arXiv subjects

Michael Vogel

Publications and source records attributed to Michael Vogel.

At least 19 recordsLinked to original sources

arXiv2026

Decoding magnetic texture

In magnetically ordered materials, magnetic field and temperature variations modify the magnetic texture through their coupling to the local energy landscape, imprinting distinct fingerprints in the resulting magnetic domain patterns. Retrieving these conditions from the pattern remains challenging, as stochastic nucleation and hysteresis produce a nonlinear, multivariate, and ambiguous relationship between magnetic domain morphology and external stimuli. To decode these fingerprints, we designed a controlled magneto-optical inference experiment that reconstructs magnetic field, temperature, and magnetic history from a single fine-scale, high-contrast, pixel-resolved optical polarization map of feature-rich magnetic domain textures in a bismuth-substituted yttrium iron garnet film. Deep convolutional neural networks are complemented by feature-based neural-network inference using hand-crafted, physically interpretable descriptors of measured magneto-optical image data, linking the decoded information to material-dependent features and exploring their contributions. Together, these results establish magnetic texture as a high-fidelity record of external conditions enabling accurate single image multiparametric sensing and paving the way for data-driven explorations of complex magnetic states. Uncovering the physically interpretable features that encode this record sheds new light on the physics of magnetic domain formation.

cond-mat.mtrl-sci↗

arXiv2025

On the Role of Internal Degrees of Freedom in Structural Relaxation of Ring-Tail Structured Liquids Across Temperature Regimes

We investigate how anisotropic molecular rotation and internal molecular flexibility influence liquid dynamics in 1-phenylalkanes. To this end, we combine depolarized dynamic light scattering, nuclear magnetic resonance spectroscopy and molecular dynamics simulations. Our results show that anisotropic rotations and internal molecular flexibility substantially contribute to structural relaxation in the liquid state. However, their influence diminishes on entering the supercooled-liquid regime, where the relaxation behavior develops towards the previously identified generic relaxation shape, likely due to the increasing cooperativity of rotational dynamics. Because 1-phenylalkanes are simple model systems with similarities to many other molecular liquids, this study suggests that effects of anisotropic rotation and internal flexibility are relevant in various liquids with similar molecular complexity, and provides a proof of concept for how these effects can be identified.

cond-mat.soft↗

arXiv2025

Structure and dynamics of ionic liquids under shear flow

We investigate the intrinsic behavior of ionic liquids under shear flow, using a coarse-grained model of C4mim-PF6 as a prototypical example. The importance of long-ranged electrostatics is assessed as a function of shear rate by comparing Ewald and reaction field treatments. An appropriate comparison is achieved through the implementation of the proper Lees-Edwards boundary conditions within the ESPResSo++ simulation software. Our results demonstrate that while structural properties are relatively insensitive to the electrostatic treatment, the more accurate treatment via the Ewald approach is essential for studies of dynamics, in particular, at lower shear rates. Furthermore, we identify a critical shear rate beyond which structural and dynamical properties begin to deviate from equilibrium behavior, while remaining largely unchanged below this threshold. Finally, we demonstrate that the dynamic heterogeneity of the liquid decreases as a function of increasing shear rate, which can be primarily explained by the faster dynamics induced by the shear flow. These results hold relevance for investigations of process-dependent properties of ionic-liquid-based materials.

physics.chem-ph↗

arXiv2025

Effects of Disorder on the Energy Landscape and Motional Mechanisms Involved in Lithium Ion Dynamics and Transport in Solid Electrolytes: Li5.5PS4.5Cl1.5 Argyrodite as a Case Study

