arXiv subjects

Hermann Stoll

Publications and source records attributed to Hermann Stoll.

At least 19 recordsLinked to original sources

arXiv2024

Identifying Switching of Antiferromagnets by Spin-Orbit Torques

Antiferromagnets are promising candidates for ultrafast spintronic applications, leveraging current-induced spin-orbit torques. However, experimentally distinguishing between different switching mechanisms of the staggered magnetization (N\'eel vector) driven by current pulses remains a challenge. In an exemplary study of the collinear antiferromagnetic compound Mn$_2$Au, we demonstrate that slower thermomagnetoelastic effects predominantly govern switching over a wide parameter range. In the regime of short current pulses in the nanosecond range, however, we observe fully N\'eel spin-orbit torque driven switching. We show that this ultrafast mechanism enables the complete directional alignment of the N\'eel vector by current pulses in device structures.

physics.app-ph↗

arXiv2021

Reconfigurable sub-micron spin-wave majority gate with electrical transducers

Spin waves are excitations in ferromagnetic media that have been proposed as information carriers in hybrid spintronic devices with much lower operation power than conventional charge-based electronics. Their wave nature can be exploited in majority gates by using interference for computation. However, a scalable spin-wave majority gate that can be co-integrated alongside conventional electronics is still lacking. Here, we demonstrate a sub-micron inline spin-wave majority gate with fan-out. Time-resolved imaging of the magnetization dynamics by scanning transmission x-ray microscopy illustrates the device operation. All-electrical spin-wave spectroscopy further demonstrates majority gates with sub-micron dimensions, reconfigurable input and output ports, and frequency-division multiplexing. Challenges for hybrid spintronic computing systems based on spin-wave majority gates are discussed.

physics.app-ph↗

arXiv2020

Quantification of competing magnetic states and switching pathways in curved nanowires by direct dynamic imaging

For viable applications, spintronic devices based e.g. on domain wall motion need to be highly reliable with stable magnetization states and highly reproducible switching pathways transforming one state to another. The existence of multiple stable states and switching pathways in a system is a definitive barrier for device operation, yet rare and stochastic events are difficult to detect and understand. We demonstrate an approach to quantify competing magnetic states and stochastic switching pathways based on time-resolved scanning electron microscopy with polarization analysis, applied to the technologically relevant control of vortex domain wall chirality via field and curvature in curved wires. While being a pump-probe technique, our analysis scheme nonetheless allows for the disentanglement of different occurring dynamic pathways and we can even identify the rare events leading to changes from one magnetization switching pathway to another pathway via temperature- and geometry-dependent measurements. The experimental imaging is supported by micromagnetic simulations to reveal the mechanisms responsible for the change of the pathway. Together the results allow us to explain the origin and details of the domain wall chirality control and to quantify the frequency and the associated energy barriers of thermally activated changes of the states and switching pathways.

cond-mat.mes-hall↗

arXiv2017

X-Ray Microscopy of Spin Wave Focusing using a Fresnel Zone Plate

Magnonics, i.e. the artificial manipulation of spin waves, is a flourishing field of research with many potential uses in data processing within reach. Apart from the technological applications the possibility to directly influence and observe these types of waves is of great interest for fundamental research. Guidance and steering of spin waves has been previously shown and lateral spin wave confinement has been achieved. However, true spin wave focusing with both lateral confinement and increase in amplitude has not been shown before. Here, we show for the first time spin wave focusing by realizing a Fresnel zone plate type lens. Using x-ray microscopy we are able to directly image the propagation of spin waves into the nanometer sized focal spot. Furthermore, we observe that the focal spot can be freely moved in a large area by small variations of the bias field. Thus, this type of lens provides a steerable intense nanometer sized spin wave source. Potentially, this could be used to selectively illuminate magnonic devices like nano oscillators with a steerable spin wave beam.

cond-mat.mes-hall↗

arXiv2017

Dealing with the exponential wall in electronic structure calculations

An alternative to Density Functional Theory are wavefunction based electronic structure calculations for solids. In order to perform them the Exponential Wall (EW) problem has to be resolved. It is caused by an exponential increase of the number of configurations with increasing electron number N. There are different routes one may follow. One is to characterize a many-electron wavefunction by a vector in Liouville space with a cumulant metric rather than in Hilbert space. This removes the EW problem. Another is to model the solid by an {\it impurity} or {\it fragment} embedded in a {\it bath} which is treated at a much lower level than the former. This is the case in Density Matrix Embedding Theory (DMET) or Density Embedding Theory (DET). The latter are closely related to a Schmidt decomposition of a system and to the determination of the associated entanglement. We show here the connection between the two approaches. It turns out that the DMET (or DET) has an identical active space as a previously used Local Ansatz, based on a projection and partitioning approach. Yet, the EW problem is resolved differently in the two cases. By studying a $H_{10}$ ring these differences are analyzed with the help of the method of increments.

physics.chem-ph↗

arXiv2016

Orbital breathing effects in the computation of x-ray d-ion spectra in solids by ab initio wave-function-based methods

