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Markus Weigand

Publications and source records attributed to Markus Weigand.

At least 55 records · Page 3Linked to original sources
arXiv2019

Real space observation of magnon interaction with driven space-time crystals

The concept of Space-Time Crystals (STC), i.e. translational symmetry breaking in time and space, was recently proposed and experimentally demonstrated for quantum systems. Here, we transfer this concept to magnons and experimentally demonstrate a driven STC at room temperature. The STC is realized by strong homogeneous micro-wave pumping of a micron-sized permalloy (Py) stripe and is directly imaged by Scanning Transmission X-ray Microscopy (STXM). For a fundamental understanding of the formation of the STC, micromagnetic simulations are carefully adapted to model the experimental findings. Beyond the mere generation of a STC, we observe the formation of a magnonic band structure due to back folding of modes at the STC's Brillouin zone boundaries. We show interactions of magnons with the STC that appear as lattice scattering. This results in the generation of ultra short spin waves down to 100 nm wavelength that cannot be described by classical dispersion relations for linear spin wave excitations. We expect that room temperature STCs will be a useful tool to investigate non-linear wave physics, as they can be easily generated and manipulated to control their spatial and temporal band structure.

cond-mat.mes-hall↗
arXiv2019

Magnons in a Quasicrystal: Propagation, Localization and Extinction of Spin Waves in Fibonacci Structures

Magnonic quasicrystals exceed the possibilities of spin wave (SW) manipulation offered by regular magnonic crystals, because of their more complex SW spectra with fractal characteristics. Here, we report the direct x-ray microscopic observation of propagating SWs in a magnonic quasicrystal, consisting of dipolarly coupled permalloy nanowires arranged in a one-dimensional Fibonacci sequence. SWs from the first and second band as well as evanescent waves from the band gap between them are imaged. Moreover, additional mini-band gaps in the spectrum are demonstrated, directly indicating an influence of the quasiperiodicity of the system. The experimental results are interpreted using numerical calculations and we deduce a simple model to estimate the frequency position of the magnonic gaps in quasiperiodic structures. The demonstrated features of SW spectra in one-dimensional magnonic quasicrystals allows utilizing this class of metamaterials for magnonics and makes them an ideal basis for future applications.

cond-mat.mes-hall↗
arXiv2018

Deterministic creation and deletion of a single magnetic skyrmion observed by direct time-resolved X-ray microscopy

Spintronic devices based on magnetic skyrmions are a promising candidate for next-generation memory applications due to their nanometre-size, topologically-protected stability and efficient current-driven dynamics. Since the recent discovery of room-temperature magnetic skyrmions, there have been reports of current-driven skyrmion displacement on magnetic tracks and demonstrations of current pulse-driven skyrmion generation. However, the controlled annihilation of a single skyrmion at room temperature has remained elusive. Here we demonstrate the deterministic writing and deleting of single isolated skyrmions at room temperature in ferrimagnetic GdFeCo films with a device-compatible stripline geometry. The process is driven by the application of current pulses, which induce spin-orbit torques, and is directly observed using a time resolved nanoscale X-ray imaging technique. We provide a current-pulse profile for the efficient and deterministic writing and deleting process. Using micromagnetic simulations, we also reveal the microscopic mechanism of the topological fluctuations that occur during this process.

cond-mat.mtrl-sci↗
arXiv2017

Direct observation of magnetic droplet solitons in all-perpendicular spin torque nano-oscillators

Magnetic droplets are non-topological dynamical solitons that can be nucleated and sustained in nano-contact based spin torque nano-oscillators (NC-STNOs) with perpendicular anisotropy free layers. While originally predicted in all-perpendicular NC-STNOs, all experimental demonstrations have so far relied on orthogonal devices with an in-plane polarizing layer that requires a strong magnetic field for droplet nucleation. Here, we instead show the nucleation and sustained operation of magnetic droplets in all-perpendicular NC-STNOs in modest perpendicular fields and over a wide range of nano-contact size. The droplet is observed electrically as an intermediate resistance state accompanied by broadband low-frequency microwave noise. Using canted fields, which introduce a non-zero relative angle between the free and fixed layer, the actual droplet precession frequency can also be determined. Finally, the droplet size, its perimeter width, and its fully reversed core are directly observed underneath a 80 nm diameter nano-contact using scanning transmission x-ray microscopy on both the Ni and Co edges. The droplet diameter is 150 nm, i.e. almost twice the nominal size of the nano-contact, and the droplet has a perimeter width of about 70 nm.

