arXiv subjects

Ingrid Mertig

Publications and source records attributed to Ingrid Mertig.

At least 55 records · Page 3Linked to original sources

arXiv2021

Thermal Hall effect of magnons in collinear antiferromagnetic insulators: signatures of magnetic and topological phase transitions

We demonstrate theoretically that the thermal Hall effect of magnons in collinear antiferromagnetic insulators is an indicator of magnetic and topological phase transitions in the magnon spectrum. The transversal heat current of magnons caused by a thermal gradient is calculated for an antiferromagnet on a honeycomb lattice. An applied magnetic field drives the system from the antiferromagnetic phase via a spin-flop phase into the field-polarized phase. Besides these magnetic phase transitions we find topological phase transitions within the spin-flop phase. Both types of transitions manifest themselves in prominent and distinguishing features in the thermal conductivities; depending on the temperature, the conductivity changes by several orders of magnitude, providing a tool to discern experimentally the two types of phase transitions. We include numerical results for the van der Waals magnet MnPS$_3$.

cond-mat.str-el↗

arXiv2021

Seebeck Effect in Nanomagnets

We present a theory of the Seebeck effect in nanomagnets with dimensions smaller than the spin diffusion length, showing that the spin accumulation generated by a temperature gradient strongly affects the thermopower. We also identify a correction arising from the transverse temperature gradient induced by the anomalous Ettingshausen effect and an induced spin-heat accumulation gradient. The relevance of these effects for nanoscale magnets is illustrated by ab initio calculations on dilute magnetic alloys.

cond-mat.mes-hall↗

arXiv2021

Atomic scale control of spin current transmission at interfaces

Spin transmission at ferromagnet/heavy metal interfaces is of vital importance for many spintronic devices. Usually the spin current transmission is limited by the spin mixing conductance and loss mechanisms such as spin memory loss. In order to understand these effects, we study the interface transmission when an insulating interlayer is inserted between the ferromagnet and the heavy metal. For this we measure the inverse spin Hall voltage generated from optically injected spin current pulses as well as the magnitude of the spin pumping using ferromagnetic resonance. From our results we conclude that significant spin memory loss only occurs for 5d metals with less than half filled d-shell.

cond-mat.mes-hall↗

arXiv2021

Control of ultrashort spin current pulses in metallic multilayers

We study the emission of femtosecond spin current pulses at Fe/Au interfaces. Using optical second harmonic generation we demonstrate the control of pulse shape and duration by varying the emitter thickness and show that it is determined by the parameters of hot electrons emission and the diffusive transport. Using a simple model of electron emission we develop a description of superdiffusive spin transport in Au allowing to extract electron velocity and scattering rates from the experimental data. The importance of carrier multiplication in Fe is demonstrated.

cond-mat.mes-hall↗

arXiv2021

Quaternary-digital data storage based on magnetic bubbles in anisotropic materials

The topologically non-trivial nano-whirls, called magnetic skyrmions, are often considered attractive for spintronic applications like the racetrack data storage device. However, skyrmions do not move parallel to applied currents and typically do not coexist with other nano-objects, making the realization of such storage concepts difficult. Herein we consider materials with an anisotropic DMI, like tetragonal Heusler materials, and show that four distinct types of topologically trivial bubbles can coexist. We show that each of them can be written by spin torques and that they can be distinguished using a single magnetic tunneling junction. Due to their trivial topology, the four types of bubbles move parallel to applied electrical currents and remain equidistant under motion. Still, the bubbles have a small size and high stability comparable to topologically non-trivial spin textures. This allows to construct a quaternary-digit-based racetrack storage device that overcomes the two mentioned drawbacks of skyrmion-based racetracks.

cond-mat.str-el↗

arXiv2021

Local spin Hall conductivity

The spin Hall conductivity is usually calculated for periodic solids, while the knowledge of local contributions from inhomogeneous regions like interfaces or surfaces would be desirable. In this work we derive a local expression for the spin Hall conductivity of two-dimensional systems in analogy to the local anomalous Hall conductivity discussed recently. Our approach is applicable to heterogeneous fully bounded systems and nanoribbons with both open and periodic boundary conditions.

