We have clarified the origin of a magnetically dead interface layer formed in yttrium iron garnet (YIG) films grown at above 700 °C onto a gadolinium gallium garnet (GGG) substrate by means of laser molecular beam epitaxy. The diffusion-assisted formation of a Ga-rich region at the YIG/GGG interface is demonstrated by means of composition depth profiling performed by x-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and x-ray and neutron reflectometry. Our finding is in sharp contrast to the earlier expressed assumption that Gd acts as a migrant element in the YIG/GGG system. We further correlate the presence of a Ga-rich transition layer with considerable quenching of ferromagnetic resonance and spin wave propagation in thin YIG films. Finally, we clarify the origin of the enigmatic low-density overlayer that is often observed in neutron and x-ray reflectometry studies of the YIG/GGG epitaxial system.

Hybridization of semiconducting and magnetic materials into a single heterostructure is believed to be potentially applicable to the design of functional spintronic devices. In the present work we report epitaxial stabilization of four magnetically ordered iron oxide phases (${\mathrm{Fe}}_3{\mathrm{O}}_4, \gamma -{\mathrm{Fe}}_2{\mathrm{O}}_3, \alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$ and most exotic metastable $\varepsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$) in the form of nanometer-sized single crystalline films on GaN(0001) surface. The epitaxial growth of four distinctly different iron oxide phases is achieved by a single-target pulsed laser deposition technology on a GaN semiconductor substrate widely used for electronic device fabrication. The discussed iron oxides belong to a family of simple formula magnetic materials exhibiting a rich variety of outstanding magnetic properties including peculiar Verwey and Morin phase transitions in ${\mathrm{Fe}}_3{\mathrm{O}}_4$ and $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$ and multiferroic behavior in metastable magnetically hard $\varepsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$ ferrite. The physical reasons standing behind the nucleation of a particular phase in an epitaxial growth process is discussed in the present paper deserving interest from the fundamental point of view. The practical side of the presented study is to exploit the tunable polymorphism of iron oxides in order to create the ferroic-on-semiconductor heterostructures usable in spintronic devices. By application of a wide range of experimental techniques the surface morphology, crystalline structure and electronic and magnetic properties of the single phase iron oxide epitaxial films on GaN have been studied. A comprehensive comparison has been made to the properties of the same ferrite materials in the bulk and nanostructured form reported by other research groups.

The metastable ε-Fe₂O₃ is known to be the most intriguing ferrimagnetic and multiferroic iron oxide phase exhibiting a bunch of exciting physical properties both below and above room temperature. The present paper unveils the structural and magnetic peculiarities of a few nm thick interface layer discovered in these films by a number of techniques. The polarized neutron reflectometry data suggests that the interface layer resembles GaFeO₃ in composition and density and is magnetically softer than the rest of the ε-Fe₂O₃ film. While the in-depth density variation is in agreement with the transmission electron microscopy measurements, the layer-resolved magnetization profiles are qualitatively consistent with the unusual wasp-waist magnetization curves observed by superconducting quantum interference device magnetometry. Interestingly a noticeable Ga diffusion into the ε-Fe₂O₃ films has been detected by secondary ion mass spectroscopy providing a clue to the mechanisms guiding the nucleation of exotic metastable epsilon ferrite phase on GaN at high growth temperature and influencing the interfacial properties of the studied films.

In this work epitaxial growth of cobalt on CaF₂(111), (110) and (001) surfaces has been extensively studied. It has been shown by atomic force microscopy that at selected growth conditions stand-alone faceted Co nanoparticles are formed on a fluorite surface. Grazing-incidence X-ray diffraction (GIXD) and reflection high-energy electron diffraction (RHEED) studies have revealed that the particles crystallize in the face-centered cubic lattice structure otherwise non-achievable in bulk cobalt under normal conditions. The particles were found to inherit lattice orientation from the underlying CaF₂ layer. Three-dimensional reciprocal space mapping carried out using X-ray and electron diffraction has revealed that there exist long bright 〈111〉 streaks passing through the cobalt Bragg reflections. These streaks are attributed to stacking faults formed in the crystal lattice of larger islands upon coalescence of independently nucleated smaller islands. Distinguished from the stacking fault streaks, crystal truncation rods perpendicular to the {111} and {001} particle facets have been observed. Finally, grazing-incidence small-angle X-ray scattering (GISAXS) has been applied to decouple the shape-related scattering from that induced by the crystal lattice defects. Particle faceting has been verified by modeling the GISAXS patterns. The work demonstrates the importance of three-dimensional reciprocal space mapping in the study of epitaxial nanoparticles.

In the present paper we discuss correlations between crystal structure and magnetic properties of epitaxial ε-Fe₂O₃ films grown on GaN. The large magnetocrystalline anisotropy and room temperature multiferroic properties of this exotic iron oxide polymorph, make it a perspective material for the development of low power consumption magnetic media storage devices. Extending our recent progress in PLD growth of ε-Fe₂O₃ on the surface of technologically important nitride semiconductors, we apply reciprocal space tomography by electron and x-ray diffraction to investigate the break of crystallographic symmetry occurring at the oxide-nitride interface resulting in the appearance of anisotropic crystallographic disorder in the sub-100 nm ε-Fe₂O₃ films. The orthorhombic-on-hexagonal nucleation scenario is shown responsible for the development of a peculiar columnar structure observed in ε-Fe₂O₃ by means of HRTEM and AFM. The complementary information on the direct and reciprocal space structure of the columnar ε-Fe₂O₃ films is obtained by various techniques and correlated to their magnetic properties. The peculiar temperature dependence of magnetization studied by the small-field magnetization derivative method and by neutron diffraction reveals the existence of a magnetic softening below 150 K, similar to the one observed earlier solely in nanoparticles. The magnetization reversal in ε-Fe₂O₃ films probed by X-ray magnetic circular dichroism is found different from the behavior of the bulk averaged magnetization measured by conventional magnetometry. The presented results fill the gap between the numerous studies performed on randomly oriented ε-Fe₂O₃ nanoparticles and much less frequent investigations of epitaxial epsilon ferrite films with lattice orientation fixed by the substrate. Graphical abstract