arXiv subjects

Dominik Kriegner

Publications and source records attributed to Dominik Kriegner.

At least 37 records · Page 2Linked to original sources

arXiv2023

Magnetic domain engineering in antiferromagnetic CuMnAs and Mn$_2$Au devices

Antiferromagnetic materials hold potential for use in spintronic devices with fast operation frequencies and field robustness. Despite the rapid progress in proof-of-principle functionality in recent years, there has been a notable lack of understanding of antiferromagnetic domain formation and manipulation, which translates to either incomplete or non-scalable control of the magnetic order. Here, we demonstrate simple and functional ways of influencing the domain structure in CuMnAs and Mn2Au, two key materials of antiferromagnetic spintronics research, using device patterning and strain engineering. Comparing x-ray microscopy data from two different materials, we reveal the key parameters dictating domain formation in antiferromagnetic devices and show how the non-trivial interaction of magnetostriction, substrate clamping and edge anisotropy leads to specific equilibrium domain configurations. More specifically, we observe that patterned edges have a significant impact on the magnetic anisotropy and domain structure over long distances, and we propose a theoretical model that relates short-range edge anisotropy and long-range magnetoelastic interactions. The principles invoked are of general applicability to the domain formation and engineering in antiferromagnetic thin films at large, which will pave the way towards realizing truly functional antiferromagnetic devices.

cond-mat.mtrl-sci↗

arXiv2023

Probing magnetic properties at the nanoscale: In-situ Hall measurements in a TEM

We report on advanced in-situ magneto-transport measurements in a transmission electron microscope. The approach allows for concurrent magnetic imaging and high resolution structural and chemical characterization of the same sample. Proof-of-principle in-situ Hall measurements on presumably undemanding nickel thin films supported by micromagnetic simulations reveal that in samples with non-trivial structures and/or compositions, detailed knowledge of the latter is indispensable for a thorough understanding and reliable interpretation of the magneto-transport data. The proposed in-situ approach is thus expected to contribute to a better understanding of the Hall signatures in more complex magnetic textures.

cond-mat.other↗

arXiv2022

Collective topological spin dynamics in a correlated spin glass

The interplay between spin-orbit interaction (SOI) and magnetic order is currently one of the most active research fields in condensed matter physics and leading the search for materials with novel and tunable magnetic and spin properties. Here we report on a variety of unexpected and unique observations in thin multiferroic \Ge$_{1-x}$Mn$_x$Te films. The ferrimagnetic order in this ferroelectric semiconductor is found to reverse with current pulses six orders of magnitude lower as for typical spin-orbit torque systems. Upon a switching event, the magnetic order spreads coherently and collectively over macroscopic distances through a correlated spin-glass state. Lastly, we present a novel methodology to controllably harness this stochastic magnetization dynamics, allowing us to detect spatiotemporal nucleation of topological spin textures we term ``skyrmiverres''.

cond-mat.mtrl-sci↗

arXiv2022

Anomalous Nernst effect in Mn$_3$NiN thin films

The observation of a sizable anomalous Hall effect in magnetic materials with vanishing magnetization has renewed interest in understanding and engineering this phenomenon. Antiferromagnetic antiperovskites are one of emerging material classes that exhibit a variety of interesting properties owing to a complex electronic band structure and magnetic ordering. Reports on the anomalous Nernst effect and its magnitude in this class of materials are, however, very limited. This scarcity may be partly due to the experimental difficulty of reliably quantifying the anomalous Nernst coefficient. Here, we report experiments on the anomalous Nernst effect in antiferromagnetic antiperovskite Mn$_3$NiN thin films. Measurement of both the anomalous Hall and Nernst effects using the same sample and measurement geometry makes it possible to directly compare these two effects and quantify the anomalous Nernst coefficient and conductivity in Mn$_3$NiN. We carefully evaluate the spatial distribution of the thermal gradient in the sample and use finite element modeling to corroborate our experimental results.