cond-mat.mes-hall↗

arXiv2021

Broadband terahertz probes of anisotropic magnetoresistance disentangle extrinsic and intrinsic contributions

Anisotropic magnetoresistance (AMR) is a ubiquitous and versatile probe of magnetic order in contemporary spintronics research. Its origins are usually ascribed to extrinsic effects (i.e. spin-dependent electron scattering), whereas intrinsic (i.e. scattering-independent) contributions are neglected. Here, we measure AMR of polycrystalline thin films of the standard ferromagnets Co, Ni, Ni81Fe19 and Ni50Fe50 over the frequency range from DC to 28 THz. The large bandwidth covers the regimes of both diffusive and ballistic intraband electron transport and, thus, allows us to separate extrinsic and intrinsic AMR components. Analysis of the THz response based on Boltzmann transport theory reveals that the AMR of the Ni, Ni81Fe19 and Ni50Fe50 samples is of predominantly extrinsic nature. However, the Co thin film exhibits a sizeable intrinsic AMR contribution, which is constant up to 28 THz and amounts to more than 2/3 of the DC AMR contrast of 1%. These features are attributed to the hexagonal structure of the Co crystallites. They are interesting for applications in terahertz spintronics and terahertz photonics. Our results show that broadband terahertz electromagnetic pulses provide new and contact-free insights into magneto-transport phenomena of standard magnetic thin films on ultrafast time scales.

cond-mat.mes-hall↗

arXiv2020

Ultrafast spin dynamics in inhomogeneous systems: a density-matrix approach applied to Co/Cu interfaces

Ultrafast spin dynamics on femto- to picosecond timescales is simulated within a density-operator approach for a Co/Cu bilayer. The electronic structure is represented in a tight-binding form; during the evolution of the density operator, optical excitation by a femtosecond laser pulse, coupling to a bosonic bath as well as dephasing are taken into account. Our simulations corroborate the importance of interfaces for ultrafast transport phenomena and demagnetisation processes. Moreover, we establish a reflow from Cu $d$ orbitals across the interface into Co $d$ orbitals, which shows up prominently in the mean occupation numbers. On top of this, this refilling manifests itself as a minority-spin current proceeding several layers into the Cu region. The present study suggests that the approach captures essential ultrafast phenomena and provides insight into microscopic processes.

cond-mat.mtrl-sci↗

arXiv2020

Skyrmion ratchet propagation: Utilizing the skyrmion Hall effect in AC racetrack storage devices

Magnetic skyrmions are whirl-like nano-objects with topological protection. When driven by direct currents (DC), skyrmions move but experience a transverse deflection. This so-called skyrmion Hall effect is often regarded a drawback for memory applications. Herein, we show that this unique effect can also be favorable for spintronic applications: We show that in a racetrack with a broken inversion symmetry, the skyrmion Hall effect allows to translate an alternating current (AC) into a directed motion along the track, like in a ratchet. We analyze several modes of the ratchet mechanism and show that it is unique for topological magnetic whirls. We elaborate on the fundamental differences compared to the motion of topologically trivial magnetic objects, as well as classical particles driven by periodic forces. Depending on the exact racetrack geometry, the ratchet mechanism can be soft or strict. In the latter case, the skyrmion propagates close to the efficiency maximum.

cond-mat.mes-hall↗

arXiv2020

Beyond skyrmions: Review and perspectives of alternative magnetic quasiparticles

Magnetic skyrmions have attracted enormous research interest since their discovery a decade ago. The non-trivial real-space topology of these nano-whirls leads to fundamentally interesting and technologically relevant consequences - the skyrmion Hall effect of the texture and the topological Hall effect of the electrons. Furthermore, it grants skyrmions in a ferromagnetic surrounding great stability even at small sizes, making skyrmions aspirants to become the carriers of information in the future. Still, the utilization of skyrmions in spintronic devices has not been achieved yet, among other reasons, due to shortcomings in their current-driven motion. In this review, we present recent trends in the field of topological spin textures that go beyond skyrmions. The majority of these objects can be considered a combination of multiple subparticles, such as the bimeron, or the skyrmion analogues in different magnetic surroundings, such as antiferromagnetic skyrmions, as well as three-dimensional generalizations, such as hopfions. We classify the alternative magnetic quasiparticles - some of them observed experimentally, others theoretical predictions - and present the most relevant and auspicious advantages of this emerging field.

cond-mat.mes-hall↗

arXiv2020

Unconventional charge-spin conversion in Weyl-semimetal WTe2

An outstanding feature of topological quantum materials is their novel spin topology in the electronic band structures with an expected large charge-to-spin conversion efficiency. Here, we report a charge current-induced spin polarization in the type-II Weyl semimetal candidate WTe2 and efficient spin injection and detection in a graphene channel up to room temperature. Contrary to the conventional spin Hall and Rashba-Edelstein effects, our measurements indicate an unconventional charge-to-spin conversion in WTe2, which is primarily forbidden by the crystal symmetry of the system. Such a large spin polarization can be possible in WTe2 due to a reduced crystal symmetry combined with its large spin Berry curvature, spin-orbit interaction with a novel spin-texture of the Fermi states. We demonstrate a robust and practical method for electrical creation and detection of such a spin polarization using both charge-to-spin conversion and its inverse phenomenon and utilized it for efficient spin injection and detection in a graphene channel up to room temperature. These findings open opportunities for utilizing topological Weyl materials as non-magnetic spin sources in allelectrical van der Waals spintronic circuits and for low-power and high-performance non-volatile spintronic technologies.

cond-mat.mes-hall↗

arXiv2020

Spin and orbital Edelstein effect in an oxide two-dimensional electron gas: theory and application to AlO$_x$/SrTiO$_{3}$

The Edelstein effect provides the purely electrical generation and control of a homogeneous magnetization in primarily nonmagnetic materials with broken inversion symmetry. Usually, only the spin density response to an external electric field is discussed. Here, we report on the electrically induced magnetization containing spin as well as orbital contributions in the topological oxide two-dimensional electron gas at the interface between SrTiO$_3$ and AlO. We find that in this particular system the orbital Edelstein effect exceeds the spin Edelstein effect by more than one order of magnitude. The main reason are orbital moments of different magnitude in the Rashba-like-split band pairs.

cond-mat.mes-hall↗

arXiv2020

Orbital Magnetic Moment of Magnons

In experiments and applications usually the spin magnetic moment of magnons is considered. In this Paper we identify an additional degree of freedom of magnons: an \emph{orbital} magnetic moment brought about by spin-orbit coupling.Our microscopic theory uncovers that spin magnetization $\vec{M}^\mathrm{S}$ and orbital magnetization $\vec{M}^\mathrm{O}$ are independent quantities. They are not necessarily collinear; thus, even when the total spin moment is compensated due to antiferromagnetism ($\vec{M}^\mathrm{S} = \vec{0}$), $\vec{M}^\mathrm{O}$ may be nonzero. This scenario of orbital weak ferromagnetism is realized in paradigmatic kagome antiferromagnets with Dzyaloshinskii-Moriya interaction. We demonstrate that magnets exhibiting a magnonic orbital moment are omnipresent and propose transport experiments for probing it.

cond-mat.str-el↗

arXiv2020

Topological Hall signatures of magnetic hopfions

Magnetic hopfions are topologically protected three-dimensional solitons that are constituted by a tube which exhibits a topologically nontrivial spin texture in the cross-section profile and is closed to a torus. Here we show that the hopfion's locally uncompensated emergent field leads to a topological Hall signature, although the topological Hall effect vanishes on the global level. The topological Hall signature is switchable by magnetic fields or electric currents and occurs independently of the anomalous and conventional Hall effects. It can therefore be exploited to electrically detect hopfions in experiments and even to distinguish them from other textures like skyrmion tubes. Furthermore, it can potentially be utilized in spintronic devices. Exemplarily, we propose a hopfion-based racetrack data storage device and simulate the electrical detection of the hopfions as carriers of information.

cond-mat.mes-hall↗

arXiv2020

Compensated Quantum and Topological Hall Effects of Electrons in Polyatomic Stripe Lattices

