arXiv subjects

Andreas Hütten

Publications and source records attributed to Andreas Hütten.

13 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 Néel 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

Tracking the Brownian motion of DNA-functionalized magnetic nanoparticles for conformation analysis beyond the optical resolution limit

Brownian motion provides access to hydrodynamic properties of nanoscale objects independent of their optical resolvability. Here, we present a diffusion-based approach to infer effective particle size distributions of DNA-functionalized magnetic nanoparticles (MNPs), consisting of a magnetic core and a polystyrene shell, in a regime where direct geometric sizing is limited by optical diffraction. Using multi-particle tracking microscopy, we analyze the Brownian dynamics of MNPs grafted with double-stranded DNA (dsDNA) of varying contour length under low-salt conditions. A physically motivated model is introduced that relates dsDNA contour length to an effective hydrodynamic diameter via an attenuated corona description. The measured diffusion coefficient distributions exhibit a systematic and monotonic dependence on dsDNA length in quantitative agreement with the model. While the tracked objects are predominantly dsDNA-mediated agglomerates rather than isolated nanoparticles, clustering does not obscure the length-dependent signal. Instead, the dsDNA corona determines the hydrodynamic scaling, whereas agglomeration mainly introduces an offset and distribution broadening. These results demonstrate that Brownian dynamics enables robust readout of biomolecular length scales even far below the optical resolution limit. The distribution-based approach is inherently tolerant to polydispersity and aggregation, making diffusion-based tracking a simple and promising strategy for future biotechnological and biomedical assays.

cond-mat.soft↗

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 ($μ$PD) apparatus. The $μ$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 $μ$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↗

arXiv2022

Scalable production of single 2D van der Waals layers through atomic layer deposition: Bilayer silica on metal foils and films

The self-limiting nature of atomic layer deposition (ALD) makes it an appealing option for growing single layers of two-dimensional van der Waals (2D-VDW) materials. In this paper it is demonstrated that a single layer of a 2D-VDW form of SiO2 can be grown by ALD on Au and Pd polycrystalline foils and epitaxial films. The silica was deposited by two cycles of bis (diethylamino) silane and oxygen plasma exposure at 525 K. Initial deposition produced a three-dimensionally disordered silica layer; however, subsequent annealing above 950 K drove a structural rearrangement resulting in 2D-VDW; this annealing could be performed at ambient pressure. Surface spectra recorded after annealing indicated that the two ALD cycles yielded close to the silica coverage obtained for 2D-VDW silica prepared by precision SiO deposition in ultra-high vacuum. Analysis of ALD-grown 2D-VDW silica on a Pd(111) film revealed the co-existence of amorphous and incommensurate crystalline 2D phases. In contrast, ALD growth on Au(111) films produced predominantly the amorphous phase while SiO deposition in UHV led to only the crystalline phase, suggesting that the choice of Si source can enable phase control.

cond-mat.mtrl-sci↗

arXiv2017

Proximity-induced superconductivity and quantum interference in topological crystalline insulator SnTe thin film devices

Topological crystalline insulators represent a new state of matter, in which the electronic transport is governed by mirror-symmetry protected Dirac surface states. Due to the helical spin-polarization of these surface states, the proximity of topological crystalline matter to a nearby superconductor is predicted to induce unconventional superconductivity and thus to host Majorana physics. We report on the preparation and characterization of Nb-based superconducting quantum interference devices patterned on top of topological crystalline insulator SnTe thin films. The SnTe films show weak antilocalization and the weak links of the SQUID fully-gapped proximity induced superconductivity. Both properties give a coinciding coherence length of 120 nm. The SQUID oscillations induced by a magnetic field show 2$π$ periodicity, possibly dominated by the bulk conductivity.

cond-mat.mes-hall↗

arXiv2016

How to enable bulk-like martensitic transformation in epitaxial films

The present study is dedicated to the influence of different substrate and buffer layer materials on the martensitic transformation in sputter deposited epitaxial shape memory Heusler alloys. For this, the magnetocaloric Heusler alloy Ni-Co-Mn-Al \cite{Teichert2015b} is grown on MgO(001), MgAl$_{2}$O$_{4}$(001), and MgO(001)/V substrates, which exhibit a lattice misfit to the Ni-Co-Mn-Al between $-1.2\%$ and $3.6\%$. By temperature dependent X-ray diffraction measurements it is shown that the optimum buffer layer for shape memory Heusler films is not one with minimum lattice misfit, but one with minimum Young's modulus and moderate misfit because an elastic buffer layer can deform during the martensitic transformation of the Heusler layer. Furthermore, epitaxial strain caused by a moderate lattice misfit does not significantly change the martensitic transformation temperatures.

