Topological Antiferromagnets Research Articles

UOTe: Kondo-Interacting Topological Antiferromagnet in a Van der Waals Lattice.

Since the initial discovery of 2D van der Waals (vdW) materials, significant effort has been made to incorporate the three properties of magnetism, band structure topology, and strong electron correlations-to leverage emergent quantum phenomena and expand their potential applications. However, the discovery of a single vdW material that intrinsically hosts all three ingredients has remained an outstanding challenge. Here, the discovery of a Kondo-interacting topological antiferromagnet is reported in the vdW 5f electron system UOTe. It has a high antiferromagnetic (AFM) transition temperature of 150K, with a unique AFM configuration that breaks the combined parity and time reversal (PT) symmetry in an even number of layers while maintaining zero net magnetic moment. This angle-resolved photoemission spectroscopy (ARPES) measurements reveal Dirac bands near the Fermi level, which combined with the theoretical calculations demonstrate UOTe as an AFM Dirac semimetal. Within the AFM order, the presence of the Kondo interaction is observed, as evidenced by the emergence of a 5f flat band near the Fermi level below 100 K and hybridization between the Kondo band and the Dirac band. The density functional theory calculations in its bilayer form predict UOTe as a rare example of a fully-compensated AFM Cherninsulator.

Multivariate growth analysis on D019–phase Mn3Ga kagome–based topological antiferromagnets

The combination of antiferromagnetism and topological properties in Mn3X (X = Sn,Ge,Ga) offers a unique platform to explore novel spin-dependent phenomena and develop innovative spintronic devices. Here, we have systematically investigated the phase transition of Mn3Ga thin films on SiO2(001)/Si substrates under various growth parameters such as seeding layer structure, annealing conditions, and film thickness. The relatively thick Mn3Ga films grown with Ru seeding exhibit a variety of polycrystalline hexagonal phases, including (002), and (201). The addition of a Ta layer to the conventional Ru seeding layer promotes the formation of nearly single-crystal antiferromagnetic (AF) Mn3Ga(002) phase from the relatively thin Mn3Ga films after annealing at 773 K. The investigation of the growth mechanism of Mn3Ga polycrystalline thin films provides a reference strategy for exploring Mn-based AF spintronic devices.

Intrinsic nonlinear Hall effect in two-dimensional honeycomb topological antiferromagnets

Intrinsic nonlinear Hall effect in two-dimensional honeycomb topological antiferromagnets

Dominance of Extrinsic Scattering Mechanisms in the Orbital Hall Effect: Graphene, Transition Metal Dichalcogenides, and Topological Antiferromagnets.

The theory of the orbital Hall effect (OHE), a transverse flow of orbital angular momentum (OAM) in response to an electric field, has concentrated on intrinsic mechanisms. Here, using a quantum kinetic formulation, we determine the full OHE in the presence of short-range disorder using 2D massive Dirac fermions as a prototype. We find that, in doped systems, extrinsic effects associated with the Fermi surface (skew scattering and side jump) provide ≈95% of the OHE. This suggests that, at experimentally relevant transport densities, the OHE is primarily extrinsic.

Effective electrical manipulation of a topological antiferromagnet by orbital torques

The electrical control of the non-trivial topology in Weyl antiferromagnets is of great interest for the development of next-generation spintronic devices. Recent studies suggest that the spin Hall effect can switch the topological antiferromagnetic order. However, the switching efficiency remains relatively low. Here, we demonstrate the effective manipulation of antiferromagnetic order in the Weyl semimetal Mn3Sn using orbital torques originating from either metal Mn or oxide CuOx. Although Mn3Sn can convert orbital current to spin current on its own, we find that inserting a heavy metal layer, such as Pt, of appropriate thickness can effectively reduce the critical switching current density by one order of magnitude. In addition, we show that the memristor-like switching behaviour of Mn3Sn can mimic the potentiation and depression processes of a synapse with high linearity—which may be beneficial for constructing accurate artificial neural networks. Our work paves a way for manipulating the topological antiferromagnetic order and may inspire more high-performance antiferromagnetic functional devices.