cond-mat.mtrl-sci↗

arXiv2021

Defect-driven antiferromagnetic domain walls in CuMnAs films

Efficient manipulation of antiferromagnetic (AF) domains and domain walls has opened up new avenues of research towards ultrafast, high-density spintronic devices. AF domain structures are known to be sensitive to magnetoelastic effects, but the microscopic interplay of crystalline defects, strain and magnetic ordering remains largely unknown. Here, we reveal, using photoemission electron microscopy combined with scanning X-ray diffraction imaging and micromagnetic simulations, that the AF domain structure in CuMnAs thin films is dominated by nanoscale structural twin defects. We demonstrate that microtwin defects, which develop across the entire thickness of the film and terminate on the surface as characteristic lines, determine the location and orientation of 180 degree and 90 degree domain walls. The results emphasize the crucial role of nanoscale crystalline defects in determining the AF domains and domain walls, and provide a route to optimizing device performance.

cond-mat.mtrl-sci↗

arXiv2021

Macroscopic time reversal symmetry breaking by staggered spin-momentum interaction

Time-reversal (T) symmetry breaking is a fundamental physics concept underpinning a broad science and technology area, including topological magnets, axion physics, dissipationless Hall currents, or spintronic memories. A best known conventional model of macroscopic T-symmetry breaking is a ferromagnetic order of itinerant Bloch electrons with an isotropic spin interaction in momentum space. Anisotropic electron interactions, on the other hand, have been a domain of correlated quantum phases, such as the T-invariant nematics or unconventional superconductors. Here we report discovery of a broken-T phase of itinerant Bloch electrons with an unconventional anisotropic spin-momentum interaction, whose staggered nature leads to the formation of two ferromagnetic-like valleys in the momentum space with opposite spin splittings. We describe qualitatively the effect by deriving a non-relativistic single-particle Hamiltonian model. Next, we identify the unconventional staggered spin-momentum interaction by first-principles electronic structure calculations in a four-sublattice antiferromagnet Mn5Si3 with a collinear checkerboard magnetic order. We show that the staggered spin-momentum interaction is set by nonrelativistic spin-symmetries which were previously omitted in relativistic physics classifications of spin interactions and topological quasiparticles. Our measurements of a spontaneous Hall effect in epilayers of antiferromagnetic Mn5Si3 with vanishing magnetization are consistent with our theory predictions. Bloch electrons with the unconventional staggered spin interaction, compatible with abundant low atomic-number materials, strong spin-coherence, and collinear antiferromagnetic order open unparalleled possibilities for realizing T-symmetry broken spin and topological quantum phases.

cond-mat.mes-hall↗

arXiv2020

Anisotropic magneto-thermal transport in Co$_2$MnGa thin films

Ferromagnetic Co$_2$MnGa has recently attracted significant attention due to effects related to non-trivial topology of its band structure, however a systematic study of canonical magneto-galvanic transport effects is missing. Focusing on high quality thin films, here we systematically measure anisotropic magnetoresistance (AMR) and its thermoelectric counterpart (AMTP). We model the AMR data by free energy minimisation within the Stoner-Wohlfarth formalism and conclude that both crystalline and non-crystalline components of this magneto-transport phenomenon are present in Co$_2$MnGa. Unlike the AMR which is small in relative terms, the AMTP is large due to a change of sign of the Seebeck coefficient as a function of temperature. This fact is discussed in the context of the Mott rule and further analysis of AMTP components is presented.

cond-mat.mtrl-sci↗

arXiv2020

Twin Domain Structure in Magnetically Doped Topological Insulators

Twin domains are naturally present in the topological insulator \BiSe{} and affect strongly its properties. While studies of its behavior for ideal \BiSe{} structure exist, little is known about their possible interaction with other defects. Extra information are needed especially for the case of artificial perturbation of topological insulator states by magnetic doping, which has attracted a lot of attention recently. Employing ab initio calculations based on layered Green's function formalism, we study the interaction between twin planes in \BiSe{}. We show the influence of various magnetic and non-magnetic chemical defects on the twin plane formation energy and discuss the related modification of their distribution. Furthermore, we examine the change of dopants' magnetic properties at sites in the vicinity of a twin plane, and the dopants' preference to occupy such sites. Our results suggest that twin planes repel each other at least over distance of $3-4$~nm. However, in the presence of magnetic Mn and Fe defects a close TP placement is preferred. Furthermore, calculated twin plane formation energies indicate that in this situation their formation becomes suppressed. Finally, we discuss the influence of twin planes on the surface band gap.

