We report on antiferromagnetic resonance experiments in bulk tetragonal NaMnAs – a room-temperature antiferromagnetic semiconductor. Our results corroborate previous ab initio studies, which propose that NaMnAs is an easy-axis antiferromagnet with the Néel vector oriented along the tetragonal axis. At B = 0, we find a single antiferromagnetic resonance line at 7 meV and associate it with a doubly degenerate (k = 0) magnon mode. Its energy softens considerably with increasing T, but remains clearly visible in the data up to room temperature. From the experimental data, we estimate the single-ion anisotropy of the Mn ions in NaMnAs to be D ≈ 0.2 meV, a value that is relatively large compared to other manganese-based antiferromagnets.

Condensed matter compounds typically form crystals, which break the rotational and translational invariance of space but remain invariant under a discrete set of symmetry operations. Understanding the effects allowed by this symmetry breaking, as well as the constraints imposed by the crystal structure, is a crucial problem in condensed matter physics. Here, we present a Python package for determining the symmetry-restricted forms of tensors describing physical properties of crystals, focusing particularly on magnetic materials. The primary focus is on response tensors; however, the program can also describe equilibrium properties and other physical properties, such as magnetic interactions. The program can describe the symmetry using the conventional magnetic space groups, as well as using the spin groups that describe the non-relativistic limit. Additional functionality includes the treatment of quantities projected onto a particular site and expansions in the magnetic order parameter. The code can be used either from the command line or via a Python API.PROGRAM SUMMARYProgram Title: SymmetrCPC Library link to program files: https://doi.org/10.17632/kxjfycsyfr.1Developer's repository link: https://github.com/zeleznyj/symmetrLicensing provisions: MPL-2.0Programming language: PythonNature of problem: Symmetr is a Python package designed to determine symmetry-restricted forms of tensors describing physical properties of crystalline materials. Based on Neumann's principle, it automates the otherwise complex task of enforcing crystal and magnetic symmetries on equilibrium and response tensors, which is essential for analyzing transport, magnetic, and other material properties. The package supports both non-magnetic and magnetic systems, including cases with broken time-reversal symmetry, and extends naturally to the non-relativistic limit through the use of spin groups, enabling symmetry analysis in the absence of spin–orbit coupling.Solution method: The program first determines the symmetry operations of the crystal and then the transformation of the tensor under the symmetry operations. This then leads to a series of linear equations. We solve these using the singular value decomposition followed by gaussian ellimation. The program is implemented in Python and is used from the command line or through a Python API.

The component of the resistivity tensor ρij corresponding to voltage transverse to both an applied current and a magnetic field can be separated into odd and even parts with respect to the applied magnetic field. The former contains information, for example, about the ordinary or anomalous Hall effect. The latter is often ascribed to experimental artifacts and ignored. Here, we show that upon suppressing these artifacts in carefully controlled experiments, useful information remains. We first investigate the well-explored ferromagnet CoFeB, where the even part of ρyx contains a contribution from the anisotropic magnetoresistance, which we confirm by Stoner–Wohlfarth modeling. We then apply our approach to magnetotransport measurements of Mn5Si3 thin films, which undergo a transition from non-collinear to an altermagnetic collinear state. In this material, the even part of the transverse signal is sizable only in the low-spin-symmetry phase below ≈80 K. Transverse resistivity measurements thus offer a simple and readily available probe of magnetic order transitions.

Anisotropic magnetoresistance (AMR) arises from symmetry lowering of the conductivity tensor induced by magnetic order. In simple ferromagnets, AMR is a relativistic effect, relying on spin-orbit interaction (SOC). Here, we demonstrate that a comparable symmetry lowering can also occur in a non-relativistic limit. Using tight-binding models, density functional theory calculations, and Boltzmann transport theory, we investigate systems with multiple magnetic sublattices, including both collinear and non-collinear antiferromagnets, as well as ferrimagnetic configurations. We show that AMR and related anisotropies can emerge purely from magnetic order, without the need for SOC, and may reach significant magnitudes. The findings are supported by case studies on toy-model lattices and real materials such as MnN, Mn_3Sn, and are further interpreted using a symmetry analysis based on Neumann's principle. Material candidates that exhibit non-relativistic anisotropic magnetoresistance are identified by symmetry analysis applied to entries in the MAGNDATA database.

