arXiv subjects

Martina Müller

Publications and source records attributed to Martina Müller.

16 recordsLinked to original sources

arXiv2026

From Film to Flakes: Electronic Properties and Magnetization Variations in Yttrium Iron Garnet

Yttrium iron garnet (YIG) is a ferrimagnetic insulator valued for its high Curie temperature, very low magnetic damping, and ability to support long-range spin-wave transport. These qualities have established it as a cornerstone material in the field of spintronics and magnonics. Most studies on YIG so far have been focused on bulk crystals, thin films, and nanoparticles, including variants with substitutions at the yttrium or iron site. New morphologies such as sub-micron flakes have drawn interest recently as their geometry and mechanical flexibility might enable different device architectures. However, detailed investigations combining their electronic structure and magnetic behavior remain scarce. In this work, we present a comparative study of the electronic and magnetic properties of a bulk-like YIG film and a sub-micron-sized YIG flake. Our results highlight the distinct behavior that emerges in sub-micron dimensions and point toward future uses for flake-based YIG in compact spintronics devices.

cond-mat.mtrl-sci↗

arXiv2025

Ge$_{1-x}$Si$_{x}$ single crystals for Ge hole spin qubit integration

Spin qubits are fundamental building blocks of modern quantum computing devices. The path of Ge-based hole-spin qubits has several advantages over Si-based electron-spin systems, such as the absence of valley band degeneracy, the possibility of efficient field control due to large spin-orbit coupling, and smaller effective masses. Among the possible Ge qubit devices, Ge/GeSi planar heterostructures have proven to be favourable for upscaling and fabrication. The Si concentration of the straining GeSi buffer serves as an important tuning parameter for the electronic structure of Ge/GeSi qubits. A particularly low Si concentration of x = 0.15 of the Ge$_{0.85}$Si$_{0.15}$ crystal should enable minimal lattice strain for spin qubit heterostructures, which is difficult to stabilize as a random alloy. We present a synchrotron-based study to investigate the chemical composition, valence band electronic structure and local atomic structure of a Ge$_{0.85}$Si$_{0.15}$ single crystal using the advanced combination of hard X-ray photoelectron spectroscopy (HAXPES), hard X-ray momentum microscopy (HarMoMic) and X-ray photoelectron diffraction (XPD). We found that the Ge$_{0.85}$Si$_{0.15}$ crystal has an individual, uniform valence band structure, with no signs of phase separation. The shapes of the valence bands resemble those of pure Ge, as do the low effective masses. XPD experiments and Bloch wave calculations, show the Si atoms located at Ge lattice sites within the crystal, forming a random alloy. This high chemical, electronic and structural quality of Ge$_{0.85}$Si$_{0.15}$ single-crystal substrates is of crucial importance for their implementation to enable long spin lifetimes in Ge-based hole-spin qubits. The results emphasise the power of combined X-ray spectromicroscopy techniques, which provide key insights into the qubit building blocks that form the basis of quantum technologies.

cond-mat.mtrl-sci↗

arXiv2025

Ferroelectricity of Wurtzite Al$_{1-x}$Hf$_{x}$N Heterovalent Alloys

Thin films of aluminum hafnium nitride (Al$_{1-x}$Hf$_{x}$N) 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 d$_{33,f,meas}$ 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.

cond-mat.mtrl-sci↗

arXiv2024

Spin polarization of the two-dimensional electron gas at the EuO/SrTiO$_3$ interface

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.

cond-mat.mtrl-sci↗

arXiv2024

Long-term stability and oxidation of ferroelectric AlScN devices: An operando HAXPES study

Aluminum scandium nitride (Al$_{1-x}$Sc$_x$N) 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 towards oxidation. In the present study, the oxidation process of tungsten-capped and uncapped Al$_{0.83}$Sc$_{0.17}$N thin films is investigated by hard X-ray photoelectron spectroscopy (HAXPES). The samples had been exposed to air for either two 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 N$_2$ spectral feature thus can be directly related to the oxidation process. We present an oxidation model that mimics these spectroscopic results of the element-specific oxidation processes within Al$_{1-x}$Sc$_x$N. Finally, in operando HAXPES data of uncapped and capped AlScN-capacitor stacks are interpreted using the proposed model.

