Antiferromagnetic Domains Research Articles

Magnetic Skyrmion Lattices in a Novel 2D‐Twisted Bilayer Magnet

AbstractMagnetic skyrmions are topologically protected spin swirling vertices, which are promising in device applications due to their particle‐like nature and excellent controlability. Magnetic skyrmions are extensively studied in a variety of materials and proposed to exist in the extreme 2D limit, i.e., in twisted bilayer CrI3 (TBCI). Unfortunately, the magnetic states of TBCIs with small twist angles are disorderly distributed ferromagnetic and antiferromagnetic (AFM) domains in recent experiments, and thus the method to get rid of disorders in TBCIs is highly desirable. Here, intralayer exchange interactions up to the third nearest neighbors without empirical parameters and very accurate interlayer exchange interactions are used to study the magnetic states of TBCIs. The functions of interlayer exchange interactions obtained using first‐principles calculations and stored in symmetry‐adapted artificial neural networks are proposed. Based on these, the subsequent Landau–Lifshitz–Gillbert equation calculations explain the disorderly distributed FM‐AFM domains in TBCIs with small twist angles and predict the orderly distributed skyrmions in TBCIs with large twist angles. This novel twisted 2D bilayer magnet can be used to design memory devices, monochromatic spin wave generators and many kinds of skyrmion lattices.

Magnetic reversal stability of spin textures in synthetic antiferromagnetic nanodots

In this work, we demonstrate the nucleation and stability of spin textures in synthetic antiferromagnetic (SAF) nanodot-shaped systems. By employing micromagnetic simulations at 300 K and zero-field, we obtain phase diagrams distinguishing antiferromagnetic (AFM) single domain, skyrmion and skyrmionium states, among other relevant spin textures, depending on the magnetic and geometric parameters of the circular disk. Furthermore, we investigate the magnetization reversal stability under out-of-plane magnetic fields, revealing that, only under specific conditions, AFM skyrmionic textures are preserved in the absence of magnetic fields. The conservation of the skyrmionic structures along the hysteresis cycle is directly linked with an energy barrier reduction for the nucleation of the spin textures. Our findings demonstrate a very promising route in stabilizing AFM chiral skyrmionic structures in nanodots, at ambient conditions, which is a step towards their use in future antiferromagnetic devices.

Observation of antiferromagnetic domains in Cr2O3 using nonreciprocal optical effects

The archetypal magnetoelectric (ME) antiferromagnet ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$, which has the diagonal components of linear ME tensor, exhibits several different types of nonreciprocal optical effects: the electric field-induced $(E\text{\ensuremath{-}}\mathrm{induced})$ Faraday effect, the electric field-induced magnetic circular dichroism ($E\text{\ensuremath{-}}\mathrm{induced}$ MCD), and the spontaneous nonreciprocal rotation of reflected light (NRR). In principle, antiferromagnetic (AFM) domains of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ are expected to be distinguished by using these effects. However, such domain observations have never been reported mainly due to the smallness of the effects studied to date. In this study, we demonstrate that all these nonreciprocal optical effects allow the observation of AFM domains in ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$. We first measured spectra of the $E\text{\ensuremath{-}}\mathrm{induced}$ Faraday effect and $E\text{\ensuremath{-}}\mathrm{induced}$ MCD, and found that relatively large effects appear in the visible light region. Then, by imaging spatial distributions of the three nonreciprocal effects via field and light-polarization modulation imaging techniques, we succeeded in visualizing AFM domains. Intriguingly, the domain patterns obtained via the bulk sensitive $E\text{\ensuremath{-}}\mathrm{induced}$ Faraday effect and MCