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Magnon eigenmodes in easy-plane antiferromagnetic insulators are linearly polarized and are not expected to carry any net spin angular momentum. Motivated by recent nonlocal spin transport experiments in the easy-plane phase of hematite, we perform a series of micromagnetic simulations in a nonlocal geometry at finite temperatures. We show that by tuning an external magnetic field, we can control the magnon eigenmodes and the polarization of the spin transport signal in these systems. We argue that a coherent beating oscillation between two orthogonal linearly polarized magnon eigenmodes is the mechanism responsible for finite spin transport in easy-plane antiferromagnetic insulators. The sign of the detected spin signal is also naturally explained by the proposed coherent beating mechanism. Our finding opens a path for on-demand control of the spin signal in a large class of easy-plane antiferromagnetic insulators.

We demonstrate how shape dependent strain can be used to control antiferromagnetic order in NiO Pt thin films. For rectangular elements patterned along the easy and hard magnetocrystalline anisotropy axes of our film, we observe different domain structures and we identify magnetoelastic interactions that are distinct for different domain configurations. We reproduce the experimental observations by modeling the magnetoelastic interactions, considering spontaneous strain induced by the domain configuration, as well as elastic strain due to the substrate and the shape of the patterns. This allows us to demonstrate and explain how the variation of the aspect ratio of rectangular elements can be used to control the antiferromagnetic ground state domain configuration. Shape dependent strain does not only need to be considered in the design of antiferromagnetic devices, but can potentially be used to tailor their properties, providing an additional handle to control antiferromagnets

We propose a concept of non-collinear spin current, whose spin polarization varies in space even in non-magnetic crystals. While it is commonly assumed that the spin polarization of the spin Hall current is uniform, asymmetric local crystal potential generally allows the spin polarization to be non-collinear in space. Based on microscopic considerations we demonstrate that such non-collinear spin Hall currents can be observed for example in layered Kagome Mn$_3$X (X = Ge, Sn) compounds. Moreover, by referring to atomistic spin dynamics simulations we show that non-collinear spin currents can be used to switch the chiral spin texture of Mn$_3$X in a deterministic way even in the absence of an external magnetic field. Our theoretical prediction can be readily tested in experiments, which will open a novel route toward electric control of complex spin structures in non-collinear antiferromagnets.

We unambiguously identify the origin of the current-induced magnetic switching of insulating antiferromagnet/heavy metal bilayers. Previously, different reorientations of the Neel order for the same current direction were reported for different device geometries and different switching mechanisms were proposed. Here, we combine concurrent electrical readout and optical imaging of the switching of antiferromagnetic domains with simulations of the current-induced temperature and strain gradients. By comparing the switching in specially engineered NiO/Pt device and pulsing geometries, we can rule out spin-orbit torque based mechanisms and identify a thermomagnetoelastic mechanism to dominate the switching of antiferromagnetic domains, reconciling previous reports.

We theoretically describe the behavior of a terahertz nano-oscillator based on an anisotropic antiferromagnetic dynamical element driven by spin torque. We consider the situation when the polarization of the spin-current is perpendicular to the external magnetic field applied along the anisotropy easy-axis. We determine the domain of the parametric space (field, current) where the oscillator demonstrates chaotic dynamics. Characteristics of the chaotic regimes are analyzed using conventional techniques such as spectra of the Lyapunov exponents. We show that the threshold current of the chaos appearance is particularly low in the vicinity of the spin-flop transition. In this regime, we consider the mechanism of the chaos appearance in detail when the field is fixed and the current density increases. We show that the appearance of chaos is preceded by a regime of quasiperiodic dynamics on the surface of a two-frequency torus arising in phase space as a result of the Neimark-Sacker bifurcation., 12 pages, 9 figures

We investigate the role of domain walls in the ultrafast magnon dynamics of an antiferromagnetic NiO single crystal in a pump-probe experiment with variable pump photon energy. Analysing the amplitude of the energy-dependent photo-induced ultrafast spin dynamics, we detect a yet unreported coupling between the material's characteristic THz- and a GHz-magnon modes. We explain this unexpected coupling between two orthogonal eigenstates of the corresponding Hamiltonian by modelling the magneto-elastic interaction between spins in different domains. We find that such interaction, in the non-linear regime, couples the two different magnon modes via the domain walls and it can be optically exploited via the exciton-magnon resonance., 6 pages, 4 figures

