Antiferromagnetic Spintronics Research Articles

Ultrafast spin current generated from an antiferromagnet

Antiferromagnets (AFMs) have the potential to push spintronic devices from a static condition or gigahertz frequency range to the terahertz range for the sake of high-speed processing. However, the insensitivity of AFMs to magnetic fields makes the manipulation of spin currents difficult. The ultrafast generation of the spin current in ferromagnet/heavy-metal (HM) structures has received a lot of attention in recent years, but whether a similar scenario can be observed in an AFM/HM system is still unknown. Here, we show the optical generation of ultrafast spin current in an AFM/HM heterostructure at zero external magnetic field and at room temperature by detecting the associated terahertz emission. We believe that this is a common phenomenon in antiferromagnets with strong nonlinear optical effects. Our results open an avenue of fundamental research into antiferromagnetism and a route to AFM spintronic devices. Spin currents are generated from an antiferromagnet/heavy-metal heterostructure using optical excitation on picosecond timescales. This will have applications in antiferromagnetic spintronics.

Spin-orbit torque switching of an antiferromagnetic metallic heterostructure

The ability to represent information using an antiferromagnetic material is attractive for future antiferromagnetic spintronic devices. Previous studies have focussed on the utilization of antiferromagnetic materials with biaxial magnetic anisotropy for electrical manipulation. A practical realization of these antiferromagnetic devices is limited by the requirement of material-specific constraints. Here, we demonstrate current-induced switching in a polycrystalline PtMn/Pt metallic heterostructure. A comparison of electrical transport measurements in PtMn with and without the Pt layer, corroborated by x-ray imaging, reveals reversible switching of the thermally-stable antiferromagnetic Néel vector by spin-orbit torques. The presented results demonstrate the potential of polycrystalline metals for antiferromagnetic spintronics.

Pure spin current phenomena

Spintronic phenomena and devices have evolved from exploiting spin-polarized current to pure spin current. A pure spin current can more efficiently deliver spin angular momentum with fewer or no charge carriers, enabling spin–orbit torque switching in ferromagnets. The prospects of antiferromagnetic spintronics, especially electrical switching, remain perplexing and controversial. We describe some recent advances, outstanding issues, challenges, and prospects of pure spin current.

Effect of antiferromagnetic order on topological electronic structure in Eu-substituted Bi2Se3 single crystals

Topological antiferromagnetic spintronics is an emerging field due to advantages such as no stray field effect and high-speed dynamics. In bismuth chalcogenide topological insulators, the coexistence of long-range antiferromagnetic order and topologically protected states has scarcely been observed. Here, an antiferromagnetic order, which sets in the direction of magnetic moments perpendicular to the c axis, is introduced by Eu substitution in Bi2Se3, and therefore it influences the topological electronic properties of Bi2Se3. Despite Eu substitution, Shubnikov–de Haas (SdH) oscillations are observed. The angle dependence of SdH oscillations shows a signature of anisotropic 3D Fermi surfaces with a nontrivial Berry phase by Eu substitution, and the temperature dependence of SdH oscillations reveals that the effective mass is comparable to the pristine Bi2Se3. These results suggest that the nontrivial topological state can survive in the antiferromagnetic order of Eu-substituted Bi2Se3. Our work expands the base of topological materials available for antiferromagnetic spintronics applications.

Exploring the potential of MnX (S, Sb) monolayers for antiferromagnetic spintronics: A theoretical investigation

In this study, we predicted new two-dimensional tetragonal structures of t-Mn2X2 (X = S, Sb) sheets on the basis of first-principles plane wave calculations within density functional theory with Hubbard U model. Stability tests such as phonon spectrum calculation and molecular dynamic simulations reveal that the 2D t-Mn2X2 structures are dynamically and thermally stable at least in room temperature. Our theoretical calculations have shown that t-Mn2X2 structures have two Raman active and seven infrared active modes. The t-Mn2Sb2 sheet exhibits metallic property, whereas t-Mn2S2 shows semiconducting property with a 0.68 eV indirect bandgap. Exploring of the favorable magnetic orientation calculations revealed that both 2D t-Mn2X2 structures prefer antiferromagnetic spin configuration. Estimated critical temperatures for the phase transition from antiferromagnetic spin order to paramagnetic case are 720 K and 545 K for t-Mn2S2 and t-Mn2Sb2, respectively. These relatively high Néel temperatures and their suitable electronic properties for many applications clearly qualify that the 2D t-Mn2X2 sheets can be a good candidate for room temperature antiferromagnetic device applications.