cond-mat.mes-hall↗
arXiv2016

Skyrmion Hall Effect Revealed by Direct Time-Resolved X-Ray Microscopy

Magnetic skyrmions are highly promising candidates for future spintronic applications such as skyrmion racetrack memories and logic devices. They exhibit exotic and complex dynamics governed by topology and are less influenced by defects, such as edge roughness, than conventionally used domain walls. In particular, their finite topological charge leads to a predicted "skyrmion Hall effect", in which current-driven skyrmions acquire a transverse velocity component analogous to charged particles in the conventional Hall effect. Here, we present nanoscale pump-probe imaging that for the first time reveals the real-time dynamics of skyrmions driven by current-induced spin orbit torque (SOT). We find that skyrmions move at a well-defined angle Θ_{SH} that can exceed 30° with respect to the current flow, but in contrast to theoretical expectations, Θ_{SH} increases linearly with velocity up to at least 100 m/s. We explain our observation based on internal mode excitations in combination with a field-like SOT, showing that one must go beyond the usual rigid skyrmion description to unravel the dynamics.

cond-mat.mtrl-sci↗
arXiv2016

Spin wave mediated unidirectional Vortex Core Reversal by Two Orthogonal Monopolar Field Pulses: The Essential Role of Three-dimensional Magnetization Dynamics

Scanning transmission x-ray microscopy is employed to investigate experimentally the reversal of the magnetic vortex core polarity in cylindrical Ni81Fe19 nanodisks triggered by two orthogonal monopolar magnetic field pulses with peak amplitude $B_0$, pulse length $τ$=60 ps and delay time $Δ$t in the range from -400 ps to +400 ps between the two pulses. The two pulses are oriented in-plane in the x- and y-direction. We have experimentally studied vortex core reversal as function of $B_0$ and $Δ$t. The resulting phase diagram shows large regions of unidirectional vortex core switching where the switching threshold is modulated due to resonant amplification of azimuthal spin waves. The switching behavior changes dramatically depending on whether the first pulse is applied in the x- or the y-direction. This asymmetry can be reproduced by three-dimensional micromagnetic simulations but not by two-dimensional simulations. This behavior demonstrates that in contrast to previous experiments on vortex core reversal the three-dimensionality in the dynamics is essential here.

cond-mat.mes-hall↗
arXiv2015

Enhanced Non-Adiabaticity in Vortex Cores due to the Emergent Hall Effect

We present a combined theoretical and experimental study, investigating the origin of the enhanced non-adiabaticity of magnetic vortex cores. Scanning transmission X-ray microscopy is used to image the vortex core gyration dynamically to measure the non-adiabaticity with high precision, including a high confidence upper bound. Using both numerical computations and analytical derivations, we show that the large non-adiabaticity parameter observed experimentally can be explained by the presence of local spin currents arising from a texture-induced emergent Hall effect. This enhanced non-adiabaticity is only present in two- and three-dimensional magnetic textures such as vortices and skyrmions and absent in one-dimensional domain walls, in agreement with experimental observations.

cond-mat.mes-hall↗
arXiv2015

Three-dimensional Character of the Magnetization Dynamics in Magnetic Vortex Structures - Hybridization of Flexure Gyromodes with Spin Waves

Three-dimensional linear spin-wave eigenmodes of a Permalloy disk having finite thickness are studied by micromagnetic simulations based on the Landau-Lifshitz-Gilbert equation. The eigenmodes found in the simulations are interpreted as linear superpositions (hybridizations) of 'approximate' three-dimensional eigenmodes, which are the fundamental gyromode $G_0$, the spin-wave modes and the higher-order gyromodes $G_N$ (flexure modes), the thickness dependence of which is represented by perpendicular standing spin waves. This hybridization leads to new and surprising dependencies of the mode frequencies on the disk thickness. The three-dimensional character of the eigenmodes is essential to explain the recent experimental results on vortex-core reversal observed in relatively thick Permalloy disks.

