously distributed Co 2+ and mainly 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.

In order to explore an alternative pathway to prepare ultrathin CoFe2O4 films, epitaxial CoO/Fe3O4 bilayers with varying film thickness of the CoO film were grown on Nb-doped SrTiO3(001) substrates via reactive molecular beam epitaxy. Thereafter, cobalt ferrite films with varying stoichiometry were prepared by post-deposition annealing at different temperatures. The thermally mediated interdiffusion resulted in the formation of vertical compressive and lateral tensile strained Co x Fe3 – x O4 films (x = 0.6 – 1.4) with homogeneous distribution of Fe and Co cations for each film. The chemical and electronic variations after each annealing step were studied by means of soft and hard X-ray photoelectron spectroscopy. The homogeneity of the cation distributions in the films were additionally verified after the last annealing step by angle-resolved hard X-ray photoelectron spectroscopy. For the cobalt ferrite film with x = 1.4, an additional crystallographic phase of Co1 – y Fe y O was observed by (grazing incidence) X-ray diffraction measurements after annealing at 600 °C. X-ray reflectivity measurements were performed to determine the film thickness of the formed Co x Fe3 – x O4 films.

We report the parallel generation of close-packed ordered silane nanodot arrays with nanodot diameters of few 100 nm and nearest-neighbor distances in the one-micron range.

The magnetooptic Kerr effect (MOKE) is a well known and handy tool to characterize ferro-, ferri-and antiferromagnetic materials. Many of the MOKE techniques employ effects solely linear in magnetization M . Nevertheless, a higher-order term being proportional to M 2 and called quadratic MOKE (QMOKE) can additionally contribute to the experimental data. Here, we present detailed QMOKE spectroscopy measurements in the range of 0.8 -5.5 eV based on a modified 8-directional method applied on ferromagnetic bcc Fe thin films grown on MgO substrates. From the measured QMOKE spectra, two further complex spectra of the QMOKE parameters Gs and 2G44 are yielded. The difference between those two parameters, known as ∆G, denotes the strength of the QMOKE anisotropy. Those QMOKE parameters give rise to the QMOKE tensor G, fully describing the perturbation of the permittivity tensor in the second order in M for cubic crystal structures. We further present experimental measurements of ellipsometry and linear MOKE spectra, wherefrom permittivity in the zeroth and the first order in M are obtained, respectively. Finally, all those spectra are described by ab-initio calculations.

A detailed understanding of ultrathin film surface properties is crucial for the proper interpretation of spectroscopic, catalytic, and spin-transport data. We present x-ray magnetic circular dichroism (XMCD) and x-ray resonant magnetic reflectivity (XRMR) measurements on ultrathin Fe 3 O 4 films to obtain magnetic depth profiles for the three resonant energies corresponding to the different cation species Fe 2+ oct , Fe 3+ tet , and Fe 3+ oct located on octahedral and tetrahedral sites of the inverse spinel structure of Fe 3 O 4 . By analyzing the XMCD spectrum of Fe 3 O 4 using multiplet calculations, the resonance energy of each cation species can be isolated. Performing XRMR on these three resonant energies yields magnetic depth profiles that each correspond to one specific cation species. The depth profiles of both kinds of Fe 3+ cations reveal a (3.9 ± 1.0)-Å-thick surface layer of enhanced magnetization, which is likely due to an excess of these ions at the expense of the Fe 2+ oct species in the surface region. The magnetically enhanced Fe 3+ tet layer is additionally shifted about 2.9 ± 0.4 Å farther from the surface than the Fe 3+ oct layer.