Anomalous Hall effect and Fermi surface reconstruction in topological antiferromagnet candidate GdAuPb

The combination of topology and magnetism has demonstrated great potential in the search of emergent quantum matters. In this study, we report the synthesis, magnetic, and electrical properties of GdAuPb, a rare-earth-based half-Heusler topological antiferromagnet candidate. Magnetization and specific heat studies reveal highly frustrated antiferromagnetic ordering of Gd at TN = 11.5 K. Interestingly, this material possesses a large, unsaturated magnetoresistance up to 800% below TN and anomalous Hall conductivity as large as 750 Ω−1cm−1 in the antiferromagnetic state. Moreover, the electric transports including the SdH quantum oscillation patterns all exhibit a sudden change around 9 T, indicating a field-driven quantum transition related to the Fermi surface reconstruction. These results suggest that GdAuPb is close to a topological critical point, which can be easily tuned via external parameters. Our study demonstrates a strong interplay between band topology and magnetism in GdAuPb and offers clues for material designs for topological antiferromagnets.

Transport characterization of magnetic phase transition in Mn<sub>3</sub>Sn thin films

In recent years, topological antiferromagnetic material with hexagonal Kagome structure has attracted great research interest due to its unique properties. Although its net magnetic moment is close to zero, the topological antiferromagnet exhibits the strong magnetoelectric, the magneto-optical, and the magnetothermal effect, with a strength comparable to that of ferromagnetic material, which makes it highly valuable for various applications. After several years of extensive studies, it has been realized that most of the unique properties of topological antiferromagnet are actually closely related to its magnetic structure. However, it has been found that the magnetic structure of the material is highly sensitive to its chemical composition and growth condition. Therefore, it is crucial to develop a universal and simple method of measuring the magnetic structure and determining the magnetic phase transition of hexagonal Kagome topological antiferromagnetic material, which can severe as a good supplement for the current high-energy neutron diffraction approach that is not accessible for ordinary laboratories. In this study, we have successfully prepared high-quality (<inline-formula><tex-math id="M3">\begin{document}$ 11\bar{2}0 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20231766_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20231766_M3.png"/></alternatives></inline-formula>)-oriented hexagonal Kagome antiferromagnetic Mn<sub>3</sub>Sn thin films on (<inline-formula><tex-math id="M4">\begin{document}$1 \bar{1} 02$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20231766_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20231766_M4.png"/></alternatives></inline-formula>)-oriented Al<sub>2</sub>O<sub>3</sub> single crystal substrates by using the pulsed laser deposition method. After systematically measuring how the magnetic and transport properties of the Mn<sub>3</sub>Sn thin film change with temperature, it is found that its magnetization curve, Hall resistivity curve, and magnetoresistance curve exhibit certain anomalous features at some or all of its three magnetic phase transition temperatures. These features can serve as good evidences of magnetic phase transitions in this hexagonal Kagome antiferromagnetic Mn<sub>3</sub>Sn thin film, or even could be used to measure the temperatures of these magnetic phase transitions. Our work contributes to the further advancement of the application of hexagonal Kagome topological antiferromagnetic materials to spin electronic devices.

Strain-tunable topological antiferromagnetism of two-dimensional magnets with negative Poisson ratio

Strain-tunable topological antiferromagnetism of two-dimensional magnets with negative Poisson ratio

Intrinsic Néel Antiferromagnetic Multimeronic Spin Textures in Ultrathin Films.

Topological antiferromagnetism is a vibrant and captivating research field, generating considerable enthusiasm with the aim of identifying topologically protected magnetic states of key importance in the hybrid realm of topology, magnetism, and spintronics. While topological antiferromagnetic (AFM) solitons bear various advantages with respect to their ferromagnetic cousins, their observation is scarce. Utilizing first-principles simulations, here we predict new chiral particles in the realm of AFM topological magnetism, exchange-frustrated multimeronic spin textures hosted by a Néel magnetic state, arising universally in single Mn layers directly grown on an Ir(111) surface or interfaced with Pd-based films. These nanoscale topological structures are intrinsic; i.e. they form in a single AFM material, can carry distinct topological charges, and can combine in various multimeronic sequences with enhanced stability against external magnetic fields. We envision the frustrated Néel AFM multimerons as exciting highly sought after AFM solitons having the potential to be utilized in novel spintronic devices hinging on nonsynthetic AFM quantum materials, further advancing the frontiers of nanotechnology and nanophysics.

Quantum-metric-induced nonlinear transport in a topological antiferromagnet.

The Berry curvature and quantum metric are the imaginary part and real part, respectively, of the quantum geometric tensor, which characterizes the topology of quantum states1. The Berry curvature is known to generate a number of important transport phenomena, such as the quantum Hall effect and the anomalous Hall effect2,3; however, the consequences of the quantum metric have rarely been probed by transport measurements. Here we report the observation of quantum-metric-induced nonlinear transport, including both a nonlinear anomalous Hall effect and a diode-like non-reciprocal longitudinal response, in thin films of a topological antiferromagnet, MnBi2Te4. Our observations reveal that the transverse and longitudinal nonlinear conductivities reverse signs when reversing the antiferromagnetic order, diminish above the Néel temperature and are insensitive to disorder scattering, thus verifying their origin in the band-structure topology. They also flip signs between electron- and hole-doped regions, in agreement with theoretical calculations. Our work provides a means to probe the quantum metric through nonlinear transport and to design magnetic nonlinear devices.

Magneto-optical Kerr and Faraday effects in bilayer antiferromagnetic insulators

Control and detection of antiferromagnetic topological materials are challenging since the total magnetization vanishes. Here we investigate the magneto-optical Kerr and Faraday effects in bilayer antiferromagnetic insulator MnBi2Te4. We find that by breaking the combined mirror symmetries with either perpendicular electric field or external magnetic moment, Kerr and Faraday effects occur. Under perpendicular electric field, antiferromagnetic topological insulators (AFMTI) show sharp peaks at the interband transition threshold, whereas trivial insulators show small adjacent positive and negative peaks. Gate voltage and Fermi energy can be tuned to reveal the differences between AFMTI and trivial insulators. We find that AFMTI with large antiferromagnetic order can be proposed as a pure magneto-optical rotator due to sizable Kerr (Faraday) angles and vanishing ellipticity. Under external magnetic moment, AFMTI and trivial insulators are significantly different in the magnitude of Kerr and Faraday angles and ellipticity. For the qualitative behaviors, AFMTI shows distinct features of Kerr and Faraday angles when the spin configurations of the system change. These phenomena provide new possibilities to optically detect and manipulate the layered topological antiferromagnets.

Theory of optical axion electrodynamics and application to the Kerr effect in topological antiferromagnets

Emergent axion electrodynamics in magneto-electric media is expected to provide novel ways to detect and control material properties with electromagnetic fields. However, despite being studied intensively for over a decade, its theoretical understanding remains mostly confined to the static limit. Here, we introduce a theory of axion electrodynamics at general frequencies. We define a proper optical axion magneto-electric coupling through its relation to optical surface Hall conductivity and provide ways to calculate it in lattice systems. By employing our formulas, we show that axion electrodynamics can lead to a significant Kerr effect in thin-film antiferromagnets at wavelengths that are seemingly too long to resolve the spatial modulation of magnetism. We identify the wavelength scale above which the Kerr effect is suppressed. Our theory is particularly relevant to materials like MnBi2Te4, a topological antiferromagnet whose magneto-electric response is shown here to be dominated by the axion contribution even at optical frequencies.

Effect of residual strain on magnetic properties and Hall effect in chiral antiferromagnet Mn3Sn

Here, the effect of residual strain (RS) generated by uniaxial stress on the magnetic properties and Hall effect of polycrystalline Mn3Sn is investigated. Contrary to the role of pressure in Mn3Sn, both Hall measurements and our theoretically calculated kagome lattice distortions suggest that RS is beneficial for suppressing the magnetic transition from an inverse triangular antiferromagnetic (AFM) state to a helical AFM state. Furthermore, the topological Hall effect (THE) is observed in Mn3Sn over the entire temperature range from 5 K to 400 K due to RS. Combined with the magnetic measurements, we speculate that the THE originates from a RS-induced non-coplanar AFM structure. Our findings point out a method to realize a chiral non-coplanar AFM structure through strain engineering, thereby providing a path for the construction of topological antiferromagnets.