Controlling spin current polarization through non-collinear antiferromagnetism

The interconversion of charge and spin currents via spin-Hall effect is essential for spintronics. Energy-efficient and deterministic switching of magnetization can be achieved when spin polarizations of these spin currents are collinear with the magnetization. However, symmetry conditions generally restrict spin polarizations to be orthogonal to both the charge and spin flows. Spin polarizations can deviate from such direction in nonmagnetic materials only when the crystalline symmetry is reduced. Here, we show control of the spin polarization direction by using a non-collinear antiferromagnet Mn3GaN, in which the triangular spin structure creates a low magnetic symmetry while maintaining a high crystalline symmetry. We demonstrate that epitaxial Mn3GaN/permalloy heterostructures can generate unconventional spin-orbit torques at room temperature corresponding to out-of-plane and Dresselhaus-like spin polarizations which are forbidden in any sample with two-fold rotational symmetry. Our results demonstrate an approach based on spin-structure design for controlling spin-orbit torque, enabling high-efficient antiferromagnetic spintronics.

Dirac fermions in the antiferromagnetic spintronics material CuMnAs

Dirac semimetals (DSMs) are the focus of study as new candidates for topological superconductivity and spintronics. Although the Dirac points are not necessarily associated with crystalline symmetries, the experimentally realized robust DSMs such as $\mathrm{C}{\mathrm{d}}_{3}\mathrm{A}{\mathrm{s}}_{2}$ have fourfold-degenerate Dirac points at generic wave vectors on high symmetry line enabled by a fourfold crystalline symmetry. The magnetic counterpart of these robust DSMs remains unknown. On the other hand, the theoretically proposed magnetic DSMs in potential antiferromagnetic (AFM) spintronics materials where both time-reversal $(\mathcal{T})$ and inversion $(\mathcal{P})$ symmetry are broken but their combination $\mathcal{P}\mathcal{T}$ is preserved have Dirac points on the boundary of the Brillouin zone protected by a low-order twofold crystalline symmetry. Here, combined with first-principles calculations and symmetry analysis, we find the Dirac fermion at generic wave vector on high symmetry line with associated nontrivial surface states in $\mathcal{P}\mathcal{T}$-symmetric tetragonal CuMnAs, which is the prototype AFM spintronics material observed in experiments. Furthermore, we reveal the hidden spin textures in this $\mathcal{P}\mathcal{T}$-symmetric magnetic system, demonstrating interesting vortex-like spin textures. Our study opens the way for designing robust magnetic DSMs that can serve as an ideal platform to study the interplay of magnetism, Dirac fermions, and spin-orbit interactions, and could stimulate a series of research in the field of topological antiferromagnetic spintronics.

Magnon dispersion relations in the noncollinear antiferromagnetIrMn3

The noncollinear $L{1}_{2}\text{\ensuremath{-}}{\mathrm{IrMn}}_{3}$ room-temperature metallic antiferromagnet is one of the most important materials for scientific investigation and for applications in antiferromagnetic spintronics. It has been subject to intense investigation for over two decades, mainly in connection with exchange bias phenomena. Here we present a theoretical calculation of the magnon dispersion relations and the experimental observation of some magnon modes using inelastic Brillouin and Raman light-scattering techniques. The fit of theory to the experimentally measured magnon frequencies yields values for the nearest-neighbor AF exchange, the next-nearest-neighbor FM exchange interaction, and the uniaxial anisotropy parameters that are in good agreement with the ones calculated with atomistic spin models. The knowledge of the magnon dispersion relations revealed here is essential for the study of possible applications of ${\mathrm{IrMn}}_{3}$ in terahertz and other spintronic devices.