Motivated by the importance of understanding competing mechanisms to current-induced spin-orbit torque in complex magnets, we develop a unified theory of current-induced spin-orbital coupled dynamics. The theory describes angular momentum transfer between different degrees of freedom in solids, e.g., the electron orbital and spin, the crystal lattice, and the magnetic order parameter. Based on the continuity equations for the spin and orbital angular momenta, we derive equations of motion that relate spin and orbital current fluxes and torques describing the transfer of angular momentum between different degrees of freedom. We then propose a classification scheme for the mechanisms of the current-induced torque in magnetic bilayers. Based on our first-principles implementation, we apply our formalism to two different magnetic bilayers, Fe/W(110) and Ni/W(110), which are chosen such that the orbital and spin Hall effects in W have opposite sign and the resulting spin- and orbital-mediated torques can compete with each other. We find that while the spin torque arising from the spin Hall effect of W is the dominant mechanism of the current-induced torque in Fe/W(110), the dominant mechanism in Ni/W(110) is the orbital torque originating in the orbital Hall effect of W. It leads to negative and positive effective spin Hall angles, respectively, which can be directly identified in experiments. This clearly demonstrates that our formalism is ideal for studying the angular momentum transfer dynamics in spin-orbit coupled systems as it goes beyond the "spin current picture" by naturally incorporating the spin and orbital degrees of freedom on an equal footing. Our calculations reveal that, in addition to the spin and orbital torque, other contributions such as the interfacial torque and self-induced anomalous torque within the ferromagnet are not negligible in both material systems., Comment: 26 pages, 13 figures

Antiferromagnets have recently surged as the prominent material platform for the next generation spintronics devices. Despite the remarkable abundance of antiferromagnets and the variety of their spin textures in nature, they share a widely common, if not ubiquitous, feature. Magnetoelasticity, which is expressed as strictions of different origin, relativistic and/or exchange, significantly contributes to the magnetic anisotropy of antiferromagnets. Crucially, a general theoretical framework able to address the role of domain walls on the spin dynamics in antiferromagnets in the presence of magnetoelasticity is lacking. Here we tackle this problem developing a very general macroscopic phenomenological model. We demonstrate that the magnetoelasticity defines both the equilibrium and dynamical magnetic properties of easy-plane antiferromagnets in linear and nonlinear regimes. We employ our model to reproduce experimental results, showing that domain walls majorly affect the optically driven ultrafast nonlinear spin dynamics. Our model can be applied to a wide group of materials and, if properly extended, can be utilised to describe also other physical scenarios, key in the establishment of concepts for antiferromagnetic spintronics. published

We unravel the origin of current-induced magnetic switching of insulating antiferromagnet/heavy metal systems. We utilize concurrent transport and magneto-optical measurements to image the switching of antiferromagnetic domains in specially engineered devices of NiO/Pt bilayers. Different electrical pulsing and device geometries reveal different final states of the switching with respect to the current direction. We can explain these through simulations of the temperature induced strain and we identify the thermomagnetoelastic switching mechanism combined with thermal excitations as the origin, in which the final state is defined by the strain distributions and heat is required to switch the antiferromagnetic domains. We show that such a potentially very versatile non-contact mechanism can explain the previously reported contradicting observations of the switching final state, which were attributed to spin-orbit torque mechanisms.

We achieve current-induced switching in collinear insulating antiferromagnetic CoO/Pt, with fourfold in-plane magnetic anisotropy. This is measured electrically by spin Hall magnetoresistance and confirmed by the magnetic field-induced spin-flop transition of the CoO layer. By applying current pulses and magnetic fields, we quantify the efficiency of the acting current-induced torques and estimate a current-field equivalence ratio of 4×10^{-11} T A^{-1} m^{2}. The Neel vector final state (n⊥j) is in line with a thermomagnetoelastic switching mechanism for a negative magnetoelastic constant of the CoO.

Comment: 16 pages (manuscript and supplementary), 12 figures, We achieve current-induced switching in collinear insulating antiferromagnetic CoO/Pt, with fourfold in-plane magnetic anisotropy. This is measured electrically by spin Hall magnetoresistance and confirmed by the magnetic field-induced spin-flop transition of the CoO layer. By applying current pulses and magnetic fields, we quantify the efficiency of the acting current-induced torques and estimate a current-field equivalence ratio of $4x10^{-11} T A^{-1} m^2$. The N\'eel vector final state ($n \perp j$) is in line with a thermomagnetoelastic switching mechanism for a negative magnetoelastic constant of the CoO.

