Emerging magnetic fields related to the presence of topologically protected spin textures such as skyrmions are expected to give rise to additional, topology-related contributions to the Hall effect. In order to doubtlessly identify this so-called topological Hall effect, it is crucial to disentangle such contributions from the anomalous Hall effect. This necessitates a direct correlation of the transversal Hall voltage with the underlying magnetic textures. A novel measurement platform is developed that allows to acquire high-resolution Lorentz transmission electron microscopy images of magnetic textures as a function of an external magnetic field and to concurrently measure the (anomalous) Hall voltage in-situ in the microscope on one and the same specimen. This approach is used to investigate the transport signatures of the chiral soliton lattice and antiskyrmions in Mn1.4PtSn. Notably, the observed textures allow to fully understand the measured Hall voltage without the need of any additional contributions due to a topological Hall effect, and the field-controlled formation and annihilation of anstiskyrmions are found to have no effect on the measured Hall voltage.

Topological states of matter exhibit unique protection against scattering by disorder. Different topological classes exhibit distinct forms and degrees of protection. Here, we investigate the response of the ferromagnetic nodal line semimetal Fe$_3$GeTe$_2$ to disorder and electronic interactions. By combining global magneto-transport with atomic-scale scanning tunneling spectroscopy we find a simultaneous onset of diverse phenomena below a common temperature scale of about 15 K: A crossover from metallic to insulating temperature dependence of the longitudinal resistivity, saturation of the anomalous Hall conductivity to its maximal value, formation of a sharp zero-bias dip in the tunneling density of state, and emergence of multi-fractal structure of the electronic wavefunction peaking at the Fermi energy. These concurrent observations reflect the emergence of a novel energy scale possibly related to the opening of a gap in the nodal line band of Fe$_3$GeTe$_2$. Our study provides overarching insight into the role of disorder, electronic interactions and Berry curvature in setting the micro- and macro-scale responses of topological semimetals.

Skyrmionic materials hold the potential for future information technologies, such as racetrack memories. Key to that advancement are systems that exhibit high tunability and scalability, with stored information being easy to read and write by means of all-electrical techniques. Topological magnetic excitations such as skyrmions and antiskyrmions, give rise to a characteristic topological Hall effect. However, the electrical detection of antiskyrmions, in both thin films and bulk samples has been challenging to date. Here, we apply magneto-optical microscopy combined with electrical transport to explore the antiskyrmion phase as it emerges in crystalline mesoscale structures of the Heusler magnet Mn_1.4PtSn. We reveal the Hall signature of antiskyrmions in line with our theoretical model, comprising anomalous and topological components. We examine its dependence on the vertical device thickness, field orientation, and temperature. Our atomistic simulations and experimental anisotropy studies demonstrate the link between antiskyrmions and a complex magnetism that consists of competing ferromagnetic, antiferromagnetic, and chiral exchange interactions, not captured by micromagnetic simulations.

Time-reversal symmetry breaking allows for a rich set of magneto-transport properties related to electronic topology. Focusing on the magnetic Weyl semimetal Co3Sn2S2, we prepared micro-ribbons and investigated their transverse and longitudinal transport properties from 100 K to 180 K in magnetic fields mu 0H up to 2 T. We establish the presence of a magnetoresistance (MR) up to 1% with a strong anisotropy depending the projection of H on the easy-axis magnetization, which exceeds all other magnetoresistive effects. Based on detailed phenomenological modeling, we attribute the observed results with unexpected form of anisotropy to magnon MR resulting from magnon-electron coupling. Moreover, a similar angular dependence is also found in the transverse resistivity which we show to originate from the combination of ordinary Hall and anomalous Hall effects. Thus the interplay of magnetic and topological properties governs the magnetotransport features of this magnetic Weyl system.

The ferromagnetic phase of Co_3Sn_2S_2 is widely considered to be a topological Weyl semimetal, with evidence for momentum-space monopoles of Berry curvature from transport and spectroscopic probes. As the bandstructure is highly sensitive to the magnetic order, attention has focused on anomalies in magnetization, susceptibility and transport measurements that are seen well below the Curie temperature, leading to speculation that a "hidden" phase coexists with ferromagnetism. Here we report spatially-resolved measurements by Kerr effect microscopy that identify this phase. We find that the anomalies coincide with a deep minimum in domain wall (DW) mobility, indicating a crossover between two regimes of DW propagation. We demonstrate that this crossover is a manifestation of a 2D phase transition that occurs within the DW, in which the magnetization texture changes from continuous rotation to unidirectional variation. We propose that the existence of this 2D transition deep within the ferromagnetic state of the bulk is a consequence of a giant quality factor for magnetocrystalline anisotropy unique to this compound. This work broadens the horizon of the conventional binary classification of DWs into Bloch and Néel walls, and suggests new strategies for manipulation of domain walls and their role in electron and spin transport.

Journal Article In-situ Magneto-Transport Measurements in a Transmission Electron Microscope Get access Darius Pohl, Darius Pohl Dresden Center for Nanoanalysis (DCN), cfaed, TU Dresden, D-01062 Dresden, Germany Search for other works by this author on: Oxford Academic Google Scholar Bernd Rellinghaus, Bernd Rellinghaus Dresden Center for Nanoanalysis (DCN), cfaed, TU Dresden, D-01062 Dresden, Germany Corresponding author: bernd.rellinghaus@tu-dresden.de Search for other works by this author on: Oxford Academic Google Scholar Sebastian Schneider, Sebastian Schneider Dresden Center for Nanoanalysis (DCN), cfaed, TU Dresden, D-01062 Dresden, Germany Search for other works by this author on: Oxford Academic Google Scholar Dominik Krieger, Dominik Krieger Leibniz Institute for Solid State and Materials Research Dresden, D-01062 Dresden, GermanyInstitute of Solid State and Materials Physics, TU Dresden, D-01062 Dresden, Germany Search for other works by this author on: Oxford Academic Google Scholar Yejin Lee, Yejin Lee Leibniz Institute for Solid State and Materials Research Dresden, D-01062 Dresden, Germany Search for other works by this author on: Oxford Academic Google Scholar Praveen Vir, Praveen Vir Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany Search for other works by this author on: Oxford Academic Google Scholar Claudia Felser, Claudia Felser Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany Search for other works by this author on: Oxford Academic Google Scholar Moritz Winter, Moritz Winter Dresden Center for Nanoanalysis (DCN), cfaed, TU Dresden, D-01062 Dresden, GermanyMax Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany Search for other works by this author on: Oxford Academic Google Scholar Andy Thomas Andy Thomas Leibniz Institute for Solid State and Materials Research Dresden, D-01062 Dresden, GermanyInstitute of Solid State and Materials Physics, TU Dresden, D-01062 Dresden, Germany Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 192–193, https://doi.org/10.1017/S1431927622001647 Published: 01 August 2022

In the lacunar spinels, with the formula AB4X8, transition-metal ions form tightly bound B4 clusters resulting in exotic physical properties such as the stabilization of Néel-type skyrmion lattices, which hold great promise for energy-efficient switching devices. These properties are governed by the symmetry of these compounds with distortion of the parent noncentrosymmetric F4̅3m space group to the polar R3m, with recent observation of a coexisting Imm2 low-temperature phase. In this study, through powder neutron diffraction, we further confirm that a metastable Imm2 coexists with the R3m phase in GaMo4Se8 and we present its structure. By applying the mode crystallography approach to the distortions together with anisotropic microstrain broadening analysis, we postulate that the formation origin of the minority Imm2 phase stems from the high compressive stress observed in the R3m phase. Bond valence sum analysis also suggests a change in electronic configuration in the transition to Imm2 which could have implications on the electrical properties of the compound. We further establish the nature of the magnetic phase transition using critical exponent analysis obtained from single-crystal magnetization measurements which shows a mixture of tricritical mean-field and 3D Heisenberg behavior [β = 0.22(4), γ = 1.19(1), and δ = 6.42(1)]. Magnetoentropic mapping performed on a single crystal reveals the signature of a positive entropy region near the magnetic phase transition which corresponds to the skyrmion phase field observed in a polycrystalline sample.

Itinerant ferromagnets constitute an important class of materials wherein spin polarization can affect the electric transport properties in nontrivial ways. One such phenomenon is anomalous Hall effect which depends on the details of the band structure such as the amount of band crossings in the valence band of the ferromagnet. Here, extraordinary anomalous Hall effect is found in an itinerant ferromagnetic metal LaCrSb 3 . The rather 2D nature of the magnetic subunit imparts large anisotropic anomalous Hall conductivity of 1250 Ω −1 cm −1 at 2 K. The investigations suggest that a strong Berry curvature by abundant momentum‐space crossings and narrow energy‐gap openings are the primary sources of the anomalous Hall conductivity. An important observation is the existence of quasi‐dispersionless bands in LaCrSb 3 which is now known to increase the anomalous Hall conductivity. After introducing f‐electrons, anomalous Hall conductivity experiences more than twofold increase and reaches 2900 Ω −1 cm −1 in NdCrSb 3 .

