arXiv subjects

H. W. Schumacher

Publications and source records attributed to H. W. Schumacher.

At least 19 recordsLinked to original sources

arXiv2025

Lithographically-controlled liquid metal diffusion in graphene: Fabrication and magneto-transport signatures of superconductivity

Metal intercalation in epitaxial graphene enables the emergence of proximity-induced superconductivity and modified quantum transport properties. However, systematic transport studies of intercalated graphene have been hindered by challenges in device fabrication, including processing-induced deintercalation and instability under standard lithographic techniques. Here, we introduce a lithographically controlled intercalation approach that enables the scalable fabrication of gallium-intercalated quasi-freestanding bilayer graphene (QFBLG) Hall bar devices. By integrating lithographic structuring with subsequent intercalation through dedicated intercalation channels, this method ensures precise control over metal incorporation while preserving device integrity. Magneto-transport measurements reveal superconductivity with a critical temperature Tc,onset ~ 3.5 K and the occurrence of a transverse resistance, including both symmetric and antisymmetric field components, which is attributed to the symmetric-in-field component to non-uniform currents. These results establish an advanced fabrication method for intercalated graphene devices, providing access to systematic investigations of confined 2D superconductivity and emergent electronic phases in van der Waals heterostructures.

cond-mat.mtrl-sci↗

arXiv2024

Influence of the magneto-volume effect on the transient reflectivity of MnSi

The magneto-volume effect is a well-established yet frequently overlooked phenomenon in magnetic materials that may affect a wide range of physical properties. Our study explores the influence of the magneto-volume effect on the transient reflectivity of MnSi, a well-known chiral magnet with strong magnetoelastic coupling. We observe a unipolar reflectivity transient in the paramagnetic phase, contrasting with a bipolar response in phases with magnetic long-range order. Comparing our findings with thermal expansion from literature, we establish that the bipolar response originates in the magneto-volume effect which dominates the thermal expansion and influences the optical reflectivity. Our results highlight not only that the magneto-volume effect must be considered when discussing transient reflectivity measurements of magnetic materials but also that such measurements permit to study the characteristic time scales of the magneto-volume effect itself, contributing to a deeper understanding of this often-neglected phenomenon.

cond-mat.mes-hall↗

arXiv2024

2024 Roadmap on Magnetic Microscopy Techniques and Their Applications in Materials Science

Considering the growing interest in magnetic materials for unconventional computing, data storage, and sensor applications, there is active research not only on material synthesis but also characterisation of their properties. In addition to structural and integral magnetic characterisations, imaging of magnetization patterns, current distributions and magnetic fields at nano- and microscale is of major importance to understand the material responses and qualify them for specific applications. In this roadmap, we aim to cover a broad portfolio of techniques to perform nano- and microscale magnetic imaging using SQUIDs, spin center and Hall effect magnetometries, scanning probe microscopies, x-ray- and electron-based methods as well as magnetooptics and nanoMRI. The roadmap is aimed as a single access point of information for experts in the field as well as the young generation of students outlining prospects of the development of magnetic imaging technologies for the upcoming decade with a focus on physics, materials science, and chemistry of planar, 3D and geometrically curved objects of different material classes including 2D materials, complex oxides, semi-metals, multiferroics, skyrmions, antiferromagnets, frustrated magnets, magnetic molecules/nanoparticles, ionic conductors, superconductors, spintronic and spinorbitronic materials.

cond-mat.mtrl-sci↗

arXiv2023

Suppression of the spin waves nonreciprocity due to interfacial Dzyaloshinskii Moriya interaction by lateral confinement in magnetic nanostructures