Macroscopic properties of multiferroics, the systems that show simultaneously two types of ordering, could be controlled by the external fields of different nature. We analyze the behavior of multiferroics with antiferro-(AFM) and ferromagnetic (FM) orderingunder the action of external magnetic and stress fields. A combination of these two fields makes it possible to achieve macroscopic states with different domain structures. The two-domain state obtained in this way shows a linear dependence of macroscopic strain vs magnetic field which is unusual for AFMs. A small but nonzero stress applied to the sample can also result in the bias of the magnetization vs magnetic field dependence., Макроскопічними властивостями мультифероїків, тобто речовин, в яких співіснують два типи впорядкування, можна керувати за допомогою полів різної природи. В даній роботі проана ізовано поведінку мультифероїків з антиферо- (АФМ) та феромагнітним (ФМ) впорядкуванням під дією зовнішнього магнітного поля та механічного напруження. Комбінація цих двох полів дозволяє отримати макроскопічні стани з різною доменною структурою. Отриманий таким способом дводоменний стан демонструє нетипову для АФМ лінійну залежність макроскопічної деформації від зовнішнього магнітного поля. Мале, але ненульове механічне напруження приводить також до зсуву кривої залежності макроскопічної намагнічуваності від магнітного поля (ефективному "підмагнічуванню").

NiO is a prototypical antiferromagnet with a characteristic resonance frequency in the THz range. From atomistic spin dynamics simulations that take into account the crystallographic structure of NiO, and in particular a magnetic anisotropy respecting its symmetry, we describe antiferromagnetic switching at THz frequency by a spin transfer torque mechanism. Sub-picosecond S-state switching between the six allowed stable spin directions is found for reasonably achievable spin currents, like those generated by laser induced ultrafast demagnetization. A simple procedure for picosecond writing of a six-state memory is described, thus opening the possibility to speed up current logic of electronic devices by several orders of magnitude., Comment: 8 pages, 11 figures

We observe a strong thermally-controlled magnon-mediated interlayer coupling of two ferromagnetic layers via an antiferromagnetic spacer in spin-valve type trilayers. The effect manifests itself as a field-induced coherent switching of the two ferromagnets, which can be controlled by varying temperature and the spacer thickness. We explain the observed behavior as due to a strong hybridization of the ferro- and antiferro-magnetic magnon modes in the trilayer at temperatures just below the N\'eel temperature of the antiferromagnetic spacer., Comment: 4 pages, 4 figures

Optical generation of complex spin textures is one of the most exciting challenges of modern spintronics. Here, we uncover a distinct physical mechanism for imprinting spin chirality into collinear magnets with short laser pulses. By simultaneously treating the laser-ignited evolution of electronic structure and magnetic order, we show that their intertwined dynamics can result in an emergence of quasi-stable chiral states. We find that laser-driven chirality does not require any auxiliary external fields or intrinsic spin-orbit interaction to exist, and it can survive on the time scale of nanoseconds even in the presence of thermal fluctuations, which makes the uncovered mechanism relevant for understanding various optical experiments on magnetic materials. Our findings open a new perspective at the interaction of complex chiral magnetism with light., Comment: 9+4 pages and 7+12 figures with supplementary materials

We probe the current-induced magnetic switching of insulating antiferromagnet/heavy metals systems, by electrical spin Hall magnetoresistance measurements and direct imaging, identifying a reversal occurring by domain wall (DW) motion. We observe switching of more than one third of the antiferromagnetic domains by the application of current pulses. Our data reveal two different magnetic switching mechanisms leading together to an efficient switching, namely the spin-current induced effective magnetic anisotropy variation and the action of the spin torque on the DWs., 26 pages, including supplementary material

In antiferromagnetic spintronics, it is essential to separate the resistance modifications of purely magnetic origin from other effects generated by current pulses intended to switch the N\'eel vector. We investigate the magnetoresistance effects resulting from magnetic field induced reorientations of the staggered magnetization of epitaxial antiferromagnetic Mn2Au(001) thin films. The samples were exposed to 60 T magnetic field pulses along different crystallographic in-plane directions of Mn2Au(001), while their resistance was measured. For the staggered magnetization aligned via a spin-flop transition parallel to the easy [110]-direction, an ansiotropic magnetoresistance of -0.15 % was measured. In the case of a forced alignment of the staggered magnetization parallel to the hard [100]-direction, evidence for a larger anisotropic magnetoresistance effect was found. Furthermore, transient resistance reductions of about 1 % were observed, which we associate with the annihilation of antiferromagnetic domain walls by the magnetic field pulses., Comment: 6 pages, 4 figures