cond-mat.mtrl-sci↗

arXiv2020

Role of magnetic exchange interactions in chiral-type Hall effects of epitaxial Mn$_{x}$PtSn films

Tetragonal Mn-based Heusler compounds feature rich exchange interactions and exotic topological magnetic textures, such as antiskyrmions, complimented by the chiral-type Hall effects. This makes the material class interesting for device applications. We report the relation of the magnetic exchange interactions to the thickness and Mn concentration of Mn$_{x}$PtSn films, grown by magnetron sputtering. The competition of the magnetic exchange interactions determines the finite temperature magnetic texture and thereby the chiral-type Hall effects in external magnetic fields. We investigate the magnetic and transport properties as a function of magnetic field and temperature. We focus on the anomalous and chiral-type Hall effects and the behavior of the dc-magnetization, in relation to chiral spin textures. We further determine the stable crystal phase for a relative Mn concentration between 1.5 and 1.85 in the $I\overline{4}2d$ structure. We observe a spin-reorientation transition in all compounds studied, which is due to the competition of exchange interactions on different Mn sublattices. We discuss our results in terms of exchange interactions and compare them with theoretical atomistic spin calculations.

cond-mat.mtrl-sci↗

arXiv2020

Thickness dependence of the anomalous Nernst effect and the Mott relation of Weyl-semimetal Co2MnGa thin films

We report a robust anomalous Nernst effect in Co2MnGa thin films in the thickness regime between 20 and 50 nm. The anomalous Nernst coefficient varied in the range of -2.0 to -3.0 uV/K at 300 K. We demonstrate that the anomalous Hall and Nernst coefficients exhibit similar behavior and fulfill the Mott relation. We simultaneously measure all four transport coefficients of the longitudinal resistivity, transversal resistivity, Seebeck coefficient, and anomalous Nernst coefficient. We connect the values of the measured and calculated Nernst conductivity by using the remaining three magneto-thermal transport coefficients, where the Mott relation is still valid. The intrinsic Berry curvature dominates the transport due to the relation between the longitudinal and transversal transport. Therefore, we conclude that the Mott relationship is applicable to describe the magneto-thermoelectric transport in Weyl semimetal Co2MnGa as a function of film thickness.

cond-mat.mes-hall↗

arXiv2020

Investigation of Nanostructures with X-ray Scattering Techniques

The structural investigations of nanomaterials motivated by their large variety and diverse set of applications have attracted considerable attention. In particular, the ever-improving machinery, both in laboratory and at large scale facilities, together with the methodical improvements available for studying nanostructures ranging from epitaxial nanomaterials, nanocrystalline thin films and coatings, to nanoparticles and colloidal nanocrystals allows us to gain a more detailed understanding of their structural properties. As the structure essentially determines the physical properties of the materials, this advances the possibilities of structural studies and also enables a deeper understanding of the structure to property relationships. In this special issue entitled "Investigation of Nanostructures with X-ray Scattering Techniques" five contributions show the recent progress in various research fields. Contributions cover topics as diverse as neutron scattering on magnetic multilayer films, epitaxial orientation of organic thin films, nanoparticle ordering and chemical composition analysis, and the combination of nanofocused X-ray beams with electrical measurements.

cond-mat.mtrl-sci↗

arXiv2019

Molecular beam epitaxy of CuMnAs

We present a detailed study of the growth of the tetragonal polymorph of antiferromagnetic CuMnAs by the molecular beam epitaxy technique. We explore the parameter space of growth conditions and their effect on the microstructural and transport properties of the material. We identify its typical structural defects and compare the properties of epitaxial CuMnAs layers grown on GaP, GaAs and Si substrates. Finally, we investigate the correlation between the crystalline quality of CuMnAs and its performance in terms of electrically induced resistance switching.