cond-mat.mes-hall↗
arXiv2015

Time-resolved imaging of pulse-induced magnetization reversal with a microwave assist field

The reversal of the magnetization under the influence of a field pulse has been previously predicted to be an incoherent process with several competing phenomena such as domain wall relaxation, spin wave-mediated instability regions, and vortex-core mediated reversal dynamics. However, there has been no study on the direct observation of the switching process with the aid of a microwave signal input. We report a time-resolved imaging study of magnetization reversal in patterned magnetic structures under the influence of a field pulse with microwave assistance. The microwave frequency is varied to demonstrate the effect of resonant microwave-assisted switching. We observe that the switching process is dominated by spin wave dynamics generated as a result of magnetic instabilities in the structures, and identify the frequencies that are most dominant in magnetization reversal.

cond-mat.mtrl-sci↗
arXiv2015

Observation of room temperature magnetic skyrmions and their current-driven dynamics in ultrathin Co films

Magnetic skyrmions are topologically-protected spin textures that exhibit fascinating physical behaviors and large potential in highly energy efficient spintronic device applications. The main obstacles so far are that skyrmions have been observed in only a few exotic materials and at low temperatures, and manipulation of individual skyrmions has not yet been achieved. Here, we report the observation of stable magnetic skyrmions at room temperature in ultrathin transition metal ferromagnets with magnetic transmission soft x-ray microscopy. We demonstrate the ability to generate stable skyrmion lattices and drive trains of individual skyrmions by short current pulses along a magnetic racetrack. Our findings provide experimental evidence of recent predictions and open the door to room-temperature skyrmion spintronics in robust thin-film heterostructures.

cond-mat.mtrl-sci↗
arXiv2014

Unidirectional sub-100 ps magnetic vortex core reversal

The magnetic vortex structure, an important ground state configuration in micron and sub-micron sized ferromagnetic thin film platelets, is characterized by a curling in-plane magnetization and, in the center, a minuscule region with out-of-plane magnetization, the vortex core, which points either up or down. It has already been demonstrated that the vortex core polarity can be reversed with external AC magnetic fields, frequency-tuned to the (sub-GHz) gyrotropic eigenmode or to (multi-GHz) azimuthal spin wave modes, where reversal times in the sub-ns regime can be realized. This fast vortex core switching may also be of technological interest as the vortex core polarity can be regarded as one data bit. Here we experimentally demonstrate that unidirectional vortex core reversal by excitation with sub-100 ps long orthogonal monopolar magnetic pulse sequences is possible in a wide range of pulse lengths and amplitudes. The application of such short digital pulses is the favourable excitation scheme for technological applications. Measured phase diagrams of this unidirectional, spin wave mediated vortex core reversal are in good qualitative agreement with phase diagrams obtained from micromagnetic simulations. The time dependence of the reversal process, observed by time-resolved scanning transmission X-ray microscopy indicates a switching time of 100 ps and fits well with our simulations. The origin of the asymmetric response to clockwise and counter clockwise excitation which is a prerequisite for reliable unidirectional switching is discussed, based on the gyromode - spin wave coupling.

cond-mat.other↗
arXiv2014

Ion beam lithography for Fresnel zone plates in X-ray microscopy

Fresnel Zone Plates (FZP) are to date very successful focusing optics for X-rays. Established methods of fabrication are rather complex and based on electron beam lithography (EBL). Here, we show that ion beam lithography (IBL) may advantageously simplify their preparation. A FZP operable from the extreme UV to the limit of the hard X-ray was prepared and tested from 450 eV to 1500 eV. The trapezoidal profile of the FZP favorably activates its 2nd order focus. The FZP with an outermost zone width of 100 nm allows the visualization of features down to 61, 31 and 21 nm in the 1st, 2nd and 3rd order focus respectively. Measured efficiencies in the 1st and 2nd order of diffraction reach the theoretical predictions.

physics.optics↗
arXiv2013

Low-amplitude magnetic vortex core reversal by non-linear interference between azimuthal spin waves and the vortex gyromode