Antiferromagnets possess a number of intriguing and promising properties for electronic devices, which include a vanishing net magnetic moment and thus insensitivity to large magnetic fields and characteristic terahertz frequency dynamics. However, probing the antiferromagnetic ordering is challenging without synchrotron-based facilities. Here, we determine the material parameters of the insulating iron oxide hematite, α-Fe2O3, using the surface sensitive spin-Hall magnetoresistance (SMR). Combined with a simple analytical model, we extract the antiferromagnetic anisotropies and the bulk Dzyaloshinskii-Moriya field over a wide range of temperatures and magnetic fields. Across the Morin phase transition, we show that the electrical response is dominated by the antiferromagnetic Néel vector rather than by the emergent weak magnetic moment. Our results highlight that the surface sensitivity of SMR enables access to the magnetic anisotropies of antiferromagnetic crystals, and also of thin films, where other methods to determine anisotropies such as bulk-sensitive magnetic susceptibility measurements do not provide sufficient sensitivity. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

As a candidate material for applications such as magnetic memory, polycrystalline antiferromagnets offer the same robustness to external magnetic fields, THz spin dynamics, and lack of stray field as their single crystalline counterparts, but without the limitation of epitaxial growth and lattice matched substrates. Here, we first report the detection of the average Neel vector orientiation in polycrystalline NiO via spin Hall magnetoresistance (SMR). Secondly, by applying strain through a piezo-electric substrate, we reduce the critical magnetic field required to reach a saturation of the SMR signal, indicating a change of the anisotropy. Our results are consistent with polycrystalline NiO exhibiting a positive sign of the in-plane magnetostriction. This method of anisotropy-tuning offers an energy efficient, on-chip alternative to manipulate a polycrystalline antiferromagnets magnetic state.

Antiferromagnetic materials promise improved performance for spintronic applications, as they are robust against external magnetic field perturbations and allow for faster magnetization dynamics compared to ferromagnets. The direct observation of the antiferromagnetic state, however, is challenging due to the absence of a macroscopic magnetization. Here, we show that the spin Hall magnetoresistance (SMR) is a versatile tool to probe the antiferromagnetic spin structure via simple electrical transport experiments by investigating the easy-plane antiferromagnetic insulators $\alpha$-Fe2O3 (hematite) and NiO in bilayer heterostructures with a Pt heavy metal top electrode. While rotating an external magnetic field in three orthogonal planes, we record the longitudinal and the transverse resistivities of Pt and observe characteristic resistivity modulations consistent with the SMR effect. We analyze both their amplitude and phase and compare the data to the results from a prototypical collinear ferrimagnetic Y3Fe5O12/Pt bilayer. The observed magnetic field dependence is explained in a comprehensive model, based on two magnetic sublattices and taking into account magnetic field-induced modifications of the domain structure. Our results show that the SMR allows us to understand the spin configuration and to investigate magnetoelastic effects in antiferromagnetic multi-domain materials. Furthermore, in $\alpha$-Fe2O3/Pt bilayers, we find an unexpectedly large SMR amplitude of $2.5 \times 10^{-3}$, twice as high as for prototype Y3Fe5O12/Pt bilayers, making the system particularly interesting for room-temperature antiferromagnetic spintronic applications.

We theoretically predict and classify the localized modes of a skyrmion in a collinear uniaxial antiferromagnet and discuss how they can be excited. As a central result, we find two branches of skyrmion eigenmodes with distinct physical properties characterized by being low or high energy excitations. The frequency dependence of the low-energy modes scales as $R_0^{-2}$ for skyrmions with large radius $R_0$. Furthermore, we predict localized high-energy eigenmodes, which have no direct ferromagnetic counterpart. Except for the breathing mode, we find that all localized antiferromagnet skyrmion modes, both in the low and high-energy branch, are doubly degenerated in the absence of a magnetic field and split otherwise. We explain our numerical results for the low-energy modes within a string model representing the skyrmion boundary., Comment: 13 pages, 9 figures

The impulsive generation of two-magnon modes in antiferromagnets by femtosecond optical pulses, so-called femto-nanomagnons, leads to coherent longitudinal oscillations of the antiferromagnetic order parameter that cannot be described by a thermodynamic Landau-Lifshitz approach. We argue that this dynamics is triggered as a result of a laser-induced modification of the exchange interaction. In order to describe the oscillations we have formulated a quantum mechanical description in terms of magnon pair operators and coherent states. Such an approach allowed us to} derive an effective macroscopic equation of motion for the temporal evolution of the antiferromagnetic order parameter. An implication of the latter is that the photo-induced spin dynamics represents a macroscopic entanglement of pairs of magnons with femtosecond period and nanometer wavelength. By performing magneto-optical pump-probe experiments with 10 femtosecond resolution in the cubic KNiF$_3$ and the uniaxial K$_2$NiF$_4$ collinear Heisenberg antiferromagnets, we observed coherent oscillations at the frequency of 22 THz and 16 THz, respectively. The detected frequencies as a function of the temperature ideally fit the two-magnon excitation up to the N\'eel point. The experimental signals are described as dynamics of magnetic linear dichroism due to longitudinal oscillations of the antiferromagnetic vector., Comment: 25 pages, 10 figures

We observe the excitation of collective modes in the terahertz (THz) range driven by the recently discovered Neel spin-orbit torques (NSOTs) in the metallic antiferromagnet Mn_{2}Au. Temperature-dependent THz spectroscopy reveals a strong absorption mode centered near 1 THz, which upon heating from 4 to 450 K softens and loses intensity. A comparison with the estimated eigenmode frequencies implies that the observed mode is an in-plane antiferromagnetic resonance (AFMR). The AFMR absorption strength exceeds those found in antiferromagnetic insulators, driven by the magnetic field of the THz radiation, by 3 orders of magnitude. Based on this and the agreement with our theory modeling, we infer that the driving mechanism for the observed mode is the current-induced NSOT. Here the electric field component of the THz pulse drives an ac current in the metal, which subsequently drives the AFMR. This electric manipulation of the Neel order parameter at high frequencies makes Mn_{2}Au a prime candidate for antiferromagnetic ultrafast memory applications.

Antiferromagnets are promising materials for spintronics because they show fast magnetic dynamics, low susceptibility to magnetic fields, and produce no stray fields. In addition, the antiferromagnetic dynamics can be efficiently manipulated by spin and charge currents. Here we discuss spin and/or charge current induced dynamics of the antiferromagnetic textures (domain walls, skyrmions) and nanoparticles. We consider and analyse four types of torques which (spin) current can generate in an antiferromagnet with two magnetic sublattices. These torques can be classified as the staggered/nonstaggered (S/ NS) according to the effective spin accumulation at the magnetic sublattices and the field-like/antidamping-like (FL/ADL) according to the produced energy dissipation (Fig. 1). The NS-FL torque is similar to the torque created by the magnetic field. The recently predicted [1] Neel spin orbit torque gives an example of the S-FL torque, while the spin transfer and antidamping Neel spin orbit torques [2, 3] represent the NS-ADL case. The S-FL and NS-ADL torques can efficiently move the antiferromagnetic domain walls [4, 5]. The symmetry matching between these torques and staggered magnetization provides high mobility compared to other torques. Due to the absence of the Walker breakdown, the velocity of the domain wall motion can reach 30 km/s at the reasonable values of currents. The NS-ADL is sensitive to the structure of the domain wall and pushes all the domain walls in the same direction, which is suitable for the race-track memory. On the contrary, the S-FL torque splits degeneracy of the domains, pushes the domain walls toward the unfavourable domain and thus is suitable for switching. Moreover, this torque is the only physical field which allows to distinguish domains with the opposite orientation of the Neel vector and to manipulate 1800 domain walls in antiferromagnets. The dynamics of the Neel vector induced by the S-FL and NS-ADL torques is assisted by large internal torques of the exchange origin which allow to achieve an ultrafast (compared to ferromagnets) magnetic dynamics. In particular, NS-ADL torque induced by dc spin current induces stable autooscillations of the Neel vector with THz frequency. This paves a way to the antiferromagnetic-based devices fitting THz gap, e.g., emmiters and detectors of THz radiation, which couples to the magnetic dynamics via Neel spin orbit (S-FL) torque [6]. The S-ADL torque, whose effect up to our knowledge was not yet discussed in the literature, can induce fast and reliable 1800 switching of the Neel vector, similar to the spin transfer torque in the ferromagnets. In contrast to NS-ADL which always competes with the internal damping and thus supports precession rather than switching of the Neel vector, the S-ADL torque can either compensate or enhance the internal damping depending on the geometry. The time-dependent S-ADL torque can also induce oscillations with THz frequency. However, the dynamics induced by this torque is not assisted by the internal exchange torques and thus is less efficient than S-FL and NS-ADL torques. From the symmetry point of view, the S-ADL torque can also compensate the Bloch damping which controls relative orientation of the magnetic sublattices and their length. Thus, this torque can play an important role in the vicinity of the Neel point. We also discuss the effects which different torques produce on an antiferromagnetic skyrmion. The field-like torques (both S-FL and NS-FL) influence the skyrmion stability. They allow to control uniaxial (via S-FL) and unidirectional (via NS-FL) anisotropy of the Neel vector. The NS-ADL torque creates a force which can induce linear motion of a skyrmion [7]. To summarise, (spin) current induced dynamics of antiferromagnets is much richer compared to ferromagnetic materials due to the variety of torques and magnetic degrees of freedom. By tuning and combining different types of torques is it possible to induce switching, precession, domain wall motion and other nontrivial dynamical regimes which give antiferromagnetic-based deivces new functionalities.