7Li NMR diffusometry and relaxometry are combined with electrochemical impedance spectroscopy to compare the mechanisms for the dynamics and transport of lithium ions in disordered and crystalline electrolytes with argyrodite composition Li5.5PS4.5Cl1.5. The dc conductivity of a disordered sample prepared by ball milling amounts to 0.76 mScm-1 at room temperature, which is substantially lower than that of two previously studied crystalline argyrodites differing in the order of the anion sublattice due to various heat treatments. However, the activation energy of the dc conductivity is smaller for ball-milled disordered Li5.5PS4.5Cl1.5 (E_dc= 0.35 eV) than for both crystalline compounds (E_dc= 0.38 eV). 7Li NMR field-gradient measurements of the self-diffusion coefficient D and its activation energy E_D confirm these findings and, furthermore, reveal different Haven ratios. 7Li NMR field-cycling relaxometry shows that the lithium ion jumps in ball-milled Li5.5PS4.5Cl1.5 are described by very broad dynamical susceptibilities arising from a temperature-independent Gaussian-like distribution of activation energies g(E_a) with a mean value of E_m= 0.43 eV, while the susceptibilities indicated a high-energy cutoff for the crystalline electrolytes. Based on different relations between the activation energies for the conductivity, diffusivity and jumps, we discuss that the shape and exploration of the energy landscapes of ball-milled and crystalline Li5.5PS4.5Cl1.5 samples strongly differ. Moreover, significant differences in the preexponential factor of the dc conductivity, the Haven ratio and the single-particle correlation factor point to distinct types of anion lattice disorder of the ball-milled disordered and heat-treated crystalline samples.

cond-mat.dis-nn↗

arXiv2023

b-Contact Structures on Tentacular Hyperboloids

This paper connects two different approaches to the analysis of Hamiltonian dynamics on non-compact energy hypersurfaces - $b$-symplectic geometry with its singular symplectic form and Floer techniques for tentacular Hamiltonians. More precisely, we show how to equip tentacular hyperboloids with a $b$-contact structure. We construct a $b^3$-symplectic manifold $(X, Z, ω_b)$, such that each connected component of $X\setminus Z$ is symplectomorphic to the standard symplectic space $(T^*\mathbb{R}^n, ω_0)$. For a tentacular hyperboloid $S \subseteq T^*\mathbb{R}^n$ we look at its copies in $X \setminus Z$ and show that their completion in $(X,Z, ω_b)$ is a smooth hypersurface of $b$-contact type.

math.SG↗

arXiv2022

Three-dimensional close-to-substrate trajectories of magnetic microparticles in dynamically changing magnetic field landscapes

The transport of magnetic nano- or microparticles in microfluidic devices using artificially designed magnetic field landscapes (MFL) is promising for the implementation of key functionalities in Lab-on-a-chip (LOC) systems. A close-to-substrate transport is hereby instrumental to use changing particle-substrate interactions upon analyte binding for analytics and diagnostics. Here, we present an essential prerequisite for such an application, namely the label-free quantitative experimental determination of the three-dimensional trajectories of superparamagnetic particles (SPP) transported by a dynamically changing MFL above a topographically flat substrate. The evaluation of the SPP sharpness within defocused video-recorded images, acquired by an optical bright-field microscope, was employed to obtain a vertical z-coordinate. This method applied to a prototypical transport scheme, using the static MFL of parallel-stripe domains superposed by a particular magnetic field pulse sequence, revealed a hopping-like motion of the magnetic particles, previously predicted by theory. Maximum vertical particle jumps of several micrometers have been observed experimentally, corroborating theoretical estimates for the particle-substrate distance. As our findings pave the way towards precise quantification of particle-substrate separations in the discussed transport system, they bear deep implications for future LOC detection schemes using only optical microscopy.

physics.app-ph↗

arXiv2021

Translatory and rotatory motion of Exchange-Bias capped Janus particles controlled by dynamic magnetic field landscapes

Magnetic Janus particles (MJPs), fabricated by covering a non-magnetic spherical particle with a hemispherical magnetic in-plane exchange-bias layer system cap, display an onion magnetization state for comparably large diameters of a few microns. In this work, the motion characteristics of these MJPs will be investigated when they are steered by a magnetic field landscape over prototypical parallel-stripe domains, dynamically varied by superposed external magnetic field pulse sequences, in an aqueous medium. We demonstrate, that due to the engineered magnetization state in the hemispherical cap, a comparably fast, directed particle transport and particle rotation can be induced. Additionally, by modifying the frequency of the applied pulse sequence and the strengths of the individual field components, we observe a possible separation between a combined or an individual occurrence of these two types of motion. Our findings bear importance for lab-on-a-chip systems, where particle immobilization on a surface via analyte bridges shall be used for low concentration analyte detection and a particle rotation over a defined position of a substrate may dramatically increase the immobilization (and therefore analyte detection) probability.

