Eel Vector Research Articles

Current-induced switching of thin film α−Fe2O3 devices imaged using a scanning single-spin microscope

Electrical switching of N\'eel order in an antiferromagnetic insulator is desirable as a basis for memory applications. Unlike electrically driven switching of ferromagnetic order via spin-orbit torques, electrical switching of antiferromagnetic order remains poorly understood. Here we investigate the low-field magnetic properties of 30-nm-thick, $c$-axis-oriented $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ Hall devices using a diamond nitrogen-vacancy center scanning microscope. Using the canted moment of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ as a magnetic handle on its N\'eel vector, we apply a saturating in-plane magnetic field to create a known initial state before letting the state relax in low field for magnetic imaging. We repeat this procedure for different in-plane orientations of the initialization field. We find that the magnetic field images are characterized by stronger magnetic textures for fields along $[\overline{1}\overline{1}20]$ and $[11\overline{2}0]$, suggesting that despite the expected 3-fold magnetocrystalline anisotropy, our $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ thin films have an overall in-plane uniaxial anisotropy. We also study current-induced switching of the magnetic order in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$. We find that the fraction of the device that switches depends on the current pulse duration, amplitude, and direction relative to the initialization field.

Third-order charge transport in a magnetic topological semimetal

Magnetic topological materials and their physical signatures are a focus of current research. By first-principles calculations and symmetry analysis, we reveal exotic topological semimetal states in an existing antiferromagnet $\mathrm{Th}\mathrm{Mn}{}_{2}{\mathrm{Si}}_{2}$. Depending on the N\'eel vector orientation, the topological band crossings near the Fermi level form either a double-nodal loop or two pairs of Dirac points, which are all fourfold degenerate and robust under spin-orbit coupling. These topological features produce large Berry connection polarizability, leading to pronounced nonlinear transport effects. Particularly, we evaluate the third-order current response, which dominates the transverse current in $\mathrm{Th}\mathrm{Mn}{}_{2}{\mathrm{Si}}_{2}$. We show that the third-order response can be much more sensitive to topological phase transitions than a linear response, which offers a powerful tool for characterizing topological states of matter.

Field-free magnetization switching in CoPt induced by noncollinear antiferromagnetic Mn3Ga

The magnetic spin Hall effect (MSHE) may produce the out-of-plane spin current with $z$-direction polarization $({\mathbit{\ensuremath{\sigma}}}_{\mathbf{z}})$, thus allowing the field-free switching of perpendicular magnetization in ferromagnetic films. Here, we fabricate the epitaxial $L{1}_{1}\text{\ensuremath{-}}\mathrm{CoPt}/D{0}_{19}\text{\ensuremath{-}}\mathrm{M}{\mathrm{n}}_{3}\mathrm{Ga}$ bilayers on MgO(111) substrates and characterize their magnetic and electrical transport properties. In the $L{1}_{1}\text{\ensuremath{-}}\mathrm{CoPt}/D{0}_{19}\text{\ensuremath{-}}\mathrm{M}{\mathrm{n}}_{3}\mathrm{Ga}$ bilayers, we observe the spin-orbit torque switching of perpendicular magnetization under zero external field. The magnetization switching ratio and polarity are specifically related to the N\'eel vector direction of the antiferromagnetic ${\mathrm{Mn}}_{3}\mathrm{Ga}$ as well as the crystalline orientations which the pulse current is applied along. The field-free switching in the bilayers is attributed to the ${\mathbit{\ensuremath{\sigma}}}_{\mathbf{z}}$ spin current generated by the MSHE, which is driven by the reduced symmetry of ${\mathrm{Mn}}_{3}\mathrm{Ga}$ film with noncollinear spin structure.