International audience; Antiferromagnets provide greater stability than their ferromagnetic counterparts, but antiferromagnetic spin textures andnanostructures also exhibit more complex, and often faster, dynamics, offering new functionalities for spintronics devices.

Antiferromagnets (AFM) with a zero, or very small macroscopic magnetization, are promising materials in spintronics. Based on generalized Landau-Lifshitz equations, we examine the magnetic dynamics of three-sublattice AFM in the presence of a spin-polarized current, and in particular, the switching processes between different equilibrium states. We found the conditions for effective switching by pulsed and DC, as well as by an external magnetic field. We examined the features of stationary dynamic states, caused by the current. The obtained results can be used to develop high-speed elements of AFM-based memory materials.

Antiferromagnets are magnetically ordered materials which exhibit no net moment and thus are insensitive to magnetic fields. Antiferromagnetic spintronics aims to take advantage of this insensitivity for enhanced stability, while at the same time active manipulation up to the natural THz dynamic speeds of antiferromagnets is possible, thus combining exceptional storage density and ultra-fast switching. However, the active manipulation and read-out of the N\'eel vector (staggered moment) orientation is challenging. Recent predictions have opened up a path based on a new spin-orbit torque, which couples directly to the N\'eel order parameter. This N\'eel spin-orbit torque was first experimentally demonstrated in a pioneering work using semimetallic CuMnAs. Here we demonstrate for Mn$_2$Au, a good conductor with a high ordering temperature suitable for applications, reliable and reproducible switching using current pulses and readout by magnetoresistance measurements. The symmetry of the torques agrees with theoretical predictions and a large read-out magnetoresistance effect of more than $\simeq 6$~$\%$ is reproduced by ab initio transport calculations., Comment: 5 pages, 4 figures

Searching for novel spin caloric effects in antiferromagnets we study the properties of thermally activated magnons in the presence of an external spin current and temperature gradient. We predict the spin Peltier effect -- generation of a heat flux by spin accumulation -- in an antiferromagnetic insulator with cubic or uniaxial magnetic symmetry. This effect is related with spin-current induced splitting of the relaxation times of the magnons with opposite spin direction. We show that the Peltier effect can trigger antiferromagnetic domain wall motion with a force whose value grows with the temperature of a sample. At a temperature, larger than the energy of the low-frequency magnons, this force is much larger than the force caused by direct spin transfer between the spin current and the domain wall. We also demonstrate that the external spin current can induce the magnon spin Seebeck effect. The corresponding Seebeck coefficient is controlled by the current density. These spin-current assisted caloric effects open new ways for the manipulation of the magnetic states in antiferromagnets., Comment: SUbmitted to J.Phus. D as a part of special issue on Spin Caloritronics

We study spin-dependent electron transport through a ferromagnetic-antiferromagnetic-normal metal tunneling junction subject to a voltage or temperature bias, in the absence of spin-orbit coupling. We derive microscopic formulas for various types of spin torque acting on the antiferromagnet as well as for charge and spin currents flowing through the junction. The obtained results are applicable in the limit of slow magnetization dynamics. We identify a parameter regime in which an unconventional damping-like torque can become comparable in magnitude to the equivalent of the conventional Slonczewski's torque generalized to antiferromagnets. Moreover, we show that the antiferromagnetic sublattice structure opens up a channel of electron transport which does not have a ferromagnetic analogue and that this mechanism leads to a pronounced field-like torque. Both charge conductance and spin current transmission through the junction depend on the relative orientation of the ferromagnetic and the antiferromagnetic vectors (order parameters). The obtained formulas for charge and spin currents allow us to identify the microscopic mechanisms responsible for this angular dependence and to assess the efficiency of an antiferromagnetic metal acting as a spin current polarizer.