Deterministic control of an antiferromagnetic spin arrangement using ultrafast optical excitation

A central prospect of antiferromagnetic spintronics is to exploit magnetic properties that are unavailable with ferromagnets. However, this poses the challenge of accessing such properties for readout and control. To this end, light-induced manipulation of the transient ground state, e.g. by changing the magnetic anisotropy potential, opens promising pathways towards ultrafast deterministic control of antiferromagnetism. Here, we use this approach to trigger a coherent rotation of the entire long-range antiferromagnetic spin arrangement about a crystalline axis in GdRh2Si2 and demonstrate deterministic control of this rotation. Our observations can be explained by a laser-induced shift of the direction of the Gd spins’ local magnetic anisotropy, and allow for a quantitative description of the transient magnetic anisotropy potential.

Tunable 3D/2D magnetism in the (MnBi2Te4)(Bi2Te3)m topological insulators family

Feasibility of many emergent phenomena that intrinsic magnetic topological insulators (TIs) may host depends crucially on our ability to engineer and efficiently tune their electronic and magnetic structures. Here we report on a large family of intrinsic magnetic TIs in the homologous series of the van der Waals compounds (MnBi2Te4)(Bi2Te3)m with m = 0, ⋯, 6. Magnetic, electronic and, consequently, topological properties of these materials depend strongly on the m value and are thus highly tunable. The antiferromagnetic (AFM) coupling between the neighboring Mn layers strongly weakens on moving from MnBi2Te4 (m = 0) to MnBi4Te7 (m = 1) and MnBi6Te10 (m = 2). Further increase in m leads to change of the overall magnetic behavior to ferromagnetic (FM) one for (m = 3), while the interlayer coupling almost disappears. In this way, the AFM and FM TI states are, respectively, realized in the m = 0, 1, 2 and m = 3 cases. For large m numbers a hitherto-unknown topologically nontrivial phase can be created, in which below the corresponding critical temperature the magnetizations of the non-interacting 2D ferromagnets, formed by the MnBi2Te4 building blocks, are disordered along the third direction. The variety of intrinsic magnetic TI phases in (MnBi2Te4)(Bi2Te3)m allows efficient engineering of functional van der Waals heterostructures for topological quantum computation, as well as antiferromagnetic and 2D spintronics.

Size-dependent anomalous Hall effect in noncollinear antiferromagnetic Mn3Sn films

The coercive field of ferromagnets generally increases with decreasing the sample size to hundreds of nanometers mainly because of the (edge) defect pinning. We investigate size-dependent anomalous Hall effect (AHE) in (112¯0)-oriented noncollinear antiferromagnetic Mn3Sn films. The switching field (coercive field) of the AHE decreases abruptly when the width of the Hall bar decreases to hundreds of nanometers, giving rise to the reduced coercive field from 445 to 30 mT for Hall bar with width from 2 μm to 100 nm. This observation is in contrast to the ferromagnetic counterpart. The transition from a multidomain to single domain-like mode and the reduction of Néel temperature are proposed to explain the coercivity variation. Our finding provides a promising candidate for the device miniaturization and adds a different dimension to antiferromagnetic spintronics.

Anisotropy in antiferromagnets

Due to the advent of antiferromagnetic (AF) spintronics, there is a burgeoning interest in AF materials for a wide range of potential and actual applications. Generally, AFs are characterized via the ordering at the Néel temperature (TN), but to have a stable AF configuration, it is necessary that the material has a sufficient level of anisotropy so as to maintain the orientation of the given magnetic state fixed in one direction. Unlike the case for ferromagnets, there are little established data on the anisotropy of AFs and, in particular, its origins, other than it being magneto-crystalline, and those factors which control it. In this perspective article, these factors are reviewed in light of recent and established experimental data. The anisotropy can be found indirectly via the exchange bias phenomenon. This technique is reviewed and, in particular, the implications for the nature of the anisotropy that is measured and its distribution. Finally, a strategy is proposed that would allow for the development of AF materials with controlled anisotropy for future applications.

Neuromorphic computing with antiferromagnetic spintronics

While artificial intelligence, capable of readily addressing cognitive tasks, has transformed technologies and daily lives, there remains a huge gap with biological systems in terms of performance per energy unit. Neuromorphic computing, in which hardware with alternative architectures, circuits, devices, and/or materials is explored, is expected to reduce the gap. Antiferromagnetic spintronics could offer a promising platform for this scheme. Active functionalities of antiferromagnetic systems have been demonstrated recently and several works indicated their potential for biologically inspired computing. In this perspective, we look through the prism of these works and discuss prospects and challenges of antiferromagnetic spintronics for neuromorphic computing. Overview and discussion are given on non-spiking artificial neural networks, spiking neural networks, and reservoir computing.

Magnetoresistance Effects in the Metallic Antiferromagnet Mn2Au

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.

Excitation of Terahertz Magnons in Antiferromagnetic Nanostructures: Theory and Experiment

The theoretical and experimental studies of the excitation, detection, and propagation of magnons in antiferromagnetic nanostructures have been reviewed. The properties of antiferromagnetic materials such as the absence of a macroscopic magnetization, the presence of strong exchange interactions, and a complex magnetocrystalline structure make it possible to implement new types of memory and functional electronic devices. The study of possible magnon effects in antiferromagnetic materials in micro- and nanoscales requires new experimental and theoretical approaches. In this review, the recent results on the excitation of magnetic oscillations—magnons—in antiferromagnetic materials induced by the current and optical radiation are described and systematized. The main theoretical results on antiferromagnets and multilayer antiferromagnetic heterostructures are presented. Models for description of phenomena induced by the current and optical pulses in nanoheterostructures including antiferromagnets are considered. Methods for studying antiferromagnetic micro- and nanostructures by means of Brillouin scattering, as well as prospects of the application of antiferromagnetic spintronics and magnonics, are briefly discussed.

Parasitic Magnetism in Magnetoelectric Antiferromagnet.

Parasitic magnetism plays an important role in magnetoelectric spin switching of antiferromagnetic oxides, but its mechanism has not been clearly investigated. Unlike the widely obtained surface boundary magnetization in magnetoelectric Cr2O3 antiferromagnet, we previously reported that Al doping could produce volume-dependent parasitic magnetism (Mpara) in Cr2O3 with the remaining magnetoelectric effect and antiferromagnetic properties. In this work, we systematically investigated the magnetic properties of Mpara in Cr2O3 through its different exchange coupling characteristics with the ferromagnet at various conditions. The columnar grain boundaries cause an antiferromagnetic sublattice breaking to produce uncompensated spins and thus are considered to be responsible for Mpara in both undoped and Al-doped Cr2O3. Finally, a model was proposed for the formation mechanism of the parasitic magnetism in Cr2O3, which explains the reported magnetic characteristics of Cr2O3, and some current topics such as the domain formation and motion in Cr2O3 during magnetoelectric spin switching. This work contributes to a deep understanding of antiferromagnetic spintronics and provides a method to realize the low-energy operation of antiferromagnetic-based magnetic random access memory.