physics.app-ph↗

arXiv2021

Concentration Gradients in Evaporating Binary Droplets Probed by Spatially Resolved Raman and NMR Spectroscopy

Understanding the evaporation process of binary sessile droplets is essential for optimizing various technical processes, such as inkjet printing or heat transfer. Liquid mixtures whose evaporation and wetting properties may differ significantly from those of pure liquids are particularly interesting. Concentration gradients may occur in these binary droplets. The challenge is to measure concentration gradients without affecting the evaporation process. Here, spectroscopic methods with spatial resolution can discriminate between the components of a liquid mixture. We show that confocal Raman microscopy and spatially resolved nuclear magnetic resonance (NMR) spectroscopy can be used as complementary methods to measure concentration gradients in evaporating 1-butanol/1-hexanol droplets on a hydrophobic surface. Deuterating one of the liquids allows analysis of the local composition through the comparison of the intensities of the CH and CD stretching bands in Raman spectra. Spatially resolved NMR spectroscopy is used to measure the composition at different positions of the droplet. Confocal Raman and spatially resolved NMR experiments show the presence of a vertical concentration gradient as the 1-butanol/1-hexanol droplet evaporates.

physics.app-ph↗

arXiv2021

Phase-resolved electrical detection of coherently coupled magnonic devices

We demonstrate the electrical detection of magnon-magnon hybrid dynamics in yttrium iron garnet/permalloy (YIG/Py) thin film bilayer devices. Direct microwave current injection through the conductive Py layer excites the hybrid dynamics consisting of the uniform mode of Py and the first standing spin wave ($n=1$) mode of YIG, which are coupled via interfacial exchange. Both the two hybrid modes, with Py or YIG dominated excitations, can be detected via the spin rectification signals from the conductive Py layer, providing phase resolution of the coupled dynamics. The phase characterization is also applied to a nonlocally excited Py device, revealing the additional phase shift due to the perpendicular Oersted field. Our results provide a device platform for exploring hybrid magnonic dynamics and probing their phases, which are crucial for implementing coherent information processing with magnon excitations

cond-mat.mes-hall↗

arXiv2021

Field-tunable interactions and frustration in underlayer-mediated artificial spin ice

Artificial spin ice systems have opened experimental windows into a range of model magnetic systems through the control of interactions among nanomagnet moments. This control has previously been enabled by altering the nanomagnet size and the geometry of their placement. Here we demonstrate that the interactions in artificial spin ice can be further controlled by including a soft ferromagnetic underlayer below the moments. Such a substrate also breaks the symmetry in the array when magnetized, introducing a directional component to the correlations. Using spatially resolved magneto-optical Kerr effect microscopy to image the demagnetized ground states, we show that the correlation of the demagnetized states depends on the direction of underlayer magnetization. Further, the relative interaction strength of nearest and next-nearest neighbors varies significantly with the array geometry. We exploit this feature to induce frustration in an inherently unfrustrated square lattice geometry, demonstrating new possibilities for effective geometries in two dimensional nanomagnetic systems.

cond-mat.mes-hall↗

arXiv2021

Dynamical properties across different coarse-grained models for ionic liquids

Room-temperature ionic liquids (RTILs) stand out among molecular liquids for their rich physicochemical characteristics, including structural and dynamic heterogeneity. The significance of electrostatic interactions in RTILs results in long characteristic length- and timescales, and has motivated the development of a number of coarse-grained (CG) simulation models. In this study, we aim to better understand the connection between certain CG parametrization strategies and the dynamical properties and transferability of the resulting models. We systematically compare five CG models: a model largely parametrized from experimental thermodynamic observables; a refinement of this model to increase its structural accuracy; and three models that reproduce a given set of structural distribution functions by construction, with varying intramolecular parametrizations and reference temperatures. All five CG models display limited structural transferability over temperature, and also result in various effective dynamical speedup factors, relative to a reference atomistic model. On the other hand, the structure-based CG models tend to result in more consistent cation-anion relative diffusion than the thermodynamic-based models, for a single thermodynamic state point. By linking short- and long-timescale dynamical behaviors, we demonstrate that the varying dynamical properties of the different coarse-grained models can be largely collapsed onto a single curve, which provides evidence for a route to constructing dynamically-consistent CG models of RTILs.

cond-mat.soft↗

arXiv2019

Phonon Transport Controlled by Ferromagnetic Resonance

The resonant coupling of phonons and magnons is important for the interconversion of phononic and spin degrees of freedom. We studied the phonon transmission in LiNbO3 manipulated by the dynamic magnetization in a Ni thin film. It was observed that the phonons could be absorbed strongly through resonant magnon-phonon coupling, which was realized by optimizing the interfacial coupling between Ni and LiNbO3. The line shapes of phonon transmission were further investigated considering the magnon-phonon interconversion in the elastically driven ferromagnetic resonance process. The results promote unique routes for phonon manipulation and detection in the presence of magnetization dynamics.

physics.app-ph↗

arXiv2019

Spin-Wave frequency division multiplexing in an yttrium iron garnet microstripe magnetized by inhomogeneous field

Spin waves are promising candidates for information processing and transmission in a broad frequency range. In the realization of magnonic devices, the frequency depended division of the spin wave frequencies is a critical function for parallel information processing. In this work, we demonstrate a proof-of-concept spin-wave frequency division multiplexing method by magnetizing a homogenous magnetic microstripe with an inhomogeneous field. The symmetry breaking additional field is introduced by a permalloy stripe simply placed in lateral proximity to the waveguide. Spin waves with different frequencies can propagate independently, simultaneously and separately in space along the shared waveguide. This work brings new potentials for parallel information transmission and processing in magnonics.

physics.app-ph↗

arXiv2019

Tuning edge localized spin waves in magnetic microstripes by proximate magnetic structures

The propagation of edge localized spin waves (E-SWs) in yttrium iron garnet (YIG) microstripes with/without the proximate magnetic microstructures is investigated by micromagnetic simulations. A splitting of the dispersion curve with the presence of permalloy (Py) stripe is also observed. The E-SWs on the two edges of YIG stripe have different wavelengths, group velocities, and decay lengths at the same frequencies. The role of the Py stripe was found to be the source of the inhomogeneous static dipolar field without dynamic coupling with YIG. This work opens new perspectives for the design of innovative SW interference-based logic devices.

cond-mat.mtrl-sci↗

arXiv2019

Controlled interconversion of quantized spin wave modes via local magnetic fields

In the emerging field of magnonics, spin waves are considered for information processing and transmission at high frequencies. Towards this end, the manipulation of propagating spin waves in nanostructured waveguides for novel functionality has recently been attracting increasing focus of research. Excitations with uniform magnetic fields in such waveguides favors symmetric spin wave modes with odd quantization numbers. Interference between multiple odd spin wave modes leads to a periodic self-focusing effect of the propagating spin waves. Here we demonstrate, how antisymmetric spin wave modes with even quantization numbers can be induced by local magnetic fields in a well-controlled fashion. The resulting interference patterns are discussed within an analytical model and experimentally demonstrated using microfocused Brillouin light scattering (μ-BLS).

cond-mat.mes-hall↗

arXiv2018

New Nuclear Magnetic Moment of $^{209}$Bi - Resolving the Bismuth Hyperfine Puzzle

A recent measurement of the hyperfine splitting in the ground state of Li-like $^{209}$Bi$^{80+}$ has established a "hyperfine puzzle" -- the experimental result exhibits a 7$σ$ deviation from the theoretical prediction [J. Ullmann et al., Nat. Commun. 8, 15484 (2017); J. P. Karr, Nat. Phys. 13, 533 (2017)]. We provide evidence that the discrepancy is caused by an inaccurate value of the tabulated nuclear magnetic moment ($μ_I$) of $^{209}$Bi. We perform relativistic density functional theory and relativistic coupled cluster calculations of the shielding constant that should be used to extract the value of $μ_I(^{209}{\rm Bi})$ and combine it with nuclear magnetic resonance measurements of Bi(NO$_3$)$_3$ in nitric acid solutions and of the hexafluoridobismuthate(V) BiF$_6^-$ ion in acetonitrile. The result clearly reveals that $μ_I(^{209}{\rm Bi})$ is much smaller than the tabulated value used previously. Applying the new magnetic moment shifts the theoretical prediction into agreement with experiment and resolves the hyperfine puzzle.

physics.atom-ph↗

arXiv2018

Battery-like Supercapacitors from Vertically Aligned Carbon Nanofibers Coated Diamond: Design and Demonstrator

Battery-like supercapacitors feature high power and energy densities as well as long-term capacitance retention. The utilized capacitor electrodes are thus better to have large surface areas, high conductivity, high stability, and importantly be of binder free. Herein, vertically aligned carbon nanofibers (CNFs) coated boron-doped diamonds (BDD) are employed as the capacitor electrodes to construct battery-like supercapacitors. Grown via a thermal chemical vapor deposition technique, these CNFs/BDD hybrid films are binder free and own porous structures, resulting in large surface areas. Meanwhile, the containment of graphene layers and copper metal catalysts inside CNFs/BDD leads to their high conductivity. Electric double layer capacitors (EDLCs) and pseudocapacitors (PCs) are then constructed in the inert electrolyte (1.0 M H2SO4 solution) and in the redox-active electrolyte (1.0 M Na2SO4 + 0.05 M Fe(CN)63-/4-), respectively. For assembled two-electrode symmetrical supercapacitor devices, the capacitances of EDLC and PC devices reach 30 and 48 mF cm-2 at 10 mV s-1, respectively. They remain constant even after 10 000 cycles. The power densities are 27.3 kW kg-1 and 25.3 kW kg-1 for EDLC and PC devices, together with their energy densities of 22.9 Wh kg-1 and 44.1 Wh kg-1, respectively. The performance of formed EDLC and PC devices is comparable to market-available batteries. Therefore, the vertically aligned CNFs/BDD hybrid film is a suitable capacitor electrode material to construct high-performance battery-like and industry-orientated supercapacitors for flexible power devices.

physics.app-ph↗

arXiv2017

Effects of Silica Surfaces on the Structure and Dynamics of Room Temperature Ionic Liquids: A Molecular Dynamics Simulation Study

Room temperature ionic liquids (ILs) at solid surfaces have been recognized for their significant interfacial properties in electrochemical and electronic devices. To ascertain the interface effects, we investigate dynamical and structural properties of two ILs in nanoscale confinement at various temperatures. Specifically, we perform all-atom molecular dynamics simulations for ILs composed of 1-butyl-3-methylimidazolium cations and hexafluorophosphate ([Bmim][PF6]) or tetrafluoroborate ([Bmim][BF4]) anions sandwiched between amorphous silica slabs. Density profiles of the ionic species across the slit reveal that [PF6] and [BF4] anions tend to stay closer to the slab wall than [Bmim] cations resulting in a bi-layered arrangement in the interfacial region. For the cations, we observe a preferred orientation at the surface with the methyl groups pointing towards the wall and the butyl tails projected inwards. Mean square displacements and incoherent scattering function reveal slowed and heterogeneous dynamics of all ionic species in the slit pore. In particular, spatially resolved analyses show that the structural relaxation times increase by about two orders of magnitude when approaching the silica surfaces, an effect to be considered when designing applications. The altered structural and dynamical features of the confined ILs can be related to an existence of preferred sites for the anions on the amorphous silica surfaces. Detailed analyses of relations between the broadly distributed site and surface properties show that particularly stable anion sites result when triangular arrangements of silanol groups enable multiple hydrogen bonds with the various fluorine atoms of a given anion, elucidating an important trapping mechanism at the silica surface.

physics.chem-ph↗