Jannis Thien

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.

Ultrathin Co${}_x$Fe${}_{3-x}$O${}_4$ films of high structural quality and with different Co content (x = 0.6–1.2) were prepared by reactive molecular beam epitaxy on MgO(001) substrates. Epitaxy of these ferrite films is extensively monitored by means of time-resolved (operando) X-ray diffraction recorded in out-of-plane geometry to characterize the temporal evolution of the film structure. The Co ferrite films show high crystalline ordering and smooth film interfaces independent of their Co content. All Co${}_x$Fe${}_{3-x}$O${}_4$ films exhibit enhanced compressive out-of-plane strain during the early stages of growth, which partly releases with increasing film thickness. When the Co content of the ferrite films increases, the vertical-layer distances increase, accompanied by slightly increasing film roughnesses. The latter result is supported by surface-sensitive low-energy electron diffraction as well as X-ray reflectivity measurements on the final films. In contrast, the substrate–film interface roughness decreases with increasing Co content, which is confirmed with X-ray reflectivity measurements. In addition, the composition and electronic structure of the ferrite films is characterized by means of hard X-ray photoelectron spectroscopy performed after film growth. The experiments reveal the expected increasing ${\mathrm{Fe}}^{3+}$/${\mathrm{Fe}}^{2+}$ cation ratios for a higher Co content.

We present time-resolved high-energy x-ray diffraction, 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 of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ ultrathin films on various substrates. Reciprocal space maps are recorded during the deposition of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ on ${\mathrm{SrTiO}}_3\left(001\right)$, 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 ${\mathrm{Fe}}_3{\mathrm{O}}_4$. Rock salt and spinel-exclusive reflections appear at different thicknesses, revealing that first, the iron oxide film grows with ${\mathrm{Fe}}_{1-\delta }\mathrm{O}$ rock salt structure with exclusive occupation of octahedral lattice sites. After reaching a film thickness of $1.1\mathrm{nm}$, 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 ${\mathrm{SrTiO}}_3\left(001\right)$ initially grows with none of these structures. Here, the formation of the rock salt structure starts when reaching a film thickness of $1.5\mathrm{nm}$. This is confirmed by tr-HAXPES data obtained during growth of iron oxide on ${\mathrm{SrTiO}}_3\left(001\right)$, which demonstrate an excess of ${\mathrm{Fe}}^{2+}$ cations compared to ${\mathrm{Fe}}_3{\mathrm{O}}_4$ in growing films thinner than $3.2\mathrm{nm}$. 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 ${\mathrm{SrTiO}}_3\left(001\right)$ substrates.

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.