Electrical control of antiferromagnetic domain walls in Weyl semimetals

The electric-field-induced angular force on the N\'eel vector in antiferromagnetic (AFM) Weyl semimetals (WSMs) is theoretically investigated. Unlike in the ferromagnetic (FM) counterparts, the magnetic textures in the AFM WSMs, such as the domain walls (DWs) appear to lack the torsion in the magnetization and, thus, unable to benefit from this highly efficient mechanism that originates from the axial magnetic effect. Contrarily, our calculations illustrate that the addition of the Dzyaloshinskii-Morya interaction can introduce a twist in the magnetization around the DW location, giving rise to a nonzero axial magnetic field. This axial magnetic field, when combined with an external electric field, can lead to an imbalance in the fermion density of Weyl cones of opposite chirality and, thus, a spatially localized net electron spin polarization. The resulting effective exchange field can exert an angular force on the AFM textures for spatial movement, which can be significant in certain AFM WSMs even under a moderate external electric field. The dynamics of the DW motion under this emergent angular force is analyzed by considering the balance of energy absorption and dissipation. Our investigation reveals the need to account for the contribution of the exchange dissipation mechanism beyond the typical Gilbert-like (relativistic) term to compensate the unusual superlinear rate of energy absorption by the AFM textures. The obtained DW velocity vs electric-field characteristics show a significant speedup for the N\'eel DWs in the AFM WSMs over the counterparts in the FM WSMs as well as those in the nontopological magnets. The analysis also elucidates the dependence of the DW motion on the DW chirality in these materials. Our results clearly indicate the significance of the energy-efficient axial magnetic effect in the dynamics of spin textures in AFM WSMs with broken inversion symmetry.

Engineering axion insulator and other topological phases in superlattices without inversion symmetry

We study theoretically the interplay between magnetism and topology in three-dimensional HgTe/MnTe superlattices stacked along the (001) axis. Our results show the evolution of the magnetic topological phases with respect to the magnetic configurations. An axion insulator phase is observed for the antiferromagnetic order with the out-of-plane N\'eel vector direction below a critical thickness of MnTe, which is the ground state amongst all magnetic configurations. Defining $T$ as the time-reversal symmetry, this axion insulator phase is protected by a magnetic two-fold rotational symmetry $C_2{\cdot}T$. The axion insulator phase evolves into a trivial insulator as we increase the thickness of the magnetic layers. By switching the N\'eel vector direction into the $ab$ plane, the system realizes different antiferromagnetic topological insulators depending on the thickness of MnTe. These phases feature gapless surface Dirac cones shifted away from high-symmetry points on surfaces perpendicular to the N\'eel vector direction of the magnetic layers. In the presence of ferromagnetism, the system realizes a magnetic Weyl semimetal and a ferromagnetic semimetal for out-of-plane and in-plane magnetization directions, respectively. We observe large anomalous Hall conductivity in the presence of ferromagnetism in the three-dimensional superlattice.

Antiferromagnetic Cavity Magnon Polaritons in Collinear and Canted Phases of Hematite

Cavity spintronics explores light-matter interactions at the interface between spintronic and quantum phenomena. Until now, studies have focused on the hybridization between magnons in ferromagnets and cavity photons. Here, we realize antiferromagnetic cavity magnon polaritons. Hybridization arises from the interaction of the collective spin motion in single hematite crystals (\ensuremath{\alpha}-${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$) and the microwave field of integrated cavities operating between 18 and 45 GHz. We show theoretically and experimentally that the photon-magnon coupling in the collinear phase is mediated by the dynamic N\'eel vector and the weak magnetic moment in the canted phase by measuring across the Morin transition. We show that the coupling strength, $\stackrel{~}{g}$, scales with the anisotropy field in the collinear phase and with the Dzyaloshinskii-Moriya field in the canted phase. We reach the strong-coupling regime in both canted (cooperativity C > 70 for selected modes at 300 K) and noncollinear phases (C > 4 at 150 K), and thus, towards coherent information-exchange-harnessing antiferromagnetic cavity magnon polaritons. These results provide evidence for a generic strategy to achieve cavity magnon polaritons in antiferromagnets for different symmetries, opening the field of cavity spintronics to antiferromagnetic materials.

Giant magnetoanisotropy in the Josephson effect and switching of staggered order in antiferromagnets

We predict that the amplitude of the Josephson current through an antiferromagnetic weak link changes by several orders of magnitude upon rotation of the N\'eel order parameter characterizing the staggered magnetic order. This occurs due to the presence of spin-orbit coupling arising from structural inversion asymmetry which makes the band gap in the antiferromagnet highly sensitive to the staggered order parameter direction. We also demonstrate that when phase-biasing the junction, magnetization dynamics is induced which switches the direction of the N\'eel vector in the antiferromagnet. These results reveal an interesting versatility of antiferromagnetic Josephson junctions as they offer both a large tunability of the supercurrent magnitude via the staggered magnetization and phase-coherent control over the N\'eel order parameter.