We demonstrate a non-linear interference due to an active 'dual frequency' excitation of both, the sub-GHz vortex gyromode and multi-GHz magneto-static spin waves in ferromagnetic micrometer sized platelets in the vortex state. When the sub-GHz vortex gyromode is excited simultaneously a significant broadband reduction of the switching threshold for spin wave mediated vortex core reversal is observed in both, experiments and micromagnetic simulations. Consequently, the magnetic field amplitudes required for vortex core reversal can be lowered by nearly one order of magnitude. Moreover, additional spin wave resonance frequencies are found which emerge only if the vortex gyromode is actively excited simultaneously which can be explained by frequency doubling and by the broken symmetry of the vortex state.

cond-mat.mes-hall↗
arXiv2013

Wave modes of collective vortex gyration in dipolar-coupled-dot-array magnonic crystals

Lattice vibration modes are collective excitations in periodic arrays of atoms or molecules. These modes determine novel transport properties in solid crystals. Analogously, in periodical arrangements of magnetic vortex-state disks, collective vortex motions have been predicted. Here, we experimentally observe wave modes of collective vortex gyration in one-dimensional (1D) chains of periodic disks using time-resolved scanning transmission x-ray microscopy. The observed modes are interpreted based on micromagnetic simulation and numerical calculation of coupled Thiele equations. Dispersion of the modes is found to be strongly affected by both vortex polarization and chirality ordering, as revealed by the explicit analytical form of 1D infinite chains. A thorough understanding thereof is fundamental both for lattice vibrations and vortex dynamics, which we demonstrate for 1D magnonic crystals. Such magnetic disk arrays with vortex-state ordering, referred to as magnetic metastructure, offer potential implementation into information processing devices.

cond-mat.mes-hall↗
arXiv2011

200 ps Vortex Core Reversal by Azimuthal Spin Waves

Spin wave mediated vortex core reversal has been investigated by time-resolved scanning transmission X-ray microscopy (STXM). Movies showing the development of the spin wave and vortex core magnetization dynamics during unidirectional vortex core reversal could be taken in Permalloy discs, 1.6 μm in diameter and 50 nm thick, during excitation with rotating ac field bursts of one period duration at 4.5 GHz and with amplitudes up to 4 mT. Unidirectional switching is achieved by taking advantage of an asymmetry for CW or CCW excitation caused by the gyrofield. The differences in the magnetization dynamics due to this asymmetry could be imaged during continuous excitation with multi-GHz rotating fields. All our experimental results are in good agreement with micromagnetic simulations. In addition, for the sample geometry given above, simulations reveal a lower limit of about 200 ps for the time of unidirectional vortex core switching, which cannot be overcome by shortening the excitation length or by increasing the excitation amplitude. We explain this limitation by the finite time needed for an energy transfer of the global excitation towards the center of the sample. Thus smaller samples will allow for much shorter vortex core reversal times.

cond-mat.other↗
arXiv2011

Observation of non-linear magnetic vortex gyration by X-ray microscopy and micromagnetic simulations

Magnetic vortex gyration in a 500 nm wide, 50 nm thick Permalloy square platelet was investigated using time-resolved scanning transmission x-ray microscopy and micromagnetic simulations. The response of the vortex core on an in-plane oscillating magnetic field was studied as a function of the excitation frequency and amplitude. Non-linear behavior was observed in the form of a redshift of the gyrotropic resonance. I.e., when the excitation amplitude was increased, the resonance frequency was found to decrease. The nonlinearity is in agreement with extensive micromagnetic simulations. This work complements previous reports on thinner nanostructures where a frequency blueshift was observed.

cond-mat.mtrl-sci↗
arXiv2011

Magnetic Vortex Core Reversal by Excitation of Spin Waves

Micron-sized magnetic platelets in the flux closed vortex state are characterized by an in-plane curling magnetization and a nanometer-sized perpendicularly magnetized vortex core. Having the simplest non-trivial configuration, these objects are of general interest to micromagnetics and may offer new routes for spintronics applications. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field core toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was established. At frequencies more than an order of magnitude higher vortex state structures possess spin wave eigenmodes arising from the magneto-static interaction. Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited. These results are confirmed by micromagnetic simulations which clearly show the selection rules for this novel reversal mechanism. Our analysis reveals that for spin wave excitation the concept of a critical velocity as the switching condition has to be modified.

cond-mat.other↗