In metallic ferromagnets, the interaction between local magnetic moments and conduction electrons renormalizes parameters of the Landau-Lifshitz-Gilbert equation such as the gyromagnetic ratio and the Gilbert damping, and makes them dependent on the magnetic configurations. Although the effects of the renormalization for nonchiral ferromagnets are usually minor and hardly detectable, we show that the renormalization does play a crucial role for chiral magnets. Here the renormalization is chiral and as such we predict experimentally identifiable effects on the phenomenology of magnetization dynamics. In particular, our theory for the self-consistent magnetization dynamics of chiral magnets allows for a concise interpretation of domain wall creep motion. We also argue that the conventional creep theory of the domain wall motion, which assumes Markovian dynamics, needs critical reexamination since the gyromagnetic ratio makes the motion non-Markovian. The non-Markovian nature of the domain wall dynamics is experimentally checkable by the chirality of the renormalization., Comment: 8 pages, 3 figures, 1 table

We investigate the spin Hall magnetoresistance in thin-film bilayer heterostructures of the heavy metal Pt and the antiferromagnetic insulator NiO. While rotating an external magnetic field in the easy plane of NiO, we record the longitudinal and the transverse resistivity of the Pt layer and observe an amplitude modulation consistent with the spin Hall magnetoresistance. In comparison to Pt on collinear ferrimagnets, the modulation is phase shifted by ${90}^{\ensuremath{\circ}}$ and its amplitude strongly increases with the magnitude of the magnetic field. We explain the observed magnetic field dependence of the spin Hall magnetoresistance in a comprehensive model taking into account magnetic-field-induced modifications of the domain structure in antiferromagnets. With this generic model, we are further able to estimate the strength of the magnetoelastic coupling in antiferromagnets. Our detailed study shows that the spin Hall magnetoresistance is a versatile tool to investigate the magnetic spin structure as well as magnetoelastic effects, even in antiferromagnetic multidomain materials.

We theoretically examine a cross effect of magnetic field and charge current on antiferromagnetic domain wall dynamics. Since antiferromagnetic materials are largely insensitive to external magnetic fields in general, charge current has been shown recently as an alternative and efficient way to manipulate antiferromagnets. We find a new role of the magnetic field in the antiferromagnetic dynamics that appears when it is combined with charge current, demonstrating a domain wall motion in the presence of both field and current. We show that a spatially varying magnetic field can shift the current-driven domain-wall velocity, depending on the domain-wall structure and the direction of the field gradient. Our result suggests a novel concept of field control of current-driven antiferromagnetic dynamics.

Spintronics seeks to functionalize antiferromagnetic materials to develop memory and logic devices operating at terahertz speed and robust against external magnetic field perturbations. To be useful, such functionality needs to be developed in thin film devices. The key functionality of long-distance spin-transport has, however, so far only been reported in bulk single crystal antiferromagnets, while in thin films, transport has so far been limited to a few nanometers. In this work, we electrically achieve a long-distance propagation of spin-information in thin films of the insulating antiferromagnet hematite. Through transport and magnetic imaging, we demonstrate a strong correlation between the efficiency of the transport of magnons, which carry spin-information, and the magnetic domain structure of the films. In thin films with large domains, magnons propagate over micrometer distances whilst they attenuate over much shorter distances in multidomain thin films. The governing factor of the attenuation is related to scattering at domain walls, and we demonstrate that we can reduce this through training by field cyclings. For the appropriate crystalline orientation of the films, spin-transport is achieved in zero applied field across micrometers, as required for integration with devices.