Topological antiferromagnetic semimetal for spintronics: A case study of a layered square-net system EuZnSb2

We use the first-principles and effective Hamiltonian methods to study the electronic structure and magnetic properties of a recently synthesized layered antiferromagnetic square-net topological semimetal ${\mathrm{EuZnSb}}_{2}$ [Phys. Rev. Res. 2, 033462 (2020)]. The main message of the paper is that effects of small changes in the band structure produced by the magnetic ordering and changes in the orientation of the N\'eel vector are amplified in such transport properties as the spin Hall conductivity. We predict that the effects of the broken symmetry introduced by the ordering of the N\'eel vector, being very weak in the bulk, are pronounced in the surface electronic dispersion, suggesting that surface probes may be more suited to measure them. The coexistence of the magnetism with many other competing phases make this material interesting and possibly useful for quantum spintronics applications.

Current switching of the antiferromagnetic Néel vector in Pd/CoO/MgO(001)

Recently, the electrical switching of antiferromagnetic (AFM) order has been intensively investigated because of its application potential in data storage technology. Herein, we report the current switching of the AFM N\'eel vector in epitaxial Pd/CoO films as a function of temperature. Using combined measurements of Hall resistance (HR) and x-ray magnetic linear dichroism (XMLD) below and above the AFM N\'eel temperature, we unambiguously identified both magnetic and nonmagnetic contributions to the current-induced HR change. Through magnetic field-induced HR measurements, we quantitatively determined the percentage of current-induced CoO spin switching. Further, we showed that the thermal effect dominated the CoO magnetic switching more in samples with a thinner Pd layer and that samples with a thicker CoO layer required higher thermal activation for current-induced magnetic switching. These results provide a clear and comprehensive picture of current-induced AFM spin switching across the AFM N\'eel temperature.

Field-induced spin reorientation transitions in antiferromagnetic ring-shaped spin chains

Easy axis antiferromagnets are robust against external magnetic fields of moderate strength. Spin reorientations in strong fields can provide an insight into more subtle properties of antiferromagnetic materials, which are often hidden by their high ground state symmetry. Here, we investigate theoretically effects of curvature in ring-shaped antiferromagnetic achiral anisotropic spin chains in strong magnetic fields. We identify the geometry-governed helimagnetic phase transition above the spin-flop field between vortex and onion states. The curvature-induced Dzyaloshinskii-Moriya interaction results in the spin-flop transition being of first- or second-order depending on the ring curvature. Spatial inhomogeneity of the N\'eel vector in the spin-flop phase generates weak ferromagnetic response in the plane perpendicular to the applied magnetic field. Our work contributes to the understanding of the physics of curvilinear antiferromagnets in magnetic fields and guides prospective experimental studies of geometrical effects relying on spin-chain-based nanomagnets.

Spin-neutral tunneling anomalous Hall effect

Anomalous Hall effect (AHE) is a fundamental spin-dependent transport property that is widely used in spintronics. It is generally expected that currents carrying net spin polarization are required to drive the AHE. Here we demonstrate that, in contrast to this common expectation, a spin-neutral tunneling AHE (TAHE), i.e. a TAHE driven by spin-neutral currents, can be realized in an antiferromagnetic (AFM) tunnel junction where an AFM electrode with a non-spin-degenerate Fermi surface and a normal metal electrode are separated by a non-magnetic barrier with strong spin-orbit coupling (SOC). The symmetry mismatch between the AFM electrode and the SOC barrier results in an asymmetric spin-dependent momentum filtering of the spin-neutral longitudinal current generating the transverse Hall current in each electrode. We predict a sizable spin-neutral TAHE in an AFM tunnel junction with a RuO$_{2}$-type AFM electrode and a SnTe-type SOC barrier and show that the Hall currents are reversible by the N\'eel vector switching. With the Hall angle being comparable to that in conventional AHE bulk materials, the predicted spin-neutral TAHE can be used for the N\'eel vector detection in antiferromagnetic spintronics.