Giant field-like torque by the out-of-plane magnetic spin Hall effect in a topological antiferromagnet

Spin-orbit torques (SOT) enable efficient electrical control of the magnetic state of ferromagnets, ferrimagnets and antiferromagnets. However, the conventional SOT has severe limitation that only in-plane spins accumulate near the surface, whether interpreted as a spin Hall effect (SHE) or as an Edelstein effect. Such a SOT is not suitable for controlling perpendicular magnetization, which would be more beneficial for realizing low-power-consumption memory devices. Here we report the observation of a giant magnetic-field-like SOT in a topological antiferromagnet Mn3Sn, whose direction and size can be tuned by changing the order parameter direction of the antiferromagnet. To understand the magnetic SHE (MSHE)- and the conventional SHE-induced SOTs on an equal footing, we formulate them as interface spin-electric-field responses and analyzed using a macroscopic symmetry analysis and a complementary microscopic quantum kinetic theory. In this framework, the large out-of-plane spin accumulation due to the MSHE has an inter-band origin and is likely to be caused by the large momentum-dependent spin splitting in Mn3Sn. Our work demonstrates the unique potential of antiferromagnetic Weyl semimetals in overcoming the limitations of conventional SOTs and in realizing low-power spintronics devices with new functionalities.

Observation of large intrinsic anomalous Hall conductivity in polycrystalline Mn3Sn films

Topological antiferromagnets have a significant role to play in the modern day electronics and spintronics due to the presence of peculiar symmetries and magnetically driven large anomalous transport behaviour. We report the observation of anomalous Hall effect in Mn3Sn polycrystalline thin films deposited on Al2O3 substrate with a large anomalous Hall conductivity of 65 (Ωcm)−1 at 3 K. The Hall and magnetic measurements show a very small hysteresis owing to a weak ferromagnetic moment present in this material. The longitudinal resistivity decreases sufficiently for the thin films as compared to the polycrystalline bulk sample used as the target for the film deposition. The anomalous Hall resistivity and conductivity decreases almost linearly with the increase in the temperature. A negative magnetoresistance is observed for all the measured temperatures with the negative decrease in the magnitude with the increase in temperature.

Imaging the spin chirality of ferrimagnetic Néel skyrmions stabilized on topological antiferromagnetic Mn3Sn

Fabricating new skyrmion-hosting materials and characterizing their topological spin textures are crucial for next-generation spintronic devices. Here, the authors successfully stabilize ferrimagnetic Neel-type skyrmions in heavy-metal-free multilayer CoTb films through interfacing with noncollinear antiferromagnet Mn${}_{3}$Sn layers, in which the spin chirality can be controlled by the opposite stacking order. Further, a new Lorentz scanning transmission electron microscopy technique is employed to resolve both the internal spin structure of the skyrmions and their chirality. Their results show how noncollinear topological antiferromagnets can be incorporated to design chiral nanomagnets, and set a possible route for bridging antiferromagnet spintronics with skyrmionics.

Layer Hall effect in a 2D topological axion antiferromagnet.

