Antiferromagnets are hard to control by external magnetic fields because of the alternating directions of magnetic moments on individual atoms and the resulting zero net magnetization. However, relativistic quantum mechanics allows for generating current-induced internal fields whose sign alternates with the periodicity of the antiferromagnetic lattice. Using these fields, which couple strongly to the antiferromagnetic order, we demonstrate room-temperature electrical switching between stable configurations in antiferromagnetic CuMnAs thin-film devices by applied current with magnitudes of order 10(6) ampere per square centimeter. Electrical writing is combined in our solid-state memory with electrical readout and the stored magnetic state is insensitive to and produces no external magnetic field perturbations, which illustrates the unique merits of antiferromagnets for spintronics.

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KAUST RepositoryWe predict that a lateral electrical current in antiferromagnets can induce nonequilibrium Néel-order fields, i.e., fields whose sign alternates between the spin sublattices, which can trigger ultrafast spin-axis reorientation. Based on microscopic transport theory calculations we identify staggered current-induced fields analogous to the intraband and to the intrinsic interband spin-orbit fields previously reported in ferromagnets with a broken inversion-symmetry crystal. To illustrate their rich physics and utility, we consider bulk Mn 2 Au with the two spin sublattices forming inversion partners, and a 2D square-lattice antiferromagnet with broken structural inversion symmetry modeled by a Rashba spin-orbit coupling. We propose an antiferromagnetic memory device with electrical writing and reading.

Ferromagnets are key materials for sensing and memory applications. In contrast, antiferromagnets which represent the more common form of magnetically ordered materials, have found less practical application beyond their use for establishing reference magnetic orientations via exchange bias. This might change in the future due to the recent progress in materials research and discoveries of antiferromagnetic spintronic phenomena suitable for device applications. Experimental demonstration of the electrical switching and detection of the Néel order open a route towards memory devices based on antiferromagnets. Apart from the radiation and magnetic-field hardness, memory cells fabricated from antiferromagnets can be inherently multilevel, which could be used for neuromorphic computing. Switching speeds attainable in antiferromagnets far exceed those of ferromagnetic and semiconductor memory technologies. Here we review the recent progress in electronic spin-transport and spin-torque phenomena in antiferromagnets that are dominantly of the relativistic quantum mechanical origin. We discuss their utility in pure antiferromagnetic or hybrid ferromagnetic/antiferromagnetic memory devices.

We have carried out a comprehensive study of the intrinsic anomalous Hall effect and spin Hall effect of several chiral antiferromagnetic compounds, Mn3X (X = Ge, Sn, Ga, Ir, Rh and Pt) by ab initio band structure and Berry phase calculations. These studies reveal large and anisotropic values of both the intrinsic anomalous Hall effect and spin Hall effect. The Mn3X materials exhibit a non-collinear antiferromagnetic order which, to avoid geometrical frustration, forms planes of Mn moments that are arranged in a Kagome-type lattice. With respect to these Kagome planes, we find that both the anomalous Hall conductivity (AHC) and the spin Hall conductivity (SHC) are quite anisotropic for any of these materials. Based on our calculations, we propose how to maximize AHC and SHC for different materials. The band structures and corresponding electron filling, that we show are essential to determine the AHC and SHC, are compared for these different compounds. We point out that Mn3Ga shows a large SHC of about 600 ( /e)(Ω · cm) −1 . Our work provides insights into the realization of strong anomalous Hall effects and spin Hall effects in chiral antiferromagetic materials.

Noncollinear antiferromagnets, such as Mn3Sn and Mn3Ir, were recently shown to be analogous to ferromagnets in that they have a large anomalous Hall effect. Here we show that these materials are similar to ferromagnets in another aspect: the charge current in these materials is spin-polarized. In addition, we show that the same mechanism that leads to the spin-polarized current also leads to a transversal spin current, which has a distinct symmetry and origin from the conventional spin Hall effect. We illustrate the existence of the spin-polarized current and the transversal spin current by performing ab initio microscopic calculations and by analyzing the symmetry. Based on the spinpolarized current we propose an antiferromagnetic tunneling junction, analogous in functionality to the magnetic tunneling junction.Introduction. Spintronics is a field that studies phenomena in which both spin and charge degree of electron play an important role. Many of the key spintronics effects are based upon the existence of spin currents. Two main types of spin currents are utilized: the spin-polarized currents in ferromagnets (FMs) and the spin currents due to the spin Hall effect (SHE) which are transveral to the charge current and appear even in non-magnetic materials. The most important effects that originate from the spin-polarized currents in FMs are the giant and the tunneling magnetoresistance effects (GMR and TMR) [1][2][3] and the spin-transfer torque (STT) [4,5]. These effects are utilized in magnetic tunneling junctions (MTJs), which form the basic building block of a new type of solid state memory, the magnetic random access memory (MRAM) [6]. This memory is non-volatite and has speed and density comparable to the widely used dynamic random access memory. The SHE on the other hand is responsible (though other effects can contribute) for the spin-orbit torque (SOT) [7,8] in multilayer heterostructures, which can be used for efficient and fast switching of FM layers. The SOT is now also being explored for use in MRAMs [9,10].While spintronics has traditionally focused on FM and non-magnetic materials, in the past few years also antiferromagnetic (AFM) materials have attracted a considerable interest. AFMs offer some unique advantages compared to FMs, but are much less explored (see reviews [11][12][13]). AFMs have a very fast dynamics, which allows for switching on ps timescale [14][15][16]. Furthermore, there exists a wide range of AFM materials, including many insulators and semiconductors, multiferroics [17] and superconductors [18]. Utilizing (and also studying) AFMs is difficult, largely because the magnetic order in AFMs is hard to detect and to manipulate. Recently, electrical detection [19][20][21][22][23] and manipulation of the AFM order has been demonstrated [23,24], however, both detection and manipulation still remain challenging from a practical point of view.

Spin-orbitronics and Dirac quasiparticles are two fields of condensed matter physics initiated independently about a decade ago. Here we predict that Dirac quasiparticles can be controlled by the spin-orbit torque reorientation of the Néel vector in an antiferromagnet. Using CuMnAs as an example, we formulate symmetry criteria allowing for the co-existence of Dirac quasiparticles and Néel spin-orbit torques. We identify the non-symmorphic crystal symmetry protection of Dirac band crossings whose on and off switching is mediated by the Néel vector reorientation. We predict that this concept, verified by minimal model and density functional calculations in the CuMnAs semimetal antiferromagnet, can lead to a topological metal-insulator transition driven by the Néel vector and to the corresponding topological anisotropic magnetoresistance.2004 was the year when the spin Hall effect was observed in GaAs [1-3] and one-atom-thick flakes of graphene were isolated [4,5]. The former discovery marked the dawn of the field of spin-orbitronics, in which the relativistic conversion between linear momentum and spin angular momentum of conducting electrons has provided new physical concepts for spintronics devices. These include the spin-orbit torque (SOT), which has opened the path to reliable and fast information writing in a ferromagnetic random access memory [6,7], and also to efficient means of the electrical switching of an antiferromagnet (AF) by the Néel SOT [8,9]. Independently, the discovery of graphene initiated intense research of Dirac fermion quasiparticles in condensed matter systems. The field includes topological insulators, semimetals, or superconductors, which host a family of quasiparticles mimicking different flavors of fermions from relativistic particle physics [10][11][12]. More recently, novel phenomena have been discovered at the intersection of these two fields, such as the quantum spin Hall effect and the quantum anomalous Hall effect in non-magnetic and magnetic topological insulators [13][14][15][16][17][18]. Dirac quasiparticles, exhibiting a strong spinmomentum locking, are also considered for enhancing the efficiency of the SOT control of magnetic moments in ferromagnetic topological insulator hetero-structures [19].In this Letter we close the loop of synergies between the fields of spin-orbitronics and Dirac quasiparticles by proposing a scheme for the electric control of Dirac band crossings via the Néel SOT in AFs. Our work addresses the outstanding problem of finding efficient means for controlling Dirac quasiparticles by external fields which may provide the desired tools for the experimental research and future practical applications in microelectronics [20]. On a specific example of the semimetal CuMnAs AF [9,21,22] we demonstrate that the Néel vector orientation is a suitable degree of freedom that can mediate on and off switching of the symmetry protection of Dirac band crossings. Based on this we also predict the topological metal-insulator transition (MIT) and the corresponding topolo...

One of the main obstacles that prevents practical applications of antiferromagnets is the difficulty of manipulating the magnetic order parameter. Recently, following the theoretical prediction [J. Zelezný et al., PRL 113, 157201 (2014)], the electrical switching of magnetic moments in an antiferromagnet has been demonstrated [P. Wadley et al., Science 351, 587 (2016)]. The switching is due to the so-called spin-orbit torque, which has been extensively studied in ferromagnets. In this phenomena a non-equilibrium spin-polarization exchange coupled to the ordered local moments is induced by current, hence exerting a torque on the order parameter. Here we give a general systematic analysis of the symmetry of the spin-orbit torque in locally and globally non-centrosymmetric crystals. We study when the symmetry allows for a nonzero torque, when is the torque effective, and its dependence on the applied current direction and orientation of magnetic moments. For comparison, we consider both antiferromagnetic and ferromagnetic orders. In two representative model crystals we perform microscopic calculations of the spin-orbit torque to illustrate its symmetry properties and to highlight conditions under which the spin-orbit torque can be efficient for manipulating antiferromagnetic moments.