Despite the huge recent interest towards chiral magnetism related to the interfacial Dzyaloshinskii Moriya interaction (iDMI) in layered systems, there is a lack of experimental data on the effect of iDMI on the spin waves eigenmodes of laterally confined nanostructures. Here we exploit Brillouin Light Scattering (BLS) to analyze the spin wave eigenmodes of non-interacting circular and elliptical dots, as well as of long stripes, patterned starting from a Pt(3.4 nm)/CoFeB(0.8 nm) bilayer, with lateral dimensions ranging from 100 nm to 400 nm. Our experimental results, corroborated by micromagnetic simulations based on the GPU-accelerated MuMax3 software package, provide evidence for a strong suppression of the frequency asymmetry between counter-propagating spin waves (corresponding to either Stokes or anti-Stokes peaks in BLS spectra), when the lateral confinement is reduced from 400 nm to 100 nm, i.e. when it becomes lower than the light wavelength. Such an evolution reflects the modification of the spin wave character from propagating to stationary and indicates that the BLS based method of quantifying the i-DMI strength from the frequency difference of counter propagating spin waves is not applicable in the case of magnetic elements with lateral dimension below about 400 nm.

cond-mat.mes-hall↗

arXiv2020

Modal frustration and periodicity breaking in artificial spin ice

Here an artificial spin ice (ASI) lattice is introduced that exhibits unique Ising and non-Ising behavior under specific field switching protocols because of the inclusion of coupled nanomagnets into the unit cell. In the Ising regime, a magnetic switching mechanism that generates a uni- or bimodal distribution of states dependent on the alignment of the field is demonstrated with respect to the lattice unit cell. In addition, a method for generating a plethora of randomly distributed energy states across the lattice, consisting of Ising and Landau states, is investigated through magnetic force microscopy and micromagnetic modeling. We demonstrate that the dispersed energy distribution across the lattice is a result of the intrinsic design and can be finely tuned through control of the incident angle of a critical field. The present manuscript explores a complex frustrated environment beyond the 16-vertex Ising model for the development of novel logic-based technologies.

cond-mat.mes-hall↗

arXiv2020

Liquid metal intercalation of epitaxial graphene: large-area gallenene layer fabrication through gallium self-propagation at ambient conditions

We demonstrate the fabrication of an ultra thin gallium film, also known as gallenene, beneath epitaxial graphene on 6H-SiC under ambient conditions triggered by liquid gallium intercalation. Gallenene has been fabricated using the liquid metal intercalation, achieving lateral intercalation and diffusion of Ga atoms at room temperature on square centimeter areas limited only by the graphene samples' size. The stepwise self-propagation of the gallenene film below the epitaxial graphene surface on the macroscopic scale was observed by optical microscopy shortly after the initial processing without further physical or chemical treatment. Directional Ga diffusion of gallenene occurs on SiC terraces since the terrace steps form an energetic barrier (Ehrlich-Schwoebel barrier),retarding the gallenene propagation. The subsequent conversion of the epitaxial graphene into quasi free-standing bilayer graphene (QFBLG) and the graphene-gallenene heterostack interactions have been analyzed by XPS and Raman measurements. The results reveal a novel approach for controlled fabrication of wafer-scale gallenene as well as for two-dimensional heterostructures and stacks based on the interaction between liquid metal and epitaxial graphene. Please note, this work was also titled as Graphene meets gallenene -- A straightforward approach to developing large-area heterostacks by gallium self-propagation https://www.researchgate.net/publication/333451130_Graphene_meets_gallenene_-_A_straightforward_approach_to_developing_large-area_heterostacks_by_gallium_self-propagation

cond-mat.mtrl-sci↗

arXiv2018

Spincaloritronic measurements: a round robin comparison of the longitudinal spin Seebeck effect

The rising field of spin caloritronics focuses on the interactions between spin and heat currents in a magnetic material; the observation of the spin Seebeck effect opened the route to this branch of research. This paper reports the results of a round robin test performed by five partners on a single device highlighting the reproducibility problems related to the measurements of the spin Seebeck coefficient, the quantity that describes the strength of the spin Seebeck effect. This work stimulated the search for more reproducible measurement methods through the analysis of the systematic effects.

cond-mat.mes-hall↗

arXiv2018

Near-field magneto-caloritronic nanoscopy on ferromagnetic nanostructures

Near-field optical microscopy by means of infrared photocurrent mapping has rapidly developed in recent years. In this letter we introduce a near-field induced contrast mechanism arising when a conducting surface, exhibiting a magnetic moment, is exposed to a nanoscale heat source. The magneto-caloritronic response of the sample to near-field excitation of a localized thermal gradient leads to a contrast determined by the local state of magnetization. By comparing the measured electric response of a magnetic reference sample with numerical simulations we derive an estimate of the field enhancement and the corresponding temperature profile induced on the sample surface.

