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Topological magnetic semimetals, like Co3Sn2S2 and Co2MnGa, display exotic transport properties, such as large intrinsic anomalous (AHE) due to uncompensated Berry curvature. The highly symmetric XPt3 compounds exhibit anti-crossing gapped nodal lines, a driving mechanism in the intrinsic Berry curvature Hall effects. Uniquely, these compounds contain two sets of gapped nodal lines that harmoniously dominate the Berry curvature in this complex multi band system. We calculate a maximum AHE of 1965 S cm-1 in the CrPt3 by first principles electronic structure. We have grown high-quality CrPt3 thin films with perpendicular magnetic anisotropy by magnetron sputtering and measured a robust AHE of 1750 S cm−1 for different sputtering growth conditions. Additionally, the cubic films display an easy magnetic axis along [111] direction. The facile and scalable fabrication of these materials is prime candidates for integration into topological devices. Topological magnetic semimetals can realise large intrinsic anomalous Hall effects using the characteristics of their electronic band structure and Berry curvature. Here, the authors predict an anomalous Hall effect for cubic CrPt3 using first principle calculations and confirm the results experimentally.

Time-reversal-symmetry-breaking Weyl semimetals (WSMs) have attracted great attention recently because of the interplay between intrinsic magnetism and topologically nontrivial electrons. Here, we present anomalous Hall and planar Hall effect studies on Co3Sn2S2 nanoflakes, a magnetic WSM hosting stacked Kagome lattice. The reduced thickness modifies the magnetic properties of the nanoflake, resulting in a 15-time larger coercive field compared with the bulk, and correspondingly modifies the transport properties. A 22% enhancement of the intrinsic anomalous Hall conductivity (AHC), as compared to bulk material, was observed. A magnetic field-modulated AHC, which may be related to the changing Weyl point separation with magnetic field, was also found. Furthermore, we showed that the PHE in a hard magnetic WSM is a complex interplay between ferromagnetism, orbital magnetoresistance, and chiral anomaly. Our findings pave the way for a further understanding of exotic transport features in the burgeoning field of magnetic topological phases.

The quantum anomalous Hall effect (QAHE) and magnetic Weyl semimetals (WSMs) are topological states induced by intrinsic magnetic moments and spin-orbit coupling. Their similarity suggests the possibility of achieving the QAHE by dimensional confinement of a magnetic WSM along one direction. In this paper, we investigate the emergence of the QAHE in the two-dimensional (2D) limit of magnetic WSMs due to finite-size effects in thin films and step edges. We demonstrate the feasibility of this approach with effective models and real materials. To this end, we have chosen the layered magnetic WSM ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$, which features a large anomalous Hall conductivity and anomalous Hall angle in its three-dimensional bulk as our material candidate. In the 2D limit of ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$, two QAHE states exist depending on the stoichiometry of the 2D layer. One is a semimetal with a Chern number of 6, and the other is an insulator with a Chern number of 3. The latter has a band gap of 0.05 eV, which is much larger than that in magnetically doped topological insulators. Our findings naturally explain the existence of chiral states in step edges of bulk ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ which have been reported in a recent experiment at $T=4\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and present a realistic avenue to realize QAH states in thin films of magnetic WSMs.

Various ${\mathrm{Co}}_{2}$-based Heusler compounds are predicted to be half-metallic ferromagnets with Weyl points. These systems with lack of time inversion symmetry possess a momentum space Berry curvature that can introduce exotic transport properties. The present study, on epitaxially grown ${\mathrm{Co}}_{2}\mathrm{TiSn}$ films, is an approach to understand and explore this possibility. The theoretical investigation shows that the Berry curvature is significant for the total anomalous Hall effect in experimentally grown ${\mathrm{Co}}_{2}\mathrm{TiSn}$ films. The deviation between the theory and experiment is due to the influence of side jump and skew scattering mechanisms. From a theoretical point of view, the intrinsic contribution to the anomalous Hall effect originates from partially gaped nodal lines due to the symmetry reduction induced by the lack of time reversal symmetry. Furthermore, from hard x-ray photoelectron spectroscopy measurements, we establish an electronic structure in the film that is comparable to the theoretical density of states calculations. These results provide intuitive insight into Heusler spintronics rooted in topological electronic structure.

