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.

In spin-transport experiments with spin currents propagating through an antiferromagnetic (AFM) material, the antiferromagnet is mainly treated as a passive spin conductor not generating nor adding any spin current to the system. The spin current transmissivity of the AFM NiO is affected by magnetic fluctuations, peaking at the Néel temperature and decreasing by lowering the temperature. To study the role of antiferromagnetism in local and nonlocal spin-transport experiments, we send spin currents through NiO of various thicknesses placed on Y 3 Fe 5 O 12 . The spin currents are injected either electrically or by thermal gradients and measured at a wide range of temperatures and magnetic field strengths. The transmissive role is reflected in the sign change of the local electrically injected signals and the decrease in signal strength of all other signals by lowering the temperature. The thermally generated signals, however, show an additional upturn below 100 K that is unaffected by an increased NiO thickness. A change in the thermal conductivity could affect these signals. The temperature and magnetic field dependence are similar to those for bulk NiO, indicating that NiO itself contributes to thermally induced spin currents.

We present time-resolved high energy x-ray diffraction (tr-HEXRD), time-resolved hard x-ray photoelectron spectroscopy (tr-HAXPES) and time-resolved grazing incidence small angle x-ray scattering (tr-GISAXS) data of the reactive molecular beam epitaxy (RMBE) of Fe3O4 ultrathin films on various substrates. Reciprocal space maps are recorded during the deposition of Fe3O4 on SrTiO3(001), MgO(001) and NiO/MgO(001) in order to observe the thickness-dependent evolution of Bragg reflections sensitive to the octahedral and tetrahedral sublattices of the inverse spinel structure of Fe3O4. Rock salt and spinel-exclusive reflections appear at different thicknesses, revealing that first, the iron oxide film grows with Fe 1-δ O rock salt structure with exclusive occupation of octahedral lattice sites. After reaching a film thickness of 1.1 nm, the further growth of the iron oxide film proceeds in the inverse spinel structure, with both octahedral and tetrahedral lattice sites being occupied. In addition, iron oxide on SrTiO3(001) initially grows with none of these structures.Here, the formation of the rock salt structure starts when reaching a film thickness of 1.5 nm. This is confirmed by tr-HAXPES data obtained during growth of iron oxide on SrTiO3(001), which demonstrate an excess of Fe 2+ cations compared to Fe3O4 in growing films thinner than 3.2 nm. This rock salt phase only appears during growth and vanishes after the supply of the Fe molecular beam is stopped. Thus, it can be concluded that the rock salt structure of the interlayer is a property of the dynamic growth process while the film is still exposed to oxygen. The tr-GISAXS data link these structural results to an island growth mode of the first 2 − 3 nm on both MgO(001) and SrTiO3(001) substrates.

In this work, we investigated the influence of oxygen plasma on the growth of nickel cobaltite (errortypeceNiCo2O4) thin films compared to growth in a molecular oxygen atmosphere. The films were grown on MgO (001), errortypeceMgAl2O4(001) and errortypeceSrTiO3(001) substrates by oxygen plasma (atmosphere of activated oxygen)-assisted and reactive molecular beam epitaxy (molecular oxygen atmosphere). Soft X-ray photoelectron spectroscopy showed that only the use of oxygen plasma led to a spectrum characteristic of errortypeceNiCo2O4. Low energy electron diffraction measurements were conducted to obtain information on the structure of the film surfaces. The results proved the formation of a spinel surface structure for films grown with oxygen plasma, while the formation of a rock salt structure was observed for growth with molecular oxygen. To determine the film thickness, X-ray reflectivity measurements were performed. If oxygen plasma were used to grow errortypeceNiCo2O4 films, this would result in lower film thicknesses compared to growth using molecular oxygen although the cation flux was kept constant during deposition. Additional X-ray diffraction experiments delivered structural information about the bulk structure of the film. All films had a rock salt bulk structure after exposure to ambient conditions. Angle-resolved hard X-ray photoelectron spectroscopy revealed a homogeneous depth distribution of cations of the grown film, but no typical errortypeceNiCo2O4 spectrum anymore. Thus, on the one hand, errortypeceNiCo2O4films with a spinel structure prepared using activated oxygen were not stable under ambient conditions. The structure of these films was transformed into NiCo oxide with a rock salt structure. On the other hand, it was not possible to form errortypeceNiCo2O4films using molecular oxygen. These films had a rock salt structure that was stable under ambient conditions.

Here, we present the (element-specific) magnetic properties and cation ordering for ultrathin Co-rich cobalt ferrite films. Two Co-rich CoxFe3−xO4 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/Fe3O4 bilayers deposited before on Nb-doped SrTiO3(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 Co3+ cations in the low-spin state on octahedral sites for the Co1.4Fe1.6O4 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.