High fraction of the surface atoms considerably enhances the influence of size and shape on the magnetic and electronic properties of nanoparticles. Shape effects in ferromagnetic nanoparticles are well understood and allow to set and control the parameters of a sample that affect its magnetic anisotropy during production. In the present paper we study the shape effects in the other widely used magnetic materials -- antiferromagnets, -- which possess vanishingly small or zero macroscopic magnetization. We take into account the difference between the surface and bulk magnetic anisotropy of a nanoparticle and show that the effective magnetic anisotropy depends on the particle shape and crystallographic orientation of its faces. Corresponding shape-induced contribution to the magnetic anisotropy energy is proportional to the particle volume, depends on magnetostriction, and can cause formation of equilibrium domain structure. Crystallographic orientation of the nanoparticle surface determines the type of domain structure. The proposed model allows to predict the magnetic properties of antiferromagnetic nanoparticles depending on their shape and treatment., Comment: 15 pages, 6 figures, to be submitted to Journal of Physics Condensed Matter

We report the observation of the three-dimensional angular dependence of the spin Hall magnetoresistance (SMR) in a bilayer of the epitaxial antiferromagnetic insulator NiO(001) and the heavy metal Pt, without any ferromagnetic element. The detected angular-dependent longitudinal and transverse magnetoresistances are measured by rotating the sample in magnetic fields up to 11 T, along three orthogonal planes (xy-, yz- and xz-rotation planes, where the z-axis is orthogonal to the sample plane). The total magnetoresistance has contributions arising from both the SMR and ordinary magnetoresistance. The onset of the SMR signal occurs between 1 and 3 T and no saturation is visible up to 11 T. The three-dimensional angular dependence of the SMR can be explained by a model considering the reversible field-induced redistribution of magnetostrictive antiferromagnetic S- and T-domains in the NiO(001), stemming from the competition between the Zeeman energy and the elastic clamping effect of the non-magnetic MgO substrate. From the observed SMR ratio, we estimate the spin mixing conductance at the NiO/Pt interface to be greater than $2\times10^{14}$ ${\Omega}^{-1}$ $m^{-2}$. Our results demonstrate the possibility to electrically detect the N\'eel vector direction in stable NiO(001) thin films, for rotations in the xy- and xz- planes. Moreover, we show that a careful subtraction of the ordinary magnetoresistance contribution is crucial to correctly estimate the amplitude of the SMR.

We investigate the existence and stability of skyrmions in a frustrated chiral ferromagnet by considering the competition between ferromagnetic (FM) nearest-neighbour (NN) interaction ($J_1$) and antiferromagnetic (AFM) next-nearest-neighbour (NNN) interaction ($J_2$). Contrary to the general wisdom that long-range ferromagnetic order is not energy preferable under frustration, the skyrmion lattice not only exists but is even stable for a large field range when $J_2 \leq J_1$ compared with frustration-free systems. We defend that the enlargement of stability window of skyrmions is a consequence of the reduced effective exchange interaction caused by the frustration. A multi-sublattice helical state is found below the skyrmion phase, which results from the competition between AFM coupling that favors a two-sublattice N\'{e}el state and the chiral interaction that prefers a helix. As a byproduct, the hysteresis loop of the frustrated chiral system shrinks as the magnetization goes to zero and then opens up again, known as wasp-waist hysteresis loop. The critical field that separates the narrow and wide part of the wasp-waist loop depends exponentially on the strength of NNN coupling. By measuring the critical field, it is possible to determine the strength of NNN coupling., Comment: 7 pages, 5 figures; references added

We study current-driven skyrmion motion in uniaxial thin film antiferromagnets in the presence of the Dzyaloshinskii-Moriya interactions and in an external magnetic field. We phenomenologically include relaxation and current-induced torques due to both spin-orbit coupling and spatially inhomogeneous magnetic textures in the equation for the N\'eel vector of the antiferromagnet. Using the collective coordinate approach we apply the theory to a two-dimensional antiferromagnetic skyrmion and estimate the skyrmion velocity under an applied DC electric current., Comment: 14 pages, 3 figures, 1 table

Antiferromagnets (AFMs), in contrast to ferromagnets, show a nontrivial magnetic structure with zero net magnetization. However, they share a number of spintronic effects with ferromagnets, including spin pumping and spin-transfer torques. Both phenomena stem from the coupled dynamics of free carriers and localized magnetic moments. In the present paper I study the adiabatic dynamics of spin-polarized electrons in a metallic AFM exhibiting a noncollinear ${120}^{\ensuremath{\circ}}$ magnetic structure. I show that the slowly varying AFM spin texture produces a non-Abelian gauge potential related to the time and space gradients of the N\'eel vectors. Corresponding emergent electric and magnetic fields induce rotation of spin and influence the orbital dynamics of free electrons. I discuss both the possibility of a topological spin Hall effect in the vicinity of topological AFM solitons with nonzero curvature and rotation of the electron spin traveling through the AFM domain wall.