arXiv subjects

Martin Wortmann

Publications and source records attributed to Martin Wortmann.

7 recordsLinked to original sources

arXiv2026

Stable Asymmetric Magnetization Reversal in Epitaxial Co(001)/CoO(001) Bilayer

The exchange bias (EB) in ferromagnetic/antiferromagnetic (FM/AFM) bilayer systems causes a shift of the magnetic hysteresis curve after field cooling through the N\'eel temperature of the AFM. In some cases, this shift is accompanied by an asymmetry between ascending and descending branches. In the past, this asymmetric magnetization reversal has been studied in different bilayer systems, including polycrystalline Co/CoO thin films. Here we investigate the asymmetric magnetization reversal in an epitaxial fcc-Co(001)/CoO(001) thin film grown on MgO(001) by molecular beam epitaxy at varying temperatures up to room temperature after field cooling along the easy and the hard axes. Room temperature measurements of the longitudinal and transverse magneto-optic Kerr effect show different magnetization reversal processes via stable intermediate states for different angles between external magnetic field and magnetic easy axes. Once the sample is cooled below the blocking temperature, a pronounced asymmetric magnetization reversal can be observed. We show that in contrast to polycrystalline bilayers, the loop asymmetry stays constant after multiple training cycles and that the magnitude of the asymmetry is directly correlated with the magnitude of the EB.

cond-mat.mtrl-sci↗

arXiv2026

Cubic magneto-optic Kerr effect in Co(111) thin films

The magneto-optic Kerr effect (MOKE) is often applied as a tool for the magnetic characterization of thin films. Here, the change in polarization upon reflection from the magnetized sample is mainly regarded as being linearly proportional to the magnetization $\mathbf{M}$ (LinMOKE). MOKE contributions of second order in $\mathbf{M}$, also known as quadratic MOKE (QMOKE), which are proportional to $\mathbf{M}^2$, have also been studied in the past and used in thin film characterization. Recently, we reported on a systematic investigation of third-order MOKE contributions, named cubic MOKE (CMOKE) in Ni(111) thin films. This CMOKE manifests itself as an anisotropic contribution to the MOKE signal (with regard to the crystallographic orientation) measured in longitudinal or transversal configuration in full magnetic saturation. While LinMOKE (odd in $\mathbf{M}$) and QMOKE (even in $\mathbf{M}$) can easily be separated by methods based on magnetization parity, this no longer holds true for LinMOKE and CMOKE (odd in $\mathbf{M}$). It is therefore crucial to be aware of CMOKE contributions in order to correctly interpret MOKE data. Here, we report on the observation of CMOKE in thin film heterostructures with structurally twinned and untwinned Co(111) layers, demonstrating that a large CMOKE is not only present in Ni thin films. Additionally, we show that the observed anisotropic contributions cannot stem from LinMOKE by analyzing their dependence on the angle of incidence (AoI) of light. While the QMOKE is almost vanishing in Co(111) using light with wavelengths of 635\,nm and 406\,nm, the CMOKE contributions reach up to about 30\% of the LinMOKE contribution at an AoI of 45 degrees and become even more dominant towards normal AoI, which emphasizes the importance of higher-order MOKE effects in magneto-optic experiments.

physics.optics↗

arXiv2026

NiTi Single Crystal Growth by Micro-Pulling-Down Method: Experimental Setup and Material Characterization

Nickel-titanium that has an austenite to martensite phase transition has been studied extensively in the past as a shape memory alloy, but a lot remains to be learned from such phase transitions. However, single crystals are needed for a detailed characterization of the emerging phase transition. In order to produce NiTi single crystals for research purposes, we have set up a micro-pulling-down ($\mu$PD) apparatus. The $\mu$PD process is a fast and flexible method for the fabrication of small single crystals. The apparatus is operated in vacuum. By pulling the crystal down through a hole in the crucible bottom, it is possible to reduce oxygen contamination, since oxides float on top of the melt due to their low density. Here we present a detailed characterization of as-grown NiTi crystals by electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), hot gas extraction method and differential scanning calorimetry (DSC). The characteristics of the phase transition in NiTi are very sensitive to dopants and alloying. The $\mu$PD method facilitates the introduction of different doping elements into the crystal.

cond-mat.mtrl-sci↗

arXiv2025

Fostering Innovation: Streamlining Magnetocaloric Materials Research by Digitalization