In existing theoretical approaches to core-level excitations of transition-metal ions in solids relaxation and polarization effects due to the inner core hole are often ignored or described phenomenologically. Here we set up an ab initio computational scheme that explicitly accounts for such physics in the calculation of x-ray absorption and resonant inelastic x-ray scattering spectra. Good agreement is found with experimental transition-metal $L$-edge data for the strongly correlated $d^9$ cuprate Li$_2$CuO$_2$, for which we determine the absolute scattering intensities. The newly developed methodology opens the way for the investigation of even more complex $d^n$ electronic structures of group VI B to VIII B correlated oxide compounds.

cond-mat.str-el↗

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

Strongly frustrated triangular spin lattice emerging from triplet dimer formation in honeycomb Li2IrO3

In quantum magnetism spin dimers are typically associated with spin-singlet states. To date, $triplet$ $dimerization$ has not been observed in any 3D or quasi-2D material. With this in mind we here discuss the electronic structure of the spin-orbit driven $5d^5$ Mott insulator Li$_2$IrO$_3$, a honeycomb-lattice system with two crystallographically inequivalent Ir-Ir bonds. From $ab$ $initio$ many-body calculations we find that, while both Heisenberg and Kitaev couplings are present, the magnetic interactions are dominated by a strong isotropic ferromagnetic exchange on only one set of bonds. This causes the formation of triplet spin dimers effectively placed on a strongly frustrated triangular lattice. The triplet dimers remain protected in a large region of the phase diagram, suggesting that Li$_2$IrO$_3$ has a long-range incommensurate magnetic ground state that is pushed by the Kitaev exchange interactions beyond a standard planar helix configuration.

cond-mat.str-el↗

arXiv2015

Strong magnetic frustration and anti-site disorder causing spin-glass behavior in honeycomb Li2RhO3

With large spin-orbit coupling, the $t_{2g}^5$ electron configuration in $d$-metal oxides is prone to highly anisotropic exchange interactions and exotic magnetic properties. In $5d^5$ iridates, given the existing variety of crystal structures, the magnetic anisotropy can be tuned from antisymmetric to symmetric Kitaev-type, with interaction strengths that outsize the isotropic terms. By many-body electronic-structure calculations we here address the nature of the magnetic exchange and the intriguing spin-glass behavior of Li$_2$RhO$_3$, a $4d^5$ honeycomb oxide. For pristine crystals without Rh-Li site inversion, we predict a dimerized ground state as in the isostructural $5d^5$ iridate Li$_2$IrO$_3$, with triplet spin dimers effectively placed on a frustrated triangular lattice. With Rh-Li anti-site disorder, we explain the observed spin-glass phase as a superposition of different, nearly degenerate symmetry-broken configurations.

cond-mat.str-el↗

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↗

arXiv2014

Electron correlation effects in diamond: a wave-function quantum chemistry study of the quasiparticle band structure

The quasiparticle bands of diamond, a prototype covalent insulator, are herein studied by means of wave-function electronic-structure theory, with emphasis on the nature of the correlation hole around a bare particle. Short-range correlations are in such a system conveniently described by using a real-space representation and many-body techniques from {\it ab initio} quantum chemistry. To account for long-range polarization effects, on the other hand, we adopt the approximation of a dielectric continuum. Having as "uncorrelated" reference the Hartree-Fock band structure, the post-Hartree-Fock treatment is carried out in terms of localized Wannier functions derived from the Hartree-Fock solution. The computed correlation-induced corrections to the relevant real-space matrix elements are important and give rise to a strong reduction, in the range of $50\%$, of the initial Hartree-Fock gap. While our final results for the indirect and direct gaps, 5.4 and 6.9 eV, respectively, compare very well with the experimental data, the width of the valence band comes out by $10$ to $15\%$ too large as compared to experiment. This overestimation of the valence-band width appears to be related to size-consistency effects in the configuration-interaction correlation treatment.

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↗

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↗

arXiv2012

CaIrO3 post-perovskite, a j = 1/2 quasi-one-dimensional antiferromagnet

The 5d5 iridate CaIrO3 is isostructural with the post-perovskite phase of MgSiO3, recently shown to occur under extreme pressure in the lower Earth's mantle. It therefore serves as an analogue of post-perovskite MgSiO3 for a wide variety of measurements at ambient conditions or achievable with conventional multianvile pressure modules. By multireference configuration-interaction calculations we here provide essential information on the chemical bonding and magnetic interactions in CaIrO3. We predict a large antiferromagnetic superexchange of 120 meV along the c axis, the same size with the interactions in the cuprate superconductors, and ferromagnetic couplings smaller by an order of magnitude along a. CaIrO3 can thus be regarded as a j = 1/2 quasi-one-dimensional antiferromagnet. While this qualitatively agrees with the stripy magnetic structure proposed by resonant x-ray diffraction, the detailed microscopic picture emerging from our study, in particular, the highly uneven admixture of t2g components, provides a clear prediction for resonant inelastic x-ray scattering experiments.

cond-mat.str-el↗

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↗