cond-mat.mtrl-sci↗

arXiv2024

Spatial mapping of magnetization fluctuations via time-lapse Kerr microscopy

Combining wide-field magneto-optical Kerr microscopy with a time-lapse analysis scheme allows investigating magnetization fluctuations with high spatial as well as temporal resolution. We here use this technique to study magnetization fluctuations in a thin ferromagnetic film prepared into a quasi-equilibrium magnetic state via a dedicated field sweep protocol. Our experiments reveal spatially localized noise hotspots distributed across the sample surface within the magnetic domains in the quasi-equilibrium state. The spatial density of the noise hot spots is very similar at different low magnetic field strengths, as expected from the fluctuation-dissipation theorem. The measurement scheme thus opens the way for the spatially resolved investigation of quasi-equilibrium noise processes in magnetic materials.

cond-mat.mes-hall↗

arXiv2024

Single-crystalline YIG flakes with uniaxial in-plane anisotropy and diverse crystallographic orientations

We study 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 nanoflakes 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 devices.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic domains in ultrathin, bulk-like and proximity-coupled Europium Oxide

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.

cond-mat.mtrl-sci↗

arXiv2024

2D synthetic ferrimagnets by magnetic proximity coupling

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.

cond-mat.mtrl-sci↗

arXiv2023

Tunable 2D Electron- and 2D Hole States Observed at Fe/SrTiO$_3$ Interfaces

Oxide electronics provide the key concepts and materials for enhancing silicon-based semiconductor technologies with novel functionalities. However, a basic but key property of semiconductor devices still needs to be unveiled in its oxidic counterparts: the ability to set or even switch between two types of carriers - either negatively (n) charged electrons or positively (p) charged holes. Here, we provide direct evidence for individually emerging n- or p-type 2D band dispersions in STO-based heterostructures using resonant photoelectron spectroscopy. The key to tuning the carrier character is the oxidation state of an adjacent Fe-based interface layer: For Fe and FeO, hole bands emerge in the empty band gap region of STO due to hybridization of Ti and Fe-derived states across the interface, while for Fe$_3$O$_4$ overlayers, an 2D electron system is formed. Unexpected oxygen vacancy characteristics arise for the hole-type interfaces, which as of yet had been exclusively assigned to the emergence of 2DESs. In general, this finding opens up the possibility to straightforwardly switch the type of conductivity at STO interfaces by the oxidation state of a redox overlayer. This will extend the spectrum of phenomena in oxide electronics, including the realization of combined n/p-type all-oxide transistors or logic gates.

cond-mat.mtrl-sci↗

arXiv2022

Temperature-dependent spin-resolved electronic structure of EuO thin films

The electronic structure of the ferromagnetic semiconductor EuO is investigated by means of spin- and angle-resolved photoemission spectroscopy (spin-ARPES) and density functional theory. EuO exhibits unique properties of hosting both weakly-dispersive nearly fully polarized Eu $4f$ bands, as well as O $2p$ levels indirectly exchange-split by the interaction with Eu nearest neighbors. Our temperature-dependent spin-ARPES data directly demonstrates the exchange splitting in O $2p$ and its vanishing at the Curie temperature. Our calculations with a Hubbard $U$ term reveal a complex nature of the local exchange splitting on the oxygen site and in conduction bands. We discuss the mechanisms of the indirect exchange in the O 2p levels by analyzing orbital-resolved band characters in ferromagnetic and antiferromagnetic phases. The directional effects due to spin-orbit coupling are predicted theoretically to be significant in particular in the Eu 4f band manifold. The analysis of the shape of spin-resolved spectra in the Eu $4f$ spectral region reveals signatures of hybridization with O $2p$, in agreement with the theoretical predictions. We also analyze spectral changes in the spin-integrated spectra throughout the Curie temperature and demonstrate they derive from both the magnetic phase transition and effects due to sample aging, unavoidable for this highly reactive material.