cond-mat.mtrl-sci↗

arXiv2016

Quantitative separation of the anisotropic magnetothermopower and planar Nernst effect by the rotation of an in-plane thermal gradient

A thermal gradient as the driving force for spin currents plays a key role in spin caloritronics. In this field the spin Seebeck effect (SSE) is of major interest and was investigated in terms of in-plane thermal gradients inducing perpendicular spin currents (transverse SSE) and out-of-plane thermal gradients generating parallel spin currents (longitudinal SSE). Up to now all spincaloric experiments employ a spatially fixed thermal gradient. Thus anisotropic measurements with respect to well defined crystallographic directions were not possible. Here we introduce a new experiment that allows not only the in-plane rotation of the external magnetic field, but also the rotation of an in-plane thermal gradient controlled by optical temperature detection. As a consequence, the anisotropic magnetothermopower and the planar Nernst effect in a permalloy thin film can be measured simultaneously and reveal a phase shift, that allows the quantitative separation of the thermopower, the anisotropic magnetothermopower and the planar Nernst effect.

cond-mat.mes-hall↗

arXiv2015

Exchange bias effect in martensitic epitaxial Ni-Mn-Sn thin films applied to pin CoFeB/MgO/CoFeB magnetic tunnel junctions

The exchange bias effect is commonly used to shift the coercive field of a ferromagnet. This technique is crucial for the use of magnetic tunnel junctions as logic or memory devices. Therefore, an independent switching of the two ferromagnetic electrodes is necessary to guarantee a reliable readout. Here, we demonstrate that the intrinsic exchange bias effect of Ni-Mn-Sn can be used to apply a unidirectional anisotropy to magnetic tunnel junctions. For this, we use epitaxial Ni-Mn-Sn films as pinning layers for microfabricated CoFeB/MgO/CoFeB magnetic tunnel junctions. We compare the exchange bias field ($H_{\text{EB}}$) measured after field cooling in $-10$\,kOe external field by magnetization measurements with $H_{\text{EB}}$ obtained from tunnel magnetoresistance measurements. Consistent for both methods we find an exchange bias of about $H_{\text{EB}}=130$\,Oe at 10\,K, which decreases with increasing temperature and vanishes above 70\,K.

cond-mat.mtrl-sci↗

arXiv2015

Structure and Giant Inverse Magnetocaloric Effect of Epitaxial Ni-Co-Mn-Al Films

The structural, magnetic, and magnetocaloric properties of epitaxial Ni-Co-Mn-Al thin films with different compositions have been studied. The films were deposited on MgO(001) substrates by co-sputtering on heated substrates. All films show a martensitic transformation, where the transformation temperatures are strongly dependent on the composition. The structure of the martensite phase is shown to be 14M. The metamagnetic martensitic transformation occurs from strongly ferromagnetic austenite to weakly magnetic martensite. The structural properties of the films were investigated by atomic force microscopy and temperature dependent X-ray diffraction. Magnetic and magnetocaloric properties were analyzed using temperature dependent and isothermal magnetization measurements. We find that Ni$_{41}$Co$_{10.4}$Mn$_{34.8}$Al$_{13.8}$ films show giant inverse magnetocaloric effects with magnetic entropy change of 17.5\,J\,kg$^{-1}$K$^{-1}$ for $μ_0 ΔH=5\,\text{T}$.

cond-mat.mtrl-sci↗

arXiv2014

Experimental realization of a semiconducting full Heusler compound: Fe2TiSi

Single-phase films of the full Heusler compound Fe2TiSi have been prepared by magnetron sputtering. The compound is found to be a semiconductor with a gap of 0.4eV. The electrical resistivity has a logarithmic temperature dependence up to room temperature due to Kondo scattering of a dilute free electron gas off superparamagnetic impurities. The origin of the electron gas is extrinsic due to disorder or off-stoichiometry. Density functional theory calculations of the electronic structure are in excellent agreement with electron energy loss, optical, and x-ray absorption experiments. Fe2TiSi may find applications as a thermoelectric material.

cond-mat.mtrl-sci↗