High-resolution diffraction reveals magnetoelastic coupling and coherent phase separation in tetragonal CuMnAs

Tetragonal CuMnAs was the first antiferromagnet where reorientation of the N\'eel vector was reported to occur by an inverse spin galvanic effect. A complicating factor in the formation of phase-pure tetragonal CuMnAs is the formation of an orthorhombic phase with nearly the same stoichiometry. Pure-phase tetragonal CuMnAs has been reported to require an excess of Cu to maintain a single phase in traditional solid state synthesis reactions. Here we show that subtle differences in diffraction patterns signal pervasive inhomogeneity and phase separation, even in Cu-rich ${\mathrm{Cu}}_{1.18}{\mathrm{Mn}}_{0.82}\mathrm{As}$. From calorimetry and magnetometry measurements, we identify two transitions corresponding to the N\'eel temperature (${T}_{N}$) and an antiferromagnet to weak ferromagnet transition in ${\mathrm{Cu}}_{1.18}{\mathrm{Mn}}_{0.82}\mathrm{As}$ and ${\mathrm{CuMn}}_{0.964}{\mathrm{As}}_{1.036}$. These transitions have clear crystallographic signatures, directly observable in the lattice parameters upon in situ heating and cooling. The immiscibility and phase separation could arise from a spinoidal decomposition that occurs at high temperatures, and the presence of a ferromagnetic transition near room temperature warrants further investigation of its effect on the electrical switching behavior.

Room-Temperature Anomalous Inverse Spin Hall Effect in an Easy-Plane Antiferromagnetic Insulator for Néel-Vector Manipulation and Detection

Metallic antiferromagnets are demonstrated to show an unconventional spin Hall effect (SHE) and the inverse spin Hall effect (ISHE), which are very promising for N\'eel-vector detection. However, the ISHE is absent in insulating antiferromagnets, as it does not conduct charge current. Here, we report our observation of the anomalous ISHE at the interface of the easy-plane insulator antiferromagnetic nickel oxide and the heavy metal platinum. We find that, at the interface of $\mathrm{Ni}\mathrm{O}/\mathrm{Pt}$, the ISHE generates an anomalous hysteretic transverse voltage in both Hall and planar Hall measurements, depending on spin-current propagation and the polarization direction. The observation of a nonzero transverse voltage in $\mathrm{Pt}$ indicates the appearance of a spin current with longitudinal polarization reflected from $\mathrm{Ni}\mathrm{O}$, implying N\'eel-vector reorientation from the transverse to longitudinal direction. Our simulation indicates that the N\'eel vector can be reoriented by a spin current injected from the heavy metal and an applied magnetic field. The observed anomalous ISHE permits the manipulation and detection of the N\'eel order in insulating antiferromagnets.

Spin Hall Nano-Oscillator Based on an Antiferromagnetic Domain Wall

We propose here a high-frequency spin-Hall nano-oscillator based on a simple magnetic texture, such as a domain wall, located in an antiferromagnet with an easy-axis anisotropy type. We show that the spin current, polarized along the anisotropy axis, excites a conical precession of the N\'eel vector in such a domain wall, which allows obtaining a robust ac output signal---contrary to the planar precession in a uniform uniaxial antiferromagnet, where ac output is hard to achieve. The frequency of the auto-oscillations is easily tunable by the applied current up to the terahertz range, and the threshold current vanishes for a pure uniaxial antiferromagnet. By micromagnetic simulations, we demonstrate that the pinning of the domain wall is crucial for the oscillator design, which can be achieved in the nanoconstriction layout of the free layer.

Néel vector driven spin current in a van der Waals antiferromagnetic insulator ( CrCl3 )/heavy metal (Pt) bilayer

The widely distributed GHz to THz spin dynamics of antiferromagnets, combined with the recent demonstration of spin pumping, underscores the potential of dynamic antiferromagnetic devices. Here, we report antiferromagnetic spin pumping from a van der Waals antiferromagnetic insulator ${\text{CrCl}}_{3}$. Spin pumping driven by the N\'eel vector and the net magnetization has been verified, by adjusting the angle between two sublattice magnetizations from ${180}^{\ensuremath{\circ}}$ to ${0}^{\ensuremath{\circ}}$ while measuring the spin pumping voltages. An enhancement of acoustic mode spin current while decreasing the angle is observed, which proves the spin current is driven by the net magnetization. However, the optical mode spin current increases first and then decreases with the decreasing angle, which proves the optical mode spin current is driven by the N\'eel vector. These findings reveal the underlying contributions of $\mathbit{n}$ in antiferromagnets, and its distinctive role in antiferromagnetic spin pumping.