cond-mat.mtrl-sci↗

arXiv2021

Quantifying the spin mixing conductance of EuO/W heterostructures by spin Hall magnetoresistance experiments

The spin Hall magnetoresistance (SMR) allows to investigate the magnetic textures of magnetically ordered insulators in heterostructures with normal metals by magnetotransport experiments. We here report the observation of the SMR in in-situ prepared ferromagnetic EuO/W thin film bilayers with magnetically and chemically well-defined interfaces. We characterize the magnetoresistance effects utilizing angle-dependent and field-dependent magnetotransport measurements as a function of temperature. Applying the established SMR model, we derive and quantify the real and imaginary parts of the complex spin mixing interface conductance. We find that the imaginary part is by one order of magnitude larger than the real part. Both decrease with increasing temperature. This reduction is in agreement with thermal fluctuations in the ferromagnet.

cond-mat.mes-hall↗

arXiv2019

Redox-controlled epitaxy and magnetism of oxide heterointerfaces: EuO/SrTiO$_3$

We demonstrate a novel route to prepare thin films of the ferromagnetic insulator Europium monoxide. Key is a redox-controlled interface reaction between metallic Eu and the substrate SrTiO$_3$ as the supplier of oxygen. The process allows tuning the electronic, magnetic and structural properties of the EuO films. Furthermore, we apply this technique to various oxidic substrates and demonstrate the universality and limits of a redox-controlled EuO film synthesis.

cond-mat.mtrl-sci↗

arXiv2017

Two-dimensional electron system at the magnetically tunable EuO/SrTiO$_3$ interface

We create a two-dimensional electron system (2DES) at the interface between EuO, a ferromagnetic insulator, and SrTiO3, a transparent non-magnetic insulator considered the bedrock of oxide-based electronics. This is achieved by a controlled in-situ redox reaction between pure metallic Eu deposited at room temperature on the surface of SrTiO3, an innovative bottom-up approach that can be easily generalized to other functional oxides and scaled to applications. Additionally, we find that the resulting EuO capping layer can be tuned from paramagnetic to ferromagnetic, depending on the layer thickness. These results demonstrate that the simple, novel technique of creating 2DESs in oxides by deposition of elementary reducing agents [T. C. Rödel et al., Adv. Mater. 28, 1976 (2016)] can be extended to simultaneously produce an active, e.g. magnetic, capping layer enabling the realization and control of additional functionalities in such oxide-based 2DESs.

cond-mat.mtrl-sci↗

arXiv2015

Functional Materials for Information and Energy Technology: Insights by Photoelectron Spectroscopy

The evolution of both information and energy technology is intimately connected to complex condensed matter systems, the properties of which are determined by electronic and chemical interactions and processes on a broad range of length and time scales. Dedicated photoelectron spectroscopy and spectromicroscopy experiments can provide important insights. We discuss some recent methodological developments with application to relevant questions in spintronics, and towards in-operando studies of resistive switching and electrochemical processes.

cond-mat.mtrl-sci↗

arXiv2014

Enhanced ferrimagnetism in auxetic NiFe2O4 in the crossover to the ultrathin film limit

We investigate the sensitive interplay between magnetic, electronic and structural properties in the ferrimagnetic oxide NiFe2O4. Emphasis is placed on the impact of reduced dimensionality in the crossover from bulk-like to ultrathin films. We observed an enhanced saturation magnetization $M_S$ for ultrathin NiFe2O4 films on Nb-SrTiO3 (001) substrates that co-occurs with a reduced out-of-plane lattice constant under compressive in-plane epitaxial strain. We found a bulk-like cationic coordination of the inverse spinel lattice independent of the NiFe2O4 film thickness -- thus ruling out a cationic inversion that nominally could account for an enhanced $M_S$. Our study instead uncovers a reduction of the unit cell volume, i.e. an auxetic behavior in ultrathin NiFe2O4 films, which may result in an enhanced magnetic exchange caused by an increased interatomic electronic localization.

cond-mat.mtrl-sci↗