Martina Müller

The control of electron spins in materials that are simultaneously ferromagnetic and insulating opens up a wealth of quantum phenomena in spin-based electronics. Thin films of europium oxide (EuO) are ideal for the generation and manipulation of spin-polarized states, but so far, there are no experimental literature reports on the magnetic domain patterns of EuO. However, at these microscopic length scales, magnetic relaxation between the remanent and demagnetized states takes place in any spintronic device. This relaxation process involves the displacement of magnetic domain walls and can therefore be strongly influenced by the film's structure and thickness. Here, we present an investigation of the temperature-dependent behavior of magnetic domains and hysteresis in bulk-like (25 nm) and ultrathin (3 nm) EuO films. Magneto-optical Kerr microscopy is used, a technique that is a valuable tool for exploring microscopic features such as spin dynamics and magnetic domain walls. Significant Kerr rotation in EuO led to high-contrast magnetic domain images in thick films, facilitating the observation of domain dynamics. The Curie temperature ( T C ) and coercivity show thickness-dependent variations. The analysis and comparison of hysteresis loops and domain imaging in EuO and EuO/Co reveal proximity effect-induced antiferromagnetic coupling of both layers. To elucidate the magnetization reversal dynamics in EuO, micromagnetic simulations using MuMax 3 were performed below and above T C . This comprehensive approach was designed to comprehend the impact of magnetism and magnetic proximity effects in EuO on the micromagnetic scale, potentially extending its magnetic ordering beyond T C .

Thin films of aluminum hafnium nitride (Al1−xHfxN) were synthesized via reactive magnetron sputtering for Hf contents up to x = 0.13. X-ray diffraction showed a single c-axis oriented wurtzite phase for all films. Hard x-ray photoelectron spectroscopy demonstrated homogeneous Al:Hf distribution through the thin films and confirmed their insulating character. A collection of complementary tests showed unambiguous polarization inversion, and thus ferroelectricity in multiple samples. Current density vs electric field hysteresis measurements showed distinct ferroelectric switching current peaks, the piezoelectric coefficient d33,f,meas measured using a double beam laser interferometer (DBLI) showed a reversal in sign with similar magnitude, and anisotropic wet etching confirmed field-induced polarization inversion. This demonstrates the possibility of using tetravalent–and not just trivalent–alloying elements to enable ferroelectricity in AlN-based thin films, highlighting the compositional flexibility of ferroelectricity in wurtzites and greatly expanding the chemistries that can be considered for future devices.

High electron mobility transistors (HEMT) based on Al_1‐xSc_xN/GaN and Al_1‐xY_xN/GaN heterostructures promise increased device performance and reliability due to the high sheet charge carrier density and the possibility to grow strain‐free layers on GaN. Metal–organic chemical vapor deposition (MOCVD) offers high throughput, high structural quality, and good electrical characteristics. The growth of GaN layers on Al_1‐xSc_xN and Al_1‐xY_xN is challenging, but at the same time crucial as passivation or for multichannel structures. GaN is observed to grow three‐dimensionally on these nitrides, exposing not‐passivated areas to surface oxidation. In this work, growth of 2–20 nm‐thick, two‐dimensional GaN layers is demonstrated. Optimization of growth conditions is enabled by understanding island formation on the atomic scale by aberration corrected scanning transmission electron microscopy (STEM) and hard X‐ray photoelectron spectroscopy (HAXPES). Increased growth temperature, an AlN interlayer, low supersaturation conditions and the carrier gas are found to be key to enhance Ga adatom mobility. Growth of single crystalline GaN layers on Al_1‐xSc_xN and Al_1‐xY_xN is unlocked and prevents oxidation of the underlying layers. Few nanometer thick GaN caps allow for depositing the gate metallization directly on the cap, whereas thicker ones allow for the growth of heterostructures for normally‐off devices and multichannel structures.

Spin-polarized two-dimensional electron gases (2DEGs) are of particular interest for functional oxide electronics applications. The redox-created 2DEG residing on the strontium titanate, SrTiO 3 (STO), side of a europium monoxide ( EuO ) / SrTiO 3 (001) interface is expected to be significantly spin polarized due to the proximity to the strong ( 7 μ B / f . u . ) Heisenberg ferromagnet EuO. We apply magnetic circular dichroism in the angular distribution (MCDAD) of photoemitted electrons to investigate whether and how the induced spin polarization of the 2DEG depends on the dimensionality of the overlaying EuO layer. The experimental data are complemented by density functional theory calculations with a Hubbard U term ( DFT + U ). We show that the EuO/STO interfacial 2DEG is spin polarized even for ultrathin EuO overlayers, starting at an EuO threshold thickness of only two monolayers. Additional EuO monolayers even increase the induced magnetic Ti moment and thus the spin polarization of the 2DEG. Our results and the potential to enhance the magnetic order of EuO by other proximity effects indicate that the EuO/STO(001) interface is an ideal template for creating (multi-)functional spin-polarized 2DEGs for application in oxide electronics. Published by the American Physical Society 2025