Signature of topological states in antiferromagnetic Sm-substituted Bi2Te3

An antiferromagnetic topological insulator has been predicted to be preserved by breaking both time-reversal symmetry and primitive lattice translational symmetry. However, the topological surface state has often been observed to disappear in an antiferromagnetic phase because the doped magnetic impurity acts as an extrinsic defect. In this study, we report the experimental signature of topological surface states coexisting with antiferromagnetic order in Sm-doped Bi2Te3. We fabricate single crystals of SmxBi2−xTe3 with x = 0.004, 0.010, and 0.025, where the Curie-Weiss law is satisfied at low temperatures but is violated at high temperatures due to the influence of the high energy states of J multiplets of Sm. For x = 0.025, e xotic physical properties are observed, such as the antiferromagnetic phase with the Néel temperature TN = 3.3 K, multi-band Hall effect with two conduction channel, and anisotropic Shubnikov-de Haas oscillations. In the antiferromagnetic phase, we detect the signature of nontrivial topological surface states with surface electron density ns = 7.9 × 1011 cm−2 and its high mobility μs = 2,200 cm2/Vs, compared to nb = 2.0 × 1019 cm−3 and μb = 2.3 cm2/Vs for bulk electrons. These observations suggest that SmxBi2−xTe3 is a candidate creating the new stage for the potential application of topological antiferromagnetic spintronics.

Interlayer exchange coupling and spin Hall effect through an Ir-doped Cu nonmagnetic layer

Metallic superlattices where the magnetization vectors in the adjacent ferromagnetic layers are antiferromagnetically coupled by the interlayer exchange coupling through nonmagnetic spacer layers are systems available for the systematic study on antiferromagnetic (AF) spintronics. As a candidate of nonmagnetic spacer layer material exhibiting remarkable spin Hall effect, which is essential to achieve spin-orbit torque switching, we selected the Ir-doped Cu in this study. The AF-coupling for the Co / Cu$_{95}$Ir$_{5}$ / Co was investigated, and was compared with those for the Co / Cu / Co and Co / Ir / Co. The maximum magnitude of AF-coupling strength was obtained to be 0.39 mJ/m$^{2}$ at the Cu$_{95}$Ir$_{5}$ thickness of about 0.75 nm. Furthermore, we found a large spin Hall angle of Cu$_{95}$Ir$_{5}$ in Co / Cu$_{95}$Ir$_{5}$ bilayers by carrying out spin Hall magnetoresistance and harmonic Hall voltage measurements, which are estimated to be 3 ~ 4 %. Our experimental results clearly indicate that Cu$_{95}$Ir$_{5}$ is a nonmagnetic spacer layer allowing us to achieve moderately strong AF-coupling and to generate appreciable spin-orbit torque via the spin Hall effect.

Concepts from the linear magnetoelectric effect that might be useful for antiferromagnetic spintronics

We discuss the correspondence between the current-induced spin polarization in non-centrosymmetric magnetic metals and the linear magnetoelectric effect in non-centrosymmetric magnetic insulators using a linear-response theory and the concept of magnetoelectric multipoles. We show that the magnetoelectric toroidal moment is a particularly useful quantity since it determines the ground-state antiferromagnetic domain of a non-centrosymmetric antiferromagnet in the presence of a steady-state electric current. We analyze two prototypical antiferromagnetic spintronic materials—Mn2Au and CuMnAs—and show that the experimentally reported domain reorientations are consistent with the alignment of their toroidal moments parallel to the applied electric current. Finally, we determine whether similar behavior should be expected in the prototypical insulating magnetoelectric materials, Cr2O3 and LiMPO4, if they could be doped into a semiconducting or metallic regime.

First-principles study of magnetism and phase stabilities of V2 based antiferromagnetic Heusler alloys

We investigate the phase stability and magnetism of V2 based antiferromagnetic semiconductors by first-principles calculations. We first analyze the phase stability of the designed alloys by calculating the convex hull and find that keeping Al atoms in the Z sites is crucial to guarantee the stability against the segregation. The V2YAl (Y=V, Nb, and Ta) systems have relatively small hull distances and Néel temperatures higher than 1000 K are evaluated. The strong antiferromagnetic coupling between V atoms is rapidly decreasing with increasing the number of electrons due to the appearance of ferromagnetic double-exchange interaction. By applying a uniaxial strain to induce tetragonal distortion, a relatively large magneto-crystalline anisotropy energy is obtained for V2TaAl. It is claimed that V2YAl are thought to be good candidates for antiferromagnetic spintronics including gate-tunable magnetic properties.