The topological Hall effect (THE) is one of the key signatures of topologically nontrivial magnetic spin textures, wherein electrons feel an additional transverse voltage to the applied current. The magnitude of THE is often small compared to the anomalous Hall effect. Here, we find a large THE of $0.9\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$ that is of the same order of the anomalous Hall effect in the single-crystalline antiskyrmion-hosting Heusler compound ${\mathrm{Mn}}_{1.4}\mathrm{PtSn}$, a noncentrosymmetric tetragonal compound. The THE is highly anisotropic and survives in the whole temperature range where the spin structure is noncoplanar (170 K). The THE is zero above the spin reorientation transition temperature of 170 K, where the magnetization will have a collinear and ferromagnetic alignment. The large value of the THE entails a significant contribution from the momentum-space Berry curvature along with real-space Berry curvature, which has never been observed earlier.

Very recently, the half-metallic compound Co3Sn2S2 was proposed to be a magnetic Weyl semimetal (WSM) with Weyl points only 60 meV above the Fermi level E-F. Owing to the low charge carrier density and large Berry curvature induced, Co3Sn2S2 possesses both a large anomalous Hall conductivity and a large anomalous Hall angle, which provide strong evidence for the existence of Weyl points in Co3Sn2S2. In this work, we theoretically study the surface topological feature of Co3Sn2S2 and its counterpart Co3Sn2Se2. By cleaving the sample at the weak Sn-S/Se bonds, one can achieve two different surfaces terminated with Sn and S/Se atoms, respectively. The resulting Fermi-arc-related states can range from the energy of the Weyl points to E-F - 0.1 eV in the Sn-terminated surface. Therefore, it should be possible to observe the Fermi arcs in angle-resolved photoemission spectroscopy (ARPES) measurements. Furthermore, in order to simulate quasiparticle interference in scanning tunneling microscopy (STM) measurements, we also calculate the joint density of states for both terminals. This work should be helpful for a comprehensive understanding of the topological properties of these two magnetic WSMs and further ARPES and STM measurements.

We predict a large anomalous Nernst effect in the inverse Heusler compensated ferrimagnets ${\mathrm{Ti}}_{2}\mathrm{Mn}X$ ($X=\mathrm{Al}$, Ga, and In) with vanishing net magnetic moments. Though the net magnetic moment is zero, the Weyl points in these systems lead to a large anomalous Nernst conductivity (ANC) due to the lack of a global time-reversal symmetry operation that inverts the sign of the Berry curvature. In comparison to the noncollinear antiferromagnets ${\mathrm{Mn}}_{3}\mathrm{Sn}$ and ${\mathrm{Mn}}_{3}\mathrm{Ge}$, the high ANC stems almost entirely from the Weyl points in this class of compounds, and thus it is topologically protected. This work shows a large ANC with zero net magnetic moments in collinear systems, which is helpful for a comprehensive understanding of the thermoelectric effect in zero-moment magnetic materials and its further applications.

The presence of non-trivial magnetic topology can give rise to non-vanishing scalar spin chirality and consequently a topological Hall or Nernst effect. In turn, topological transport signals can serve as indicators for topological spin structures. This is particularly important in thin films or nanopatterned materials where the spin structure is not readily accessible. Conventionally, the topological response is determined by combining magnetotransport data with an independent magnetometry experiment. This approach is prone to introduce measurement artifacts. In this study, we report the observation of large topological Hall and Nernst effects in micropatterned thin films of Mn$_{1.8}$PtSn below the spin reorientation temperature $T_\mathrm{SR} \approx 190$K. The magnitude of the topological Hall effect $\rho_\mathrm{xy}^\mathrm{T} = 8$ n$\Omega$m is close to the value reported in bulk Mn$_2$PtSn, and the topological Nernst effect $S_\mathrm{xy}^\mathrm{T} = 115$ nV K$^{-1}$ measured in the same microstructure has a similar magnitude as reported for bulk MnGe ($S_\mathrm{xy}^\mathrm{T} \sim 150$ nV K$^{-1}$), the only other material where a topological Nernst was reported. We use our data as a model system to introduce a topological quantity, which allows to detect the presence of topological transport effects without the need for independent magnetometry data. Our approach thus enables the study of topological transport also in nano-patterned materials without detrimental magnetization related limitations., Comment: 8 pages, 3 figures