Topological magnets comprising 2D magnetic layers with Curie temperatures ( T C ) exceeding room temperature are key for dissipationless quantum transport devices. However, the identification of a material with 2D ferromagnetic planes that exhibits an out‐of‐plane‐magnetization remains a challenge. This study reports a ferromagnetic, topological, nodal‐line, and semimetal MnAlGe composed of square‐net Mn layers that are separated by nonmagnetic Al–Ge spacers. The 2D ferromagnetic Mn layers exhibit an out‐of‐plane magnetization below T C ≈ 503 K. Density functional calculations demonstrate that 2D arrays of Mn atoms control the electrical, magnetic, and therefore topological properties in MnAlGe. The unique 2D distribution of the Berry curvature resembles the 2D Fermi surface of the bands that form the topological nodal line near the Fermi energy. A large anomalous Hall conductivity of ≈700 S cm –1 is obtained at 2 K and related to this nodal‐line‐induced 2D Berry curvature distribution. The high transition temperature, large anisotropic out‐of‐plane magnetism, and natural heterostructure‐type atomic arrangements consisting of magnetic Mn and nonmagnetic Al/Ge elements render nodal‐line MnAlGe one of the few, unique, and layered topological ferromagnets that have ever been observed.

Lacunar spinel is a class of compounds that are derivative of the spinel family, AB2X4, with some vacancies at the A-site.They are very interesting both crystallographically and with respect to the physical properties as several members exhibit structural phase transition from F-43m to R3m and long-range magnetic ordering at low-temperature.[1,2]Having R3m (C3v symmetry) space group along with long-range magnetism make these compounds interesting in the aspect of spintronics, as they may host Néel-type skyrmions.[3] One such very well-studied compound is GaV4S8 that hosts skyrmion with individual size of 22 nm.[3] Here, we report a study on a different member of the lacunar spinel family, GaMo4Se8 that is expected to have smaller skyrmions size.We performed high-resolution powder neutron diffraction across the structural phase transition (TS = 51 K).Through Rietveld refinement, it is found out that there are two coexisting low-temperature crystal structures with space group R3m (major phase) and Imm2 (minor phase) (Fig. 1), which is very unique only for GaMo4Se8.[4] We propose an explanation for the coexisting of both crystal structures through mode-crystallographic and bond-valence sum analysis and postulate that the large strain in the rhombohedral structure is alleviated by the formation of the orthorhombic phase with larger displacive distortion amplitude.Furthermore, we have carried out magnetization measurements and performed magnetic critical behavior analysis.We find that the magnetic transition in GaMo4Se8 is close to a tricritical mean-field model, and the analysis of the magnetic phase diagram using magneto-entropic map revealed a positive phasefield which might be an indication of the presence of complex magnetic structures such as cycloid or skyrmions states.Figure 1.The percentage change in cubic (F-43m), rhombohedral (R3m), and orthorhombic (Imm2) phases of GaMo4Se8 as a function of temperature.The Mo4 tetrahedron (green color) corresponding to each phase is shown along with the possible distortion direction (red color arrow).

In article number 2006301, Satya N. Guin, Claudia Felser, and co-workers report a layered topological nodal-line magnet MnAlGe, which is composed of magnetic Mn square-net layers that are separated by nonmagnetic Al–Ge spacers. The material exhibits a large anomalous Hall conductivity driven by the unique 2D Berry curvature distribution. From this work the search for new layered topological magnets and exploration of magnetism and topological properties are enabled.

Correlated topological magnets are emerging as a new class of quantum materials, exhibiting exotic interacting fermions and unconventional phase transitions. Despite considerable interest, direct observation of the magnetic manipulation of topological quasiparticles remains limited. Here we report a correlated topological phase transition in a kagome spin-orbit semimetal, examined by high-resolution photoemission spectroscopy. By modulating the magnetic order, we observe a clear exchange gap collapse in our spectra, associated with a large renormalization of a spin-orbit-gapped Weyl loop at the Fermi level. This unexpected response suggests the collapse of opposite-spin partner ferromagnetic Weyl loops into a paramagnetic Dirac loop. Taken together with $ab$ $initio$ calculation, our results further indicate that oppositely-charged Weyl points pair up and annihilate under collapse, and the Fermi arc surface states are removed. Our findings suggest a novel topological phase transition driven by magnetic interactions, guiding future exploration of renormalized topology under correlated order parameters.

The manipulation of topological states in quantum matter is an essential pursuit of fundamental physics and next-generation quantum technology. Here we report the magnetic manipulation of Weyl fermions in the kagome spin-orbit semimetal Co_{3}Sn_{2}S_{2}, observed by high-resolution photoemission spectroscopy. We demonstrate the exchange collapse of spin-orbit-gapped ferromagnetic Weyl loops into paramagnetic Dirac loops under suppression of the magnetic order. We further observe that topological Fermi arcs disappear in the paramagnetic phase, suggesting the annihilation of exchange-split Weyl points. Our findings indicate that magnetic exchange collapse naturally drives Weyl fermion annihilation, opening new opportunities for engineering topology under correlated order parameters.

In magnetic Weyl semimetals, fluctuations of the local magnetization may generate gauge fields that couple to the chiral charge of emergent Weyl fermions. Recent theoretical studies have proposed that the temporal and spatial-dependent magnetization associated with propagating domain walls (DWs) generates pseudo electric and magnetic fields that drive novel phenomena such as a current of real charge. Here we report a key step in testing these predictions: characterizing the propagation of DWs in the Weyl semimetal Co3Sn2S2 using scanning magneto-optic Kerr microscopy. We observe an unexpected deep minimum in the temperature dependence of the DW mobility, $\mu$, indicating a crossover between two regimes of propagation. The nonmonotonic $\mu(T)$ is evidence of a phase transition in the topology of the DW well below the Curie temperature, in which the magnetization texture changes from continuous rotation (elliptical wall) to a linear wall whose unidirectional magnetization passes through zero at the wall center.

The physical realization of Chern insulators is of fundamental and practical interest, as they are predicted to host the quantum anomalous Hall (QAH) effect and topologically protected chiral edge states which can carry dissipationless current. Current realizations of the QAH state often require complex heterostructures and sub-Kelvin temperatures, making the discovery of intrinsic, high temperature QAH systems of significant interest. In this work we show that time-reversal symmetry breaking Weyl semimetals, being essentially stacks of Chern insulators with inter-layer coupling, may provide a new platform for the higher temperature realization of robust chiral edge states. We present combined scanning tunneling spectroscopy and theoretical investigations of the magnetic Weyl semimetal, Co3Sn2S2. Using modeling and numerical simulations we find that depending on the strength of the interlayer coupling, chiral edge states can be localized on partially exposed kagome planes on the surfaces of a Weyl semimetal. Correspondingly, our dI/dV maps on the kagome Co3Sn terraces show topological states confined to the edges which display linear dispersion. This work provides a new paradigm for realizing chiral edge modes and provides a pathway for the realization of higher temperature QAH effect in magnetic Weyl systems in the two-dimensional limit.

The nontrivial band structure of semimetals has attracted substantial research attention in condensed matter physics and materials science in recent years owing to its intriguing physical properties. Within this class, a group of nontrivial materials known as nodal‐line semimetals is particularly important. Nodal‐line semimetals exhibit the potential effects of electronic correlation in nonmagnetic materials, whereas they enhance the contribution of the Berry curvature in magnetic materials, resulting in high anomalous Hall conductivity (AHC). In this study, two ferromagnetic compounds, namely ZrMnP and HfMnP, are selected, wherein the abundance of mirror planes in the crystal structure ensures gapped nodal lines at the Fermi energy. These nodal lines result in one of the largest AHC values of 2840 Ω−1 cm−1, with a high anomalous Hall angle of 13.6% in these compounds. First‐principles calculations provide a clear and detailed understanding of nodal line‐enhanced AHC. The finding suggests a guideline for searching large AHC compounds.

Skyrmions and antiskyrmions are magnetic nano-objects with distinct chiral, noncollinear spin textures that are found in various magnetic systems with crystal symmetries that give rise to specific Dzyaloshinskii-Moriya exchange vectors. These magnetic nano-objects are associated with closely related helical spin textures that can form in the same material. The skyrmion size and the period of the helix are generally considered as being determined, in large part, by the ratio of the magnitude of the Heisenberg to that of the Dzyaloshinskii-Moriya exchange interaction. In this work, it is shown by real-space magnetic imaging that the helix period λ and the size of the antiskyrmion daSk in the D2d compound Mn1.4 PtSn can be systematically tuned by more than an order of magnitude from ≈100 nm to more than 1.1 µm by varying the thickness of the lamella in which they are observed. The chiral spin texture is verified to be preserved even up to micrometer-thick layers. This extreme size tunability is shown to arise from long-range magnetodipolar interactions, which typically play a much less important role for B20 skyrmions. This tunability in size makes antiskyrmions very attractive for technological applications.

Topological Weyl semimetals have recently attracted considerable attention among materials scientists as their properties are predicted to be protected against perturbations such as lattice distortion and chemical substitution. However, any experimental proof of such robustness is still lacking. In this study, we experimentally demonstrate that the topological properties of the ferromagnetic kagomé compound Co3Sn2S2 are preserved upon Ni substitution. We systematically vary the Ni content in Co3Sn2S2 single crystals and study their magnetic and anomalous transport properties. For the intermediate Ni substitution, we observe a remarkable increase in the coercive field while still maintaining significant anomalous Hall conductivity. The large anomalous Hall conductivity of these compounds is intrinsic, consistent with first-principles calculations, which proves its topological origin. Our results can guide further studies on the chemical tuning of topological materials for better understanding.