cond-mat.mtrl-sci↗

arXiv2019

Thickness dependence of the anomalous Hall effect in thin films of the topological semimetal Co$_2$MnGa

Topological magnetic semimetals promise large Berry curvature through the distribution of the topological Weyl nodes or nodal lines and further novel physics with exotic transport phenomena. We present a systematic study of the structural and magnetotransport properties of Co$_2$MnGa films from thin (20 nm) to bulk like behavior (80 nm), in order to understand the underlying mechanisms and the role on the topology. The magnetron sputtered Co$_2$MnGa films are $L$$2_{\mathrm {1}}$-ordered showing very good heteroepitaxy and a strain-induced tetragonal distortion. The anomalous Hall conductivity was found to be maximum at a value of 1138 S/cm, with a corresponding anomalous Hall angle of 13 %, which is comparatively larger than topologically trivial metals. There is a good agreement between the theoretical calculations and the Hall conductivity observed for the 80 nm film, which suggest that the effect is intrinsic. Thus, the Co$_2$MnGa compound manifests as a promising material towards topologically-driven spintronic applications.

cond-mat.mtrl-sci↗

arXiv2019

Imaging and writing magnetic domains in the non-collinear antiferromagnet Mn$_{\text{3}}$Sn

Harnessing the unique properties of non-collinear antiferromagnets (AFMs) will be essential for exploiting the full potential of antiferromagnetic spintronics. Indeed, many of the effects enabling ferromagnetic spintronic devices have a corresponding counterpart in materials with non-collinear spin structure. In addition, new phenomena such as the magnetic spin Hall effect were experimentally observed in non-collinear AFMs, and the presence of the equivalent to the ferromagnetic spin transfer torque via spin polarized currents was theoretically predicted. In spite of these developments, an interpretation of the rich physical phenomena observed in non-collinear antiferromagnets is challenging, since the microscopic spin arrangement, the magnetic domain distribution, and the domain orientations have proven notoriously difficult to access experimentally. This is all the more problematic, as imaging and writing magnetic domains is of central importance for applications. Successful imaging is a basic requirement to experimentally confirm the spin transfer torque acting on non-collinear domain walls and therefore of eminent interest. Here, we demonstrate that the local magnetic structure of the non-collinear AFM Mn3Sn films can be imaged by scanning thermal gradient microscopy (STGM). The technique is based on scanning a laser spot over the sample's surface, and recording the ensuing thermo-voltage. We image the magnetic structure at a series of different temperatures and show that at room temperature, the domain structure is not affected by the application of moderate magnetic fields. In addition to imaging, we establish a scheme for heat-assisted magnetic recording, using local laser heating in combination with magnetic fields to intentionally write domain patterns into the antiferromagnet.

cond-mat.mes-hall↗

arXiv2018

Topological Hall effect in thin films of Mn$_{1.5}$PtSn

Spin chirality in metallic materials with non-coplanar magnetic order can give rise to a Berry phase induced topological Hall effect. Here, we report the observation of a large topological Hall effect in high-quality films of Mn$_{1.5}$PtSn that were grown by means of magnetron sputtering on MgO(001). The topological Hall resistivity is present up to $μ_{0}H \approx 4~$T below the spin reorientation transition temperature, $T_{s}=185$~K. We find, that the maximum topological Hall resistivity is of comparable magnitude as the anomalous Hall resistivity. Owing to the size, the topological Hall effect is directly evident prior to the customarily performed subtraction of magnetometry data. Further, we underline the robustness of the topological Hall effect in Mn\textsubscript{2-x}PtSn by extracting the effect for multiple stoichiometries (x~=~0.5, 0.25, 0.1) and film thicknesses (t = 104, 52, 35~nm) with maximum topological Hall resistivities between $0.76~μΩ$cm and $1.55~μΩ$cm at 150~K.

