Antiferromagnetic Domains Research Articles

On the sign of the linear magnetoelectric coefficient in Cr2O3

We establish the sign of the linear magnetoelectric (ME) coefficient, α, in chromia, Cr2O3. Cr2O3 is the prototypical linear ME material, in which an electric (magnetic) field induces a linearly proportional magnetization (polarization), and a single magnetic domain can be selected by annealing in combined magnetic (H) and electric (E) fields. Opposite antiferromagnetic (AFM) domains have opposite ME responses, and which AFM domain corresponds to which sign of response has previously been unclear. We use density functional theory (DFT) to calculate the magnetic response of a single AFM domain of Cr2O3 to an applied in-plane electric field at zero kelvin. We find that the domain with nearest neighbor magnetic moments oriented away from (towards) each other has a negative (positive) in-plane ME coefficient, α⊥ , at zero kelvin. We show that this sign is consistent with all other DFT calculations in the literature that specified the domain orientation, independent of the choice of DFT code or functional, the method used to apply the field, and whether the direct (magnetic field) or inverse (electric field) ME response was calculated. Next, we reanalyze our previously published spherical neutron polarimetry data to determine the AFM domain produced by annealing in combined E and H fields oriented along the crystallographic symmetry axis at room temperature. We find that the AFM domain with nearest-neighbor magnetic moments oriented away from (towards) each other is produced by annealing in (anti-)parallel E and H fields, corresponding to a positive (negative) axial ME coefficient, α∥ , at room temperature. Since α⊥ at zero kelvin and α∥ at room temperature are known to be of opposite sign, our computational and experimental results are consistent.

Mechanisms of Electrical Switching of Ultrathin CoO/Pt Bilayers.

We study current-induced switching of the Néel vector in CoO/Pt bilayers to understand the underlying antiferromagnetic switching mechanism. Surprisingly, we find that for ultrathin CoO/Pt bilayers electrical pulses along the same path can lead to an increase or decrease of the spin Hall magnetoresistance signal, depending on the current density of the pulse. By comparing these results to XMLD-PEEM imaging of the antiferromagnetic domain structure before and after the application of current pulses, we reveal the details of the reorientation of the Néel vector in ultrathin CoO(4 nm). This allows us to understand how opposite resistance changes can result from a thermomagnetoelastic switching mechanism. Importantly, our spatially resolved imaging shows that regions where the current pulses are applied and regions further away exhibit different switched spin structures, which can be explained by a spin-orbit torque-based switching mechanism that can dominate in very thin films.

Cationic mismatch effect on structural and magnetic behavior of La0.5-xEuxCa0.5-yBayMnO3 charge-ordered manganites

In this research, we have tried to investigate the effect of the cationic disorder on charge ordering and magnetic phase separation in La0.5-xEuxCa0.5-yBayMnO3 (x = 0, 0.05, and 0.1) compounds synthesized by using the sol-gel method. All the samples possess the same average ionic radius at A site <rA> and the same Mn3+/Mn4+ ratio. Our analysis confirmed the chemical composition by energy dispersive X-ray spectroscopy and verified the structural integrity and purity of the phase by X-ray diffraction, revealing an orthorhombic structure with Pnma space group, which is in agreement with the tolerance factor. The important mismatch effect observed in the substituted specimens induces drastic effects on the structural and microstructural properties. Magnetic measurements indicate a great reduction of Curie temperature for substituted samples (from 250 K for x = 0 to 90 K for x = 0.05 and 85 K for x = 0.1), which indicates a weakening of ferromagnetic double exchange interactions induced by cationic disorder. The Arrott plots and the magnetocaloric study of the Eu−based samples indicate the persistence of the antiferromagnetic charge-ordered state characterizing the pristine compound (x = 0) as well as the presence of a first-ordered metamagnetic transition for all the studied samples, which is ascribed to the presence of antiferromagnetic domains. Such transition induced the appearance of a shoulder peak near 100 K in the magnetic entropy change curves for high magnetic field values, resulting in an extra cooling efficiency.

Detecting Magnetoelectric Effect in a Metallic Antiferromagnet via Nonreciprocal Rotation of Reflected Light.

Certain types of media breaking both space-inversion (P) and time-reversal (T) symmetries but preserving their combination PT exhibit the polarization rotation of reflected light even when that of transmitted light is prohibited. Such an effect is termed nonreciprocal rotation of reflected light (NRR). Although NRR shows nearly the same phenomenon as the magnetooptical Kerr effect or, equivalently, the Hall effect at optical frequencies, its origin is distinct and ascribed to a magnetoelectric (ME) effect at optical frequencies, i.e., the optical ME effect. Here we show the observation of NRR in a metallic antiferromagnet TbB_{4}. The result demonstrates that the ME effect in a metallic system, which is considered to be ill defined, can be detected using reflected light. Furthermore, we spatially resolve antiferromagnetic domains in TbB_{4} by microscope observations of NRR. Our work offers a unique way to probe the ME effect in metallic systems.