Antiferromagnetic materials show a remarkable variety of magnetic orders and hold promises for spintronics devices. Zhang et al. propose a new type of antiferromagnetic order where the antiferromagnetic sublattices are stacked along orthogonal lines. This configuration allows addressing individual sublattices using currents along orthogonal directions, resulting in highly spin-polarized currents and efficient torques of interest for spintronics.

The resistance of a heavy metal can be modulated by an adjacent magnetic material through the combined effects of the spin Hall effect, inverse spin Hall effect, and dissipation of the spin accumulation at the interface. This phenomenon is known as the spin Hall magnetoresistance. The dissipation of the spin accumulation can occur via various mechanisms, with spin-transfer torque being the most extensively studied. In this work, we report the observation of spin Hall magnetoresistance at the interface between platinum and an insulating altermagnetic candidate, Ba_2CoGe_2O_7. Our findings reveal that this heterostructure exhibits a relatively large spin Hall magnetoresistance signal, which is anisotropic with respect to the crystal orientation of the current channel. We explore and rule out several potential explanations for this anisotropy and propose that our results may be understood in the context of the anisotropies of the spin current channels across the Pt/altermagnetic Ba_2CoGe_2O_7 interface.

We demonstrate that epitaxial thin film antiferromagnet Mn2As exhibits the quench-switching effect, which was previously reported only in crystallographically similar antiferromagnetic CuMnAs thin films. Quench switching in Mn2As shows stronger increase in resistivity, reaching hundreds of percent at 5K, and significantly longer retention time of the metastable high-resistive state before relaxation towards the low-resistive uniform magnetic state. Qualitatively, Mn2As and CuMnAs show analogous parametric dependence of the magnitude and relaxation of the quench-switching signal. Quantitatively, relaxation dynamics in both materials show direct proportionality to the Néel temperature. This confirms that the quench switching has magnetic origin in both materials. The presented results suggest that the antiferromagnets crystalizing in the Cu2Sb structure are well suited for exploring and exploiting the intriguing physics of highly non-uniform magnetic states associated with the quench switching.

Following the recent observation of anomalous Hall effect in antiferromagnetic hexagonal MnTe thin films, related phenomena at finite frequencies have come into focus. Magnetic circular dichroism (MCD) is the key material property here. In the X‐ray range, the X‐ray MCD has already been demonstrated and used to visualize domains via photoemission electron microscopy. Herein, MCD in optical range is reported on and its microscopic mechanism is discussed.

Following the recent observation of anomalous Hall effect in antiferromagnetic hexagonal MnTe thin films, related phenomena at finite frequencies have come into focus. Magnetic circular dichroism (MCD) is the key material property here. In the x-ray range, the XMCD has already been demonstrated and used to visualise domains via photoemission electron microscopy (PEEM). Here we report on MCD in optical range and discuss its microscopic mechanism.

Recently, MnTe was established as an altermagnetic material that hosts spin-polarized electronic bands as well as anomalous transport effects like the anomalous Hall effect. In addition to these effects arising from altermagnetism, MnTe also hosts other magnetoresistance effects. Here, we study the manipulation of the magnetic order by an applied magnetic field and its impact on the electrical resistivity. In particular, we establish which components of anisotropic magnetoresistance are present when the magnetic order is rotated within the hexagonal basal plane. Our experimental results, which are in agreement with our symmetry analysis of the magnetotransport components, showcase the existence of an anisotropic magnetoresistance linked to both the relative orientation of current and magnetic order, as well as crystal and magnetic order. Altermagnetism is manifested as a three-fold component in the transverse magnetoresistance which arises due to the anomalous Hall effect.