Whereas ferromagnets have been known and used for millennia, antiferromagnets were only discovered in the 1930s1. At large scale, because of the absence of global magnetization, antiferromagnets may seem to behave like any non-magnetic material. At the microscopic level, however, the opposite alignment of spins forms a rich internal structure. In topological antiferromagnets, this internal structure leads to the possibility that the property known as the Berry phase can acquire distinct spatial textures2,3. Here we study this possibility in an antiferromagnetic axion insulator-even-layered, two-dimensional MnBi2Te4-in which spatial degrees of freedom correspond to different layers. We observe a type of Hall effect-the layer Hall effect-in which electrons from the top and bottom layers spontaneously deflect in opposite directions. Specifically, under zero electric field, even-layered MnBi2Te4 shows no anomalous Hall effect. However, applying an electric field leads to the emergence of a large, layer-polarized anomalous Hall effect of about 0.5e2/h (where e is the electron charge and h is Planck's constant). This layer Hall effect uncovers an unusual layer-locked Berry curvature, which serves to characterize the axion insulator state. Moreover, we find that the layer-locked Berry curvature can be manipulated by the axion field formed from the dot product of the electric and magnetic field vectors. Our results offer new pathways to detect and manipulate the internal spatial structure of fully compensated topological antiferromagnets4-9. The layer-locked Berry curvature represents a first step towards spatial engineering of the Berry phase through effects such as layer-specific moiré potential.

Metamagnetism of few-layer topological antiferromagnets

MnBi$_2$Te$_4$ (MBT) is a promising antiferromagnetic topological insulator whose films provide access to novel and technologically important topological phases, including quantum anomalous Hall states and axion insulators. MBT device behavior is expected to be sensitive to the various collinear and non-collinear magnetic phases that are accessible in applied magnetic fields. Here, we use classical Monte Carlo simulations and electronic structure models to calculate the ground state magnetic phase diagram as well as topological and optical properties for few layer films with thicknesses up to six septuple layers. Using magnetic interaction parameters appropriate for MBT, we find that it is possible to prepare a variety of different magnetic stacking sequences, some of which have sufficient symmetry to disallow non-reciprocal optical response and Hall transport coefficients. Other stacking arrangements do yield large Faraday and Kerr signals, even when the ground state Chern number vanishes.

Giant Effective Damping of Octupole Oscillation in an Antiferromagnetic Weyl Semimetal

A magnetic Weyl semimetal is a recent focus of extensive research as it may exhibit large and robust transport phenomena associated with topologically protected Weyl points in momentum space. Since a magnetic texture provides a handle for the configuration of the Weyl points and its transport response, understanding of magnetic dynamics forms the basis for future control of a topological magnet. Mn3Sn is an example of an antiferromagnetic Weyl semimetal that exhibits a large response comparable to the one observed in ferromagnets despite a vanishingly small magnetization. The noncollinear spin order in Mn3Sn can be viewed as a ferroic order of cluster magnetic octupole and breaks the time‐reversal symmetry, stabilizing Weyl points and the significantly enhanced Berry curvature near the Fermi energy. Herein, the first observation of time‐resolved octupole oscillation in Mn3Sn is reported. In particular, the giant effective damping of the octupole dynamics is found, and it is feasible to conduct an ultrafast switching at <10 ps, a hundred times faster than the case of spin‐magnetization in a ferromagnet. Moreover, high domain wall velocity over 10 km s−1 is theoretically predicted. This work paves the path toward realizing ultrafast electronic devices using the topological antiferromagnet.

Omnidirectional Control of Large Electrical Output in a Topological Antiferromagnet

AbstractControl of magnetization direction is essential for the wide application of ferromagnets; it defines the signal size of memory and sensor. However, the magnetization itself causes a dilemma. While its size matters to obtain strong responses upon its reversal, the large magnetization concomitantly suppresses the range of its directional control because of the demagnetizing field. On the other hand, realization of the desired magnetic anisotropy requires careful engineering of crystalline and interfacial effects to overcome the demagnetization barrier. Thus, it would be ideal if one could find alternative magnets that carry no magnetization but strong responses. The discovery of a topological metallic state in the antiferromagnet Mn3Sn is significant; they host a large Berry curvature in momentum space, enabling the observation of disproportionately large transverse responses such as anomalous Hall and Nernst effects, the key functionalities for replacing ferromagnets in the magnetic devices. Here, the experimental realization of omnidirectional control of the large responses in an antiferromagnet is reported. In particular, it is demonstrated that the absence of shape anisotropy enables the omnidirectional control, and lifts the shape constraint in designing the magnetic devices. This work lays the technological foundation for developing simple‐structured high‐performance devices including multi‐level memory and heat flux sensor.