cond-mat.mes-hall↗

arXiv2018

Excitation and coherent control of magnetization dynamics in magnetic tunnel junctions using acoustic pulses

We experimentally study magnetization dynamics in magnetic tunnel junctions driven by femtosecond-laser-induced surface acoustic waves. The acoustic pulses induce a magnetization precession in the free layer of the magnetic tunnel junction through magnetoelastic coupling. The frequency and amplitude of the precession shows a pronounced dependence on the applied magnetic field and the laser excitation position. Comparing the acoustic-wave-induced precession frequencies with precession induced by charge currents and with micromagnetic simulations we identify spatially non-uniform magnetization modes localized close the edge regions as being responsible for the optically induced magnetization dynamics. The experimental scheme even allows us to coherently control the magnetization precession using two acoustic pulses. This might prove important for future applications requiring ultrafast spin manipulation. Additionally, our results directly pinpoint the importance of acoustic pulses since they could be relevant when investigating optically-induced temperature effects in magnetic structures.

cond-mat.mes-hall↗

arXiv2018

MnSi-nanostructures obtained from thin films: magnetotransport and Hall effect

We present a comparative study of the (magneto)transport properties, including Hall effect, of bulk, thin film and nanostructured MnSi. In order to set our results in relation to published data we extensively characterize our materials, this way establishing a comparatively good sample quality. Our analysis reveals that in particular for thin film and nanostructured material, there are extrinsic and intrinsic contributions to the electronic transport properties, which by modeling the data we separate out. Finally, we discuss our Hall effect data of nanostructured MnSi under consideration of the extrinsic contributions and with respect to the question of the detection of a topological Hall effect in a skyrmionic phase.

cond-mat.mes-hall↗

arXiv2017

Nonmonotonous classical magneto-conductivity of a two-dimensional electron gas in a disordered array of obstacles

Magnetotransport measurements in combination with molecular dynamics (MD) simulations on two-dimensional disordered Lorentz gases in the classical regime are reported. In quantitative agreement between experiment and simulation, the magnetoconductivity displays a pronounced peak as a function of perpendicular magnetic field $B$ which cannot be explained in the framework of existing kinetic theories. We show that this peak is linked to the onset of a directed motion of the electrons along the contour of the disordered obstacle matrix when the cyclotron radius becomes smaller than the size of the obstacles. This directed motion leads to transient superdiffusive motion and strong scaling corrections in the vicinity of the insulator-to-conductor transitions of the Lorentz gas.

cond-mat.dis-nn↗

arXiv2015

Lissajous rocking ratchet

Breaking time-reversal symmetry (TRS) in the absence of a net bias can give rise to directed steady-state non-equilibrium transport phenomena such as ratchet effects. Here we present, theoretically and experimentally, the concept of a Lissajous rocking ratchet as an instrument based on breaking TRS. Our system is a semiconductor quantum dot (QD) with periodically modulated dot-lead tunnel barriers. Broken TRS gives rise to single electron tunneling current. Its direction is fully controlled by exploring frequency and phase relations between the two barrier modulations. The concept of Lissajous ratchets can be realized in a large variety of different systems, including nano-electrical, nano-electromechanical or superconducting circuits. It promises applications based on a detailed on-chip comparison of radio-frequency signals.

cond-mat.mes-hall↗

arXiv2015

Commensurability resonances in two-dimensional magneto-electric lateral superlattices

Hybrid lateral superlattices composed of a square array of antidots and a periodic one-dimensional magnetic modulation are prepared in $\mathrm{Ga[Al]As}$ heterostructures. The two-dimensional electron gases exposed to these superlattices are characterized by magnetotransport experiments in vanishing average perpendicular magnetic fields. Despite the absence of closed orbits, the diagonal magnetoresistivity in the direction perpendicular to the magnetic modulation shows pronounced classical resonances. They are located at magnetic fields where snake trajectories exist which are quasi-commensurate with the antidot lattice. The diagonal magnetoresistivity in the direction of the magnetic modulation increases sharply above a threshold magnetic field and shows no fine structure. The experimental results are interpreted with the help of numerical simulations based on the semiclassical Kubo model.