Infrared imaging of magnetic octupole domains in non-collinear antiferromagnets

ABSTRACT Magnetic structure plays a pivotal role in the functionality of antiferromagnets (AFMs), which not only can be employed to encode digital data but also yields novel phenomena. Despite its growing significance, visualizing the antiferromagnetic domain structure remains a challenge, particularly for non-collinear AFMs. Currently, the observation of magnetic domains in non-collinear antiferromagnetic materials is feasible only in Mn3Sn, underscoring the limitations of existing techniques that necessitate distinct methods for in-plane and out-of-plane magnetic domain imaging. In this study, we present a versatile method for imaging the antiferromagnetic domain structure in a series of non-collinear antiferromagnetic materials by utilizing the anomalous Ettingshausen effect (AEE), which resolves both the magnetic octupole moments parallel and perpendicular to the sample surface. Temperature modulation due to AEE originating from different magnetic domains is measured by lock-in thermography, revealing distinct behaviors of octupole domains in different antiferromagnets. This work delivers an efficient technique for the visualization of magnetic domains in non-collinear AFMs, which enables comprehensive study of the magnetization process at the microscopic level and paves the way for potential advancements in applications.

Tailoring elastic and inelastic collisions of relativistic antiferromagnetic domain walls

Soliton-based computing relies on their unique properties for transporting energy and emerging intact from head-on collisions. Magnetic domain walls are often referred to as solitons disregarding the strict mathematical definition requiring the above scattering property. Here we demonstrate the conditions of elastic and inelastic scattering for spin–orbit torque-induced dynamics of relativistic domain walls on the technologically relevant Mn2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}Au antiferromagnetic material. We show that even domain walls with opposite winding numbers can experience elastic scattering and we present the corresponding phase diagram as a function of the spin–orbit field strength and duration. The elastic collision requires minimum domain walls speed, which we explain assuming an attractive potential created by domain wall pair. On the contrary, when the domain walls move at lower speeds, their collision is inelastic and results in a dispersing breather. Our findings will be important for the development of soliton-based computing using antiferromagnetic spintronics and we discuss their prospects for building NOT and XOR gates.

Antiferromagnetic topological insulator with selectively gapped Dirac cones

Antiferromagnetic (AF) topological materials offer a fertile ground to explore a variety of quantum phenomena such as axion magnetoelectric dynamics and chiral Majorana fermions. To realize such intriguing states, it is essential to establish a direct link between electronic states and topology in the AF phase, whereas this has been challenging because of the lack of a suitable materials platform. Here we report the experimental realization of the AF topological-insulator phase in NdBi. By using micro-focused angle-resolved photoemission spectroscopy, we discovered contrasting surface electronic states for two types of AF domains; the surface having the out-of-plane component in the AF-ordering vector displays Dirac-cone states with a gigantic energy gap, whereas the surface parallel to the AF-ordering vector hosts gapless Dirac states despite the time-reversal-symmetry breaking. The present results establish an essential role of combined symmetry to protect massless Dirac fermions under the presence of AF order and widen opportunities to realize exotic phenomena utilizing AF topological materials.

Generation of electric current and electromotive force by an antiferromagnetic domain wall

Generation of electric current and electromotive force by an antiferromagnetic domain wall

Large anomalous Hall effect and unusual domain switching in an orthorhombic antiferromagnetic material NbMnP

Specific antiferromagnetic (AF) spin configurations generate large anomalous Hall effects (AHEs) even at zero magnetic field through nonvanishing Berry curvature in momentum space. In addition to restrictions on AF structures, suitable control of AF domains is essential to observe this effect without cancellations among its domains; therefore, compatible materials remain limited. Here we show that an orthorhombic noncollinear AF material, NbMnP, acquired AF structure-based AHE and controllability of the AF domains. Theoretical calculations indicated that a large Hall conductivity of ~230 Ω−1cm−1 originated from the AF structure of NbMnP. Symmetry considerations explained the production of a small net magnetization, whose anisotropy enabled the generation and cancellation of the Hall responses using magnetic fields in different directions. Finally, asymmetric hysteresis in NbMnP shows potential for the development of controllability of responses in AF materials.