The Edelstein effect is the origin of the spin-orbit torque: a current-induced torque that is used for the electrical control of ferromagnetic and antiferromagnetic materials. This effect originates from the relativistic spin-orbit coupling, which necessitates utilizing materials with heavy elements. Here, we show that in magnetic materials with non-collinear magnetic order, the Edelstein effect and, consequently, a current-induced torque can exist even in the absence of the spin-orbit coupling. Using group symmetry analysis, model calculations, and realistic simulations on selected compounds, we identify large classes of non-collinear magnet candidates and demonstrate that the current-driven torque is of similar magnitude as the celebrated spin-orbit torque in conventional transition metal structures. We also show that this torque can exist in an insulating material, which could allow for highly efficient electrical control of magnetic order.

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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.

The anomalous Hall effect, commonly observed in metallic magnets, has been established to originate from the time-reversal symmetry breaking by an internal macroscopic magnetization in ferromagnets or by a non-collinear magnetic order. Here we observe a spontaneous anomalous Hall signal in the absence of an external magnetic field in an epitaxial film of MnTe, which is a semiconductor with a collinear antiparallel magnetic ordering of Mn moments and a vanishing net magnetization. The anomalous Hall effect arises from an unconventional phase with strong time-reversal symmetry breaking and alternating spin polarization in real-space crystal structure and momentum-space electronic structure. The anisotropic crystal environment of magnetic Mn atoms due to the non-magnetic Te atoms is essential for establishing the unconventional phase and generating the anomalous Hall effect.

Despite its long history, the anomalous Hall continues to attract attention due to its complex origins, its connection to topology, and its use as a probe of magnetic order. In this work we investigate the anomalous Hall effect in 2871 ferromagnetic materials using an automatic high-throughput calculation scheme. We analyze general properties of the effect, such as its reliance on spin-orbit coupling strength and magnetization. In materials with the largest anomalous Hall effect, we find that symmetry-protected band degeneracies in the non-relativistic electronic structure, such as mirror symmetry-protected nodal lines, are typically responsible for the large effect. Furthermore, we examine the dependence of the anomalous Hall effect on magnetization direction and demonstrate deviations from the commonly assumed expression j AHE ~ M × E .

Ferromagnetic spin valves and tunneling junctions are crucial for spintronics applications and are one of the most fundamental spintronics devices. Motivated by the potential unique advantages of antiferromagnets for spintronics, we theoretically study here junctions built out of noncollinear antiferromagnets. We demonstrate a large and robust magnetoresistance and spin-transfer torque capable of ultrafast switching between parallel and antiparallel states of the junction. In addition, we show that a new type of self-generated torque appears in the noncollinear junctions.

The interest in understanding scaling limits of magnetic textures such as domain walls spans the entire field of magnetism from its physical fundamentals to applications in information technologies. Here, we explore antiferromagnetic CuMnAs in which imaging by x-ray photoemission reveals the presence of magnetic textures down to nanoscale, reaching the detection limit of this established microscopy in antiferromagnets. We achieve atomic resolution by using differential phase-contrast imaging within aberration-corrected scanning transmission electron microscopy. We identify abrupt domain walls in the antiferromagnetic film corresponding to the Néel order reversal between two neighboring atomic planes. Our work stimulates research of magnetic textures at the ultimate atomic scale and sheds light on electrical and ultrafast optical antiferromagnetic devices with magnetic field–insensitive neuromorphic functionalities.

Recently, a distinct mechanism of spontaneous anomalous Hall effect, the so-called crystal anomalous Hall effect, was predicted to arise from antiparallel magnetic moments surrounded by cages of nonmagnetic atoms [\v{S}mejkal et al., Sci. Adv.6, eaaz8809(2020)]. Here, we observe this spontaneous anomalous Hall effect in thin films of semiconducting MnTe with well-established room-temperature collinear antiparallel ordering. We show that this Hall signal is consistent with the measured experimental magnetic easy axis and with the calculated unconventional time-reversal symmetry broken band structure.