cond-mat.mtrl-sci↗

arXiv2018

Giant magnetic response of a two-dimensional antiferromagnet

A fundamental difference between antiferromagnets and ferromagnets is the lack of linear coupling to a uniform magnetic field due to the staggered order parameter. Such coupling is possible via the Dzyaloshinskii-Moriya (DM) interaction but at the expense of reduced antiferromagnetic (AFM) susceptibility due to the canting-induced spin anisotropy. We solve this long-standing problem with a top-down approach that utilizes spin-orbit coupling in the presence of a hidden SU(2) symmetry. We demonstrate giant AFM responses to sub-Tesla external fields by exploiting the extremely strong two-dimensional critical fluctuations preserved under a symmetry-invariant exchange anisotropy, which is built into a square-lattice artificially synthesized as a superlattice of SrIrO3 and SrTiO3. The observed field-induced logarithmic increase of the ordering temperature enables highly efficient control of the AFM order. As antiferromagnets promise to afford switching speed and storage security far beyond ferromagnets, our symmetry-invariant approach unleashes the great potential of functional antiferromagnets.

cond-mat.str-el↗

arXiv2017

Magnetic structure of NdMn$_{0.8}$Fe$_{0.2}$O$_{3+δ}$; neutron powder diffraction experiment

The magnetic structure of the mixed antiferromagnet NdMn$_{0.8}$Fe$_{0.2}$O$_3$ was resolved. Neutron powder diffraction data definitively resolve the Mn-sublattice with a magnetic propagation vector ${\bf k} = (000)$ and with the magnetic structure (A$_x$, F$_y$, G$_z$) for 1.6~K~$< T < T_N (\approx 59$~K). The Nd-sublattice has a (0, f$_y$, 0) contribution in the same temperature interval. The Mn sublattice undergoes spin-reorientation transition at $T_1 \approx 13$~K while the Nd magnetic moment keep ordered abruptly increases at this temperature. Powder X-ray diffraction shows a strong magnetoelastic effect at $T_N$ but no additional structural phase transitions from 2~K to 300~K. Density functional theory calculations confirm the magnetic structure of the undoped NdMnO$_3$ as part of our analysis. Taken together, these results show the magnetic structure of Mn-sublattice in NdMn$_{0.8}$Fe$_{0.2}$O$_3$ is a combination of the Mn and Fe parent compounds, but the magnetic ordering of Nd sublattice spans over broader temperature interval than in case of NdMnO$_3$ and NdFeO$_3$. This result is a consequence of the fact that the Nd ions do not order independently, but via polarization from Mn/Fe sublattice.

cond-mat.mtrl-sci↗

arXiv2017

Unusual ferroelectric and magnetic phases in multiferroic 2H-BaMnO$_3$ ceramics

The structural phase transition in hexagonal BaMnO$_3$ occurring at $T_c$=130 K was studied in ceramic samples using electron and X-ray diffraction, second harmonic generation as well as by dielectric and lattice dynamic spectroscopies. The low-temperature phase (space group $P6_{3}cm$) is ferroelectric with a triplicated unit cell. The phase transition is driven by an optical soft mode from the Brillouin-zone boundary [$q = (\frac{1}{3},\frac{1}{3},0)$]; this mode activates in infrared and Raman spectra below $T_c$ and it hardens according to the Cochran law. Upon cooling below $T_c$, the permittivity exhibits an unusual linear increase with temperature; below 60 K, in turn, a frequency-dependent decrease is observed, which can be explained by slowing-down of ferroelectric domain wall motions. Based on our data we could not distinguish whether the high-temperature phase is paraelectric or polar (space groups $P6_{3}/mmc$ or $P6_{3}mc$, respectively). Both variants of the phase transition to the ferroelectric phase are discussed based on the Landau theory. Electron paramagnetic resonance and magnetic susceptibility measurements reveal an onset of one-dimensional antiferromagnetic ordering below $\approx220\,\rm K$ which develops fully near 140 K and, below $T_{n} \approx 59\,\rm K$, it transforms into a three-dimensional antiferromagnetic order.

cond-mat.mtrl-sci↗