We carry out density functional theory calculations which demonstrate that the electron dynamics in the Skyrmion phase of Fe-rich Mn_{1-x}Fe_{x}Ge alloys is governed by Berry phase physics. We observe that the magnitude of the Dzyaloshinskii-Moriya interaction directly related to the mixed space-momentum Berry phases, changes sign and magnitude with concentration x in direct correlation with the data of Shibata et al. [Nat. Nanotechnol. 8, 723 (2013)]. The computed anomalous and topological Hall effects in FeGe are also in good agreement with available experiments. We further develop a simple tight-binding model able to explain these findings. Finally, we show that the adiabatic Berry phase picture is violated in the Mn-rich limit of the alloys.

Materials that crystalize in diamond-related lattices, with Si and GaAs as their prime examples, are at the foundation of modern electronics. Simultaneously, the two atomic sites in the unit cell of these crystals form inversion partners which gives rise to relativistic non-equilibrium spin phenomena highly relevant for magnetic memories and other spintronic devices. When the inversion-partner sites are occupied by the same atomic species, electrical current can generate local spin polarization with the same magnitude and opposite sign on the two inversion-partner sites. In CuMnAs, which shares this specific crystal symmetry of the Si lattice, the effect led to the demonstration of electrical switching in an antiferromagnetic memory at room temperature. When the inversion-partner sites are occupied by different atoms, a non-zero global spin-polarization is generated by the applied current which can switch a ferro-magnet, as reported at low temperatures in the diluted magnetic semiconductor (Ga,Mn)As. Here we demonstrate the effect of the global current-induced spin polarization in a counterpart crystal-symmetry material NiMnSb which is a member of the broad family of magnetic Heusler compounds. It is an ordered high-temperature ferromagnetic metal whose other favorable characteristics include high spin-polarization and low damping of magnetization dynamics. Our experiments are performed on strained single-crystal epilayers of NiMnSb grown on InGaAs. By performing all-electrical ferromagnetic resonance measurements in microbars patterned along different crystal axes we detect room-temperature spin-orbit torques generated by effective fields of the Dresselhaus symmetry. The measured magnitude and symmetry of the current-induced torques are consistent with our relativistic density-functional theory calculations.

We demonstrate that biological molecules such as Watson-Crick DNA base pairs can behave as biological Aviram-Ratner electrical rectifiers because of the spatial separation and weak hydrogen bonding between the nucleobases. We have performed a parallel computational implementation of the ab-initio non-equilibrium Green's function (NEGF) theory to determine the electrical response of graphene---base-pair---graphene junctions. The results show an asymmetric (rectifying) current-voltage response for the Cytosine-Guanine base pair adsorbed on a graphene nanogap. In sharp contrast we find a symmetric response for the Thymine-Adenine case. We propose applying the asymmetry of the current-voltage response as a sensing criterion to the technological challenge of rapid DNA sequencing via graphene nanogaps.

The anomalous Hall effect soars when Dirac quasiparticles meet frustrated magnetism., The anomalous Hall effect (AHE) is one of the most fundamental phenomena in physics. In the highly conductive regime, ferromagnetic metals have been the focus of past research. Here, we report a giant extrinsic AHE in KV3Sb5, an exfoliable, highly conductive semimetal with Dirac quasiparticles and a vanadium Kagome net. Even without report of long range magnetic order, the anomalous Hall conductivity reaches 15,507 Ω−1 cm−1 with an anomalous Hall ratio of ≈ 1.8%; an order of magnitude larger than Fe. Defying theoretical expectations, KV3Sb5 shows enhanced skew scattering that scales quadratically, not linearly, with the longitudinal conductivity, possibly arising from the combination of highly conductive Dirac quasiparticles with a frustrated magnetic sublattice. This allows the possibility of reaching an anomalous Hall angle of 90° in metals. This observation raises fundamental questions about AHEs and opens new frontiers for AHE and spin Hall effect exploration, particularly in metallic frustrated magnets.