Visualization of antiferromagnetic domains by nonreciprocal directional dichroism and related optical responses

Nonreciprocal directional dichroism (NDD) is a phenomenon in which the optical absorption is changed by reversing the direction of light propagation or the sign of the magnetic order parameters. While the NDD has mostly been observed in materials with macroscopic magnetization, recent experiments have shown that the NDD can be induced by a specific antiferromagnetic (AFM) spin structure that breaks both space-inversion and time-reversal symmetries. This opens the possibility of visualizing the spatial distribution of AFM domains via the NDD effect. This article reviews the basic features of the NDD, a brief history of the NDD in AFM materials, and recent achievements in visualizing AFM domains via the NDD and related optical responses, and finally provides a perspective on applications of this method for future AFM spintronics research.

Single spin magnetometry and relaxometry applied to antiferromagnetic materials

Despite the considerable interest for antiferromagnets that appeared with the perspective of using them for spintronics, their experimental study, including the imaging of antiferromagnetic textures, remains a challenge. To address this issue, quantum sensors, and, in particular, the nitrogen-vacancy (NV) defects in diamond have become a widespread technical solution. We review here the recent applications of single NV centers to study a large variety of antiferromagnetic materials, from quantitative imaging of antiferromagnetic domains and non-collinear states, to the detection of spin waves confined in antiferromagnetic textures and the non-perturbative measurement of spin transport properties. We conclude with recent developments improving further the magnetic sensitivity of scanning NV microscopy, opening the way to detailed investigations of the internal texture of antiferromagnetic objects.

Fourfold magnetic anisotropy varied by interfacial exchange coupling in epitaxial Fe/IrMn bilayers

We report variable in-plane fourfold magnetic anisotropy in epitaxial exchange-biased Fe/IrMn bilayers with different layer thicknesses. The fourfold magnetic anisotropy of the bilayers continuously decreases and even appears negative as the thickness of the Fe layer decreases, indicating a change in the easy axes from Fe 100 to Fe 110, which can be interpreted by the competition between the intrinsic magnetocrystalline anisotropy along Fe 100 and the extrinsic fourfold magnetic anisotropy along Fe 110 induced by the interfacial exchange coupling to IrMn. As the IrMn thickness increases, the fourfold magnetic anisotropy of the bilayers decreases rapidly from a value close to the intrinsic magnetocrystalline anisotropy of Fe and then gradually reaches saturation. The extrinsic fourfold magnetic anisotropy depends on the amount of the induced uncompensated IrMn spins near the edge of antiferromagnetic domains. In contrast, the exchange bias is determined by the net difference of the opposite uncompensated IrMn spins.

Nanosized antiferromagnetic domains in NiO/Ag(001) films probed by polarised X-ray and unpolarised low-energy electrons

Surface antiferromagnetism of NiO film, epitaxially deposited on Ag(001), is investigated using the absorption of linearly polarised X-ray and the diffraction of low-energy unpolarised electrons. Polarisation-dependent X-ray absorption studies reveal a preferential stabilisation of the magnetic domains with the main components of magnetic axes parallel to the interface. Furthermore, the role of interfacial strain-induced magnetocrystalline anisotropy below a critical film thickness (∼ 20 ML) could be demonstrated by the magnetic linear dichroism results. Utilising the exchange scattered electrons from various antiferromagnetic (111) planes, the magnetic twin domains could be probed by a low energy electron microscope operating in the dark-field mode. The linearly polarised X-ray, on the other hand, is sensitive to the magnetic axes orientations of Ni2+ ions and therefore, provides access to the spin domains as well as the twin domains. Comparison of results of these two independent imaging techniques demonstrates that the in-plane component of the magnetic easy axes contained by the complementary T-domains are parallel to each other.

Low-temperature hysteresis broadening emerging from domain-wall creep dynamics in a two-phase competing system

Hysteretic behaviour accompanies any first-order phase transition, forming a basis for many applications. However, its quantitative understanding remains challenging, and even a qualitative understanding of pronounced hysteresis broadening at low temperature, which is often observed in magnetic-field-induced first-order phase transition materials, is unclear. Here, we show that such pronounced hysteresis broadening emerges if the phase-front velocity during the first-order phase transition exhibits an activated behaviour as a function of both temperature and magnetic field. This is demonstrated by using real-space magnetic imaging techniques, for the magnetic-field-induced first-order phase transition between antiferromagnetic and ferrimagnetic phases in (Fe0.95Zn0.05)2Mo3O8. When combined with the Kolmogorov-Avrami-Ishibashi model, the observed activated temperature- and field-dependences of the growth velocity of the emerging antiferromagnetic domain quantitatively reproduce the pronounced hysteresis broadening. Furthermore, the same approach also reproduces the field-sweep-rate dependence of the transition field observed in the experiment. Our findings thus provide a quantitative and comprehensive understanding of pronounced hysteresis broadening from the microscopic perspective of domain growth.