Aluminum scandium nitride (Al_1−xSc_xN) is a promising material for ferroelectric devices due to its large remanent polarization, scalability, and compatibility with semiconductor technology. By doping AlN with Sc, the bonds in the polar AlN structure are weakened, which enables ferroelectric switching below the dielectric breakdown field. However, one disadvantage of Sc doping is that it increases the material's tendency toward oxidation. Herein, the oxidation process of tungsten‐capped and uncapped Al_0.83Sc_0.17N thin films is investigated by hard X‐ray photoelectron spectroscopy (HAXPES). The samples is exposed to air for either 2 weeks or 6 months. HAXPES spectra indicate the replacement of nitrogen by oxygen and the tendency of oxygen to favor oxidation with Sc rather than Al. The appearance of an N₂ spectral feature thus can be directly related to the oxidation process. An oxidation model that mimics these spectroscopic results of the element‐specific oxidation processes within Al_1−xSc_xN is presented. Finally, in operando HAXPES data of uncapped and capped AlScN‐capacitor stacks are interpreted using the proposed model.

The control of electron spins in materials that are simultaneously ferromagnetic and insulating opens up a wealth of quantum phenomena in spin-based electronics. Thin films of europium oxide (EuO) are ideal for the generation and manipulation of spin-polarized states, but so far there are no experimental literature reports on the magnetic domain patterns for EuO. However, at these microscopic length scales, magnetic relaxation between the remanent and demagnetized states takes place in any spintronic device. This relaxation process involves displacements of magnetic domain walls and can therefore be strongly influenced by the film structure and thickness. Here we present an investigation of the temperature-dependent behavior of magnetic domains and hysteresis in bulk-like (25 nm) and ultrathin (3 nm) EuO films. Magneto-optical Kerr microscopy is used, a technique that is a valuable tool to explore microscopic features such as spin dynamics and magnetic domain walls. Significant Kerr rotation in EuO led to high-contrast magnetic domain images in thick films, facilitating observation of domain dynamics. The critical temperature (TC) and coercivity shows strong thickness-dependent variations. The analysis and comparison of hysteresis loops and domain imaging in EuO and EuO/Co reveal proximity effect-induced antiferromagnetic coupling of both layers. To elucidate the magnetization reversal dynamics in EuO, micromagnetic simulations using MuMax3 were performed below and above TC. This comprehensive approach aims to comprehend the impact of magnetism and magnetic proximity effect in EuO on the micromagnetic scale, potentially extending its magnetic ordering beyond TC.

Proximity effects allow for the adjustment of magnetic properties in a physically elegant way. If two thin ferromagnetic (FM) films are brought into contact, electronic coupling alters their magnetic exchange interaction at their interface. For a low-TC rare-earth FM coupled to a 3d transition metal FM, even room temperature magnetism is within reach. In addition, magnetic proximity coupling is particularly promising for increasing the magnetic order of metastable materials such as europium monoxide (EuO) beyond their bulk TC, since neither the stoichiometry nor the insulating properties are modified. We investigate the magnetic proximity effect at Fe/EuO and Co/EuO interfaces using hard X-ray photoelectron spectroscopy. By exciting the FM layers with circularly polarized light, magnetic dichroism is observed in angular dependence on the photoemission geometry. In this way, the depth-dependence of the magnetic signal is determined element-specifically for the EuO and 3d FM parts of the bilayers. In connection with atomistic spin dynamics simulations, the thickness of EuO layer is found to be crucial, indicating that the observed antiferromagnetic proximity coupling is a short-ranged and genuine interface phenomenon. This fact turns the bilayer into a strong synthetic ferrimagnet. The increase in magnetic order in EuO occurs in a finite spatial range and is therefore particularly strong in the 2D limit-a counterintuitive but very useful phenomenon for spin-based device applications.

We study sub-micron Y3Fe5O12 (YIG) flakes that we produce via mechanical cleaving and exfoliation of YIG single crystals. By characterizing their structural and magnetic properties, we find that these YIG flakes have surfaces oriented along unusual crystallographic axes and uniaxial in-plane magnetic anisotropy due to their shape, both of which are not commonly available in YIG thin films. These physical properties, combined with the possibility of picking up the YIG flakes and stacking them onto flakes of other van der Waals materials or pre-patterned electrodes or waveguides, open unexplored possibilities for magnonics and for the realization of novel YIG-based heterostructures and spintronic devices.