Nonlinear spin Hall effect in PT -symmetric collinear magnets

We theoretically investigate a nonlinear spin Hall effect in $\mathcal{PT}$-symmetric antiferromagnetic metals, which serve as an efficient spin current generator. We elucidate that an emergent spin-dependent Berry curvature dipole is a microscopic origin of the nonlinear spin Hall effect, which becomes nonzero with neither relativistic spin-orbit coupling, uniform magnetization, nor spin-split band structure. By analyzing a microscopic antiferromagnetic model without spin-orbit coupling for an intuitive understanding of the phenomena, we elucidate essential hopping processes and a condition to enhance the nonlinear spin Hall conductivity. We also provide a complete table to include useful correspondence among the N\'eel vector, odd-parity multipoles, nonlinear spin conductivity tensor, and candidate materials in all the $\mathcal{PT}$-symmetric black-and-white magnetic point groups.

Control of Néel Vector with Spin-Orbit Torques in an Antiferromagnetic Insulator with Tilted Easy Plane.

Injecting spin currents into antiferromagnets and realizing efficient spin-orbit-torque switching represents a challenging topic. Because of the diminishing magnetic susceptibility, current-induced antiferromagnetic dynamics remain poorly characterized, complicated by spurious effects. Here, by growing a thin film antiferromagnet, α-Fe_{2}O_{3}, along its nonbasal plane orientation, we realize a configuration where the spin-orbit torque from an injected spin current can unambiguously rotate and switch the Néel vector within the tilted easy plane, with an efficiency comparable to that of classical ferrimagnetic insulators. Our study introduces a new platform for quantitatively characterizing switching and oscillation dynamics in antiferromagnets.

Spin waves in layered antiferromagnets with honeycomb structure

We develop a description of spin waves in a $3D$ quantum $XY$ antiferromagnet (AFM) in terms of macroscopic variables, magnetization and N\'eel vector densities. We consider a layered AFM with spins located on the honeycomb lattice. In the discussed system, the spectrum of spin waves consists of four modes, all well captured by our macroscopic description. The gapless mode of the spin waves, i.e., magnons, is described by a system of equations, which has a structure general for the Goldstone mode in AFMs. We demonstrate that the parameters in the spin Hamiltonian can be evaluated by fitting the experimental data with the results obtained for the four modes using the macroscopic variable approach. The description of AFM in terms of macroscopic variables can be easily extended to the case when the lattice of the magnetic substance is deformed by an external strain or acoustic wave.

Observation of magnetization surface textures of the van der Waals antiferromagnet FePS3 by spin Hall magnetoresistance

The van der Waals materials are a new platform to study two-dimensional systems, including magnetic order. Since the number of spins is relatively small, measuring the magnetization is challenging. Here, we report spin Hall magnetoresistance (SMR) up to room temperature caused by the magnetic surface texture of exfoliated flakes of magnetic van der Waals materials. For the antiferromagnet ${\mathrm{FePS}}_{3}$ the SMR amounts to $0.1%$ for an applied magnetic field of 7 T at 5 K, which implies a substantial canting of the magnetic moments in a perpendicular antiferromagnetic order. The canting is substantial even for a magnetic field along the N\'eel vector, which illustrates the unique power of the SMR to detect magnetic surface textures in van der Waals magnets.

Detection and manipulation of the antiferromagnetic Néel vector in Cr2O3

Detection and manipulation of the N\'eel vector in antiferromagnetic materials are promising for more stable, faster, and much higher-density spintronic devices. However, the electrical manipulation of the antiferromagnetic N\'eel vector remains challenging and controversial due to the difficulty in the detection of the zero net magnetization and the unavoidable complications from thermal artifacts. In this work, by utilizing the uniaxial antiferromagnet (AFM) ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$, we demonstrate the detection and manipulation of the antiferromagnetic N\'eel vector. We reveal unambiguously the spin-dependent electrical responses of the coherent ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ N\'eel vector switching, where a symmetric Hall signal and a fourfold angular-dependent magnetoresistance are captured. We also demonstrate the in-plane arbitrary manipulation of the ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ N\'eel vector when the magnetocrystalline anisotropy energy is compensated. Our work for detecting and manipulating N\'eel vectors offers a critical guide for antiferromagnetic-based N\'eel vector switching exploration.