Refrigeration based on the magnetocaloric effect (MCE) can contribute to energysaving, environmentally friendly cooling in private households, or industrial application. The cooling is based on the reversible heat release or uptake during a phase-transformation of the materials that can be controlled by a magnetic field. This process could replace conventional compression-based refrigeration, which often relies on environmentally harmful refrigerants. Here we show, how to digitalize the process chain for the synthesis, theoretical and experimental characterization, and prototypical application of magnetocaloric alloy. Different Heusler alloys are examined experimentally as model systems for potential application in magnetic cooling. OTTR templates are used for the acquisition and semantic representation of knowledge in the development of an ontology. The ontology, when combined with unstructured data, can be exploited to train a model that can then be used to predict missing facts, which can help to gain new insights and to generate new hypotheses. Furthermore, tools are developed that automate data acquisition into ontological structures and workflows are implemented that provide an easy-to-use theoretical and experimental evaluation of the MCE from first principles and raw data.

cond-mat.mtrl-sci↗

arXiv2025

Depth Profiling of Oxygen Migration in Ta/HfO2 Stacks During Ionic Liquid Gating

Ionic liquid (IL) gating has emerged as a powerful tool to control the structural, electronic, optical, and magnetic properties of materials by driving ion motion at solid interfaces. In magneto-ionic systems, electric fields are used to move ions, typically oxygen, from a donor layer into an underlying magnetic metal. Although oxygen distribution is key to enabling precise and stable control in magneto-ionic systems, the spatial distribution and voltage-dependence of oxygen incorporation in such nanoscale stacks remain unknown. Here, we quantify oxygen depth profiles and oxide formation in Si/ SiO2/ Ta (15)/ HfO2 (t) films after IL gating as a function of the gate voltage and HfO2 capping thickness (2 and 3 nm). X-ray reflectivity and X-ray photoelectron spectroscopy measurements revealed a threshold electric field of ~ -2.8 MV/cm to initiate oxygen migration from HfO2 into metallic Ta. The resulting Ta2O5 thickness increases linearly with gate voltage, reaching up to 4 nm at -3 V gating. Notably, the required electric field rises with oxide thickness, indicating a progressively growing barrier for thicker oxide films. The Ta/Ta2O5 interface remains atomically sharp for all gate voltages. This suggests that complete Ta2O5 layers form sequentially before further oxygen penetration, with no sign of deeper diffusion into bulk Ta. Thinner capping layers enhance oxidation, relevant for optimized stack design. Additionally, indium migration from the indium tin oxide electrode to the sample surface was observed, which should be considered for surface-sensitive applications. These insights advance design principles for magneto-ionic and nanoionic devices requiring precise interface engineering.

cond-mat.mtrl-sci↗

arXiv2025

Surface Oxidation of SnTe Analyzed by Self-Consistent Fitting of all Emission Peaks in its X-ray Photoelectron Spectrum

X-ray photoelectron spectroscopy (XPS) is among the most widely used methods for surface characterization. Currently, the analysis of XPS data is almost exclusively based on the main emission peak of a given element and the rest of the spectrum is discarded. This makes quantitative chemical state analyses by peak fitting prone to substantial error, especially in light of incomplete and flawed reference literature. However, most elements give rise to multiple emission peaks in the x-ray energy range, which are virtually never analyzed. For samples with an inhomogeneous depth distribution of chemical states, these peaks show different but interdependent ratios of signal components, as they correspond to different information depths. In this work, we show that self-consistent fitting of all emission peaks lends additional reliability to the depth profiling of chemical states by angular-resolved (AR-)XPS. We demonstrate this using a natively oxidized thin film of the topological crystalline insulator tin telluride (SnTe). This approach is not only complementary to existing depth profiling methods, but may also pave the way towards complete deconvolution of the XPS spectrum, facilitating a more comprehensive and holistic understanding of the surface chemistry of solids.

cond-mat.mes-hall↗

arXiv2024

Stabilizing perpendicular magnetic anisotropy with strong exchange bias in PtMn/Co by magneto-ionics

Electric field control of magnetic properties offers a broad and promising toolbox for enabling ultra-low power electronics. A key challenge with high technological relevance is to master the interplay between the magnetic anisotropy of a ferromagnet and the exchange coupling to an adjacent antiferromagnet. Here, we demonstrate that magneto-ionic gating can be used to achieve a very stable out-of-plane (OOP) oriented magnetization with strong exchange bias in samples with as-deposited preferred in-plane (IP) magnetization. We show that the perpendicular interfacial anisotropy can be increased by more than a factor 2 in the stack Ta/Pt/PtMn/Co/HfO2 by applying -2.5 V gate voltage over 3 nm HfO2, causing a reorientation of the magnetization from IP to OOP with a strong OOP exchange bias of more than 50 mT. Comparing two thicknesses of PtMn, we identify a notable trade-off: while thicker PtMn yields a significantly larger exchange bias, it also results in a slower response to ionic liquid gating within the accessible gate voltage window. These results pave the way for post-deposition electrical tailoring of magnetic anisotropy and exchange bias in samples requiring significant exchange bias.

cond-mat.mtrl-sci↗