cond-mat.mes-hall↗

arXiv2013

Magneto-thermoelectric figure of merit of Co/Cu multilayers

The switching of the magnetization configurations of giant magnetoresistance multilayer stacks not only changes the electric and thermal conductivities, but also the thermopower. We study the magnetotransport and the magneto-thermoelectric properties of Co/Cu multilayer devices in a lateral thermal gradient. We derive values of the Seebeck coefficient, the thermoelectric figure of merit, and the thermoelectric power factor. The Seebeck coefficient reaches values up to -18 microvolt/K at room temperature and shows a magnetic field dependence up to 28.6 % upon spin reversal. In combination with thermal conductivity data of the same Co/Cu stack, we find a spin dependence of the thermoelectric figure of merit of up to 65 %. Furthermore, a spin dependence of the power factor of up to 110 % is derived.

cond-mat.mtrl-sci↗

arXiv2013

Influence of sample geometry on inductive damping measurement methods

We study the precession frequency and effective damping of patterned permalloy thin films of different geometry using integrated inductive test structures. The test structures consist of coplanar wave guides fabricated onto patterned permalloy stripes of different geometry. The width, length and position of the permalloy stripe with respect to the center conductor of the wave guide are varied. The precession frequency and effective damping of the different devices is derived by inductive measurements in time and frequency domain in in-plane magnetic fields. While the precession frequencies do not reveal a significant dependence on the sample geometry we find a decrease of the measured damping with increasing width of the permalloy centered underneath the center conductor of the coplanar wave guide. We attribute this effect to an additional damping contribution due to inhomogeneous line broadening at the edges of the permalloy stripes which does not contribute to the inductive signal provided the permalloy stripe is wider than the center conductor. Consequences for inductive determination of the effective damping using such integrated reference samples are discussed.

cond-mat.mes-hall↗

arXiv2013

Towards quantized current arbitrary waveform synthesis

The generation of ac modulated quantized current waveforms using a semiconductor non-adiabatic single electron pump is demonstrated. In standard operation the single electron pump generates a quantized output current of I = ef where e is the charge of the electron and f is the pumping frequency. Suitable frequency modulation of f allows the generation of ac modulated output currents with different characteristics. By sinusoidal and saw tooth like modulation of f accordingly modulated quantized current waveforms with kHz modulation frequencies and peak currents up to 100 pA are obtained. Such ac quantized current sources could find applications ranging from precision ac metrology to on-chip signal generation.

cond-mat.mes-hall↗

arXiv2013

The influence of individual lattice defects on the domain structure in magnetic antidot lattices

We numerically and experimentally investigate the influence of single defects consisting of a missing antidot on the spin configurations in rectangular permalloy antidot lattices. The introduction of such lattice defects leads to the nucleation of complex domain structures after the decay of a saturating magnetic field. Micromagnetic simulations yield four typical domain configurations around the defect having distinct energy densities. The existence of the four spin configurations is confirmed by magnetic force microscopy on antidot lattices containing individual defects.

cond-mat.mes-hall↗

arXiv2012

Measurement of heavy-hole spin dephasing in (InGa)As quantum dots

We measure the spin dephasing of holes localized in self-assembled (InGa)As quantum dots by spin noise spectroscopy. The localized holes show a distinct hyperfine interaction with the nuclear spin bath despite the p-type symmetry of the valence band states. The experiments reveal a short spin relaxation time τ_{fast}^{hh} of 27 ns and a second, long spin relaxation time τ_{slow}^{hh} which exceeds the latter by more than one order of magnitude. The two times are attributed to heavy hole spins aligned perpendicular and parallel to the stochastic nuclear magnetic field. Intensity dependent measurements and numerical simulations reveal that the long relaxation time is still obscured by light absorption, despite low laser intensity and large detuning. Off-resonant light absorption causes a suppression of the spin noise signal due to the creation of a second hole entailing a vanishing hole spin polarization.

cond-mat.mes-hall↗