The quantum Hall effect is generally understood for free electron gases, in which topologically protected edge states between Landau levels (LLs) form conducting channels at the edge of the sample. In periodic crystals, the LLs are imprinted with lattice properties; plateaus in the transverse Hall conductivity are not equidistant in energy anymore. Herein, crystals with a polyatomic basis are considered. For a stripe arrangement of different atoms, the band structure resorts nontrivially and exhibits strong oscillations that form a salient pattern with very small bandgaps. The Hall conductivity strongly decreases for energies within these bands, and only sharp peaks remain for energies in the gap. These effects are traced back to open orbits in the initial band structure; the corresponding LLs are formed from states with positive and negative effective mass. The partial cancellation of transverse charge conductivity also holds for different polyatomic stripe lattices and even when the magnetic field is replaced by a topologically nontrivial spin texture. The topological Hall effect is suppressed in the presence of magnetic skyrmions. The discussion is complemented by calculations of Hofstadter butterflies and orbital magnetization.

cond-mat.str-el↗

arXiv2019

Origin of the Magnetic Spin Hall Effect: Spin Current Vorticity in the Fermi Sea

The interplay of spin-orbit coupling (SOC) and magnetism gives rise to a plethora of charge-to-spin conversion phenomena that harbor great potential for spintronics applications. In addition to the spin Hall effect, magnets may exhibit a magnetic spin Hall effect (MSHE), as was recently discovered [Kimata \textit{et al.}, Nature \textbf{565}, 627-630 (2019)]. To date, the MSHE is still awaiting its intuitive explanation. Here we relate the MSHE to the vorticity of spin currents in the Fermi sea, which explains pictorially the origin of the MSHE. For all magnetic Laue groups that allow for nonzero spin current vorticities the related tensor elements of the MSH conductivity are given. Minimal requirements for the occurrence of a MSHE are compatibility with either a magnetization or a magnetic toroidal quadrupole. This finding implies in particular that the MSHE is expected in all ferromagnets with sufficiently large SOC. To substantiate our symmetry analysis, we present various models, in particular a two-dimensional magnetized Rashba electron gas, that corroborate an interpretation by means of spin current vortices. Considering thermally induced spin transport and the magnetic spin Nernst effect in magnetic insulators, which are brought about by magnons, our findings for electron transport can be carried over to the realm of spincaloritronics, heat-to-spin conversion, and energy harvesting.

cond-mat.mes-hall↗

arXiv2019

Electrical writing, deleting, reading, and moving of magnetic skyrmioniums in a racetrack device

A magnetic skyrmionium (also called 2$π$-skyrmion) can be understood as a skyrmion - a topologically non-trivial magnetic whirl - which is situated in the center of a second skyrmion with reversed magnetization. Here, we propose a new optoelectrical writing and deleting mechanism for skyrmioniums in thin films, as well as a reading mechanism based on the topological Hall voltage. Furthermore, we point out advantages for utilizing skyrmioniums as carriers of information in comparison to skyrmions with respect to the current-driven motion. We simulate all four constituents of an operating skyrmionium-based racetrack storage device: creation, motion, detection and deletion of bits. The existence of a skyrmionium is thereby interpreted as a '1' and its absence as a '0' bit.

cond-mat.str-el↗

arXiv2019

Forming individual magnetic biskyrmions by merging two skyrmions in a centrosymmetric nanodisk

When two magnetic skyrmions - whirl-like, topologically protected quasiparticles - form a bound pair, a biskyrmion state with a topological charge of $N_\mathrm{Sk}=\pm 2$ is constituted. Recently, especially the case of two partially overlapping skyrmions has brought about great research interest. Since for its formation the individual skyrmions need to posses opposite in-plane magnetizations, such a biskyrmion cannot be stabilized by the Dzyaloshinskii-Moriya-interaction (DMI), which is the interaction that typically stabilizes skyrmions in non-centrosymmetric materials and at interfaces. Here, we show that these biskyrmions can be stabilized by the dipole-dipole interaction in centrosymmetric materials in which the DMI is forbidden. Analytical considerations indicate that the bound state of a biskyrmion is energetically preferable over two individual skyrmions. As a result, when starting from two skyrmions in a micromagnetic simulation, a biskyrmion is formed upon relaxation. We propose a scheme that allows to control this biskyrmion formation in nanodisks and analyze the individual steps.

cond-mat.str-el↗