Electrical control of 180° domain walls in an antiferromagnet

We demonstrate the reversible current-induced motion of 180° antiferromagnetic domain walls in a CuMnAs device. By controlling the magnitude and direction of the current pulse, the position of a domain wall can be switched between three distinct pinning sites. The domain wall motion is attributed to a field-like spin–orbit torque that induces the same sense of rotation on each magnetic sublattice, owing to the crystal symmetry of CuMnAs. Domain wall motion is observed for current densities down to ≈2.5×1010 A/m2 at room temperature.

Magnetic and magnetoelastic properties of antiferromagnet FeGe2

The magnetic susceptibility χ of single-crystal antiferromagnet FeGe2 (TN = 289 K) was measured in the temperature range from 4.2 to 300 K in a magnetic field H up to 40 kOe applied along the main crystallographic axes. For the antiferromagnetic (AFM) state at low temperatures, a strong increase in χ is observed with increasing H applied along the [100] axis, which reaches saturation at H≥H0∼ 11 kOe. It is assumed that this behavior is associated with a field-induced change in the AFM domain structure, and the single-domain state is realized in a field above H0 with the AFM axis perpendicular to the field direction. The study of χ under the uniaxial Pa and uniform P pressures in a wide temperature range has revealed an anomalously large increase in χ in AFM state with increasing uniaxial pressure for Pa||H||[100] at H≪H0. The observed growth of χ saturates at pressure Pa≥Pa∗≃1.5 kbar and suggests the formation of the single-domain state for the above conditions. The values of uniform pressure effect on χ was found to lie in the range dln⁡χ/dP = − (2–3) Mbar–1, being weakly dependent on the magnetic state and field direction. In addition, the uniform pressure effect on the transition temperature between two AFM structures in FeGe2, TM≃263 K, was found to be weak.

Antiferromagnetic domain distribution carried by stoichiometry

Antiferromagnetic domain distribution carried by stoichiometry

A proposal for leaky integrate-and-fire neurons by domain walls in antiferromagnetic insulators

Brain-inspired neuromorphic computing is a promising path towards next generation analogue computers that are fundamentally different compared to the conventional von Neumann architecture. One model for neuromorphic computing that can mimic the human brain behavior are spiking neural networks (SNNs), of which one of the most successful is the leaky integrate-and-fire (LIF) model. Since conventional complementary metal-oxide-semiconductor (CMOS) devices are not meant for modelling neural networks and are energy inefficient in network applications, recently the focus shifted towards spintronic-based neural networks. In this work, using the advantage of antiferromagnetic insulators, we propose a non-volatile magnonic neuron that could be the building block of a LIF spiking neuronal network. In our proposal, an antiferromagnetic domain wall in the presence of a magnetic anisotropy gradient mimics a biological neuron with leaky, integrating, and firing properties. This single neuron is controlled by polarized antiferromagnetic magnons, activated by either a magnetic field pulse or a spin transfer torque mechanism, and has properties similar to biological neurons, namely latency, refraction, bursting and inhibition. We argue that this proposed single neuron, based on antiferromagnetic domain walls, is faster and has more functionalities compared to previously proposed neurons based on ferromagnetic systems.

Long range motion of domain wall in antiferromagnetic [formula omitted] curved nanowire

The theoretical study of domain wall dynamics in a 3D curved nanowire with torsion in an antiferromagnetic environment is presented. The equations of motion for the antiferromagnetic domain wall are derived using the collective variable approach. The curvature induces the Dzyaloshinskii–Moriya interaction, while the torsion induces an anisotropy term. The forces resulting from their gradients drive the domain wall in opposite directions in a stable state. A curve has been designed to allow the domain wall to move spontaneously over long distances. All analytical predictions are confirmed through numerical simulations.

Perspective on imaging antiferromagnetic domains in thin films with the magneto-optical birefringence effect

Antiferromagnets are considered to be a promising host material for the next generation of magnetic storage due to their high stability and stray-field-free property. However, the absence of net magnetization in antiferromagnets renders conventional magnetometry ineffective, posing a great challenge in investigating microscopic antiferromagnetic (AFM) properties under magnetic or electric fields. In this Perspective, we provide an overview of various AFM domain imaging techniques and discuss the most promising optical imaging method based on the magneto-optical birefringence (MOB) effect. Additionally, we highlight recent advances in imaging AFM domains utilizing the MOB technique. This Perspective aims to provide a comprehensive review of the current research and potential future directions based on the MOB imaging technique, which could pave the way for the development of more efficient and reliable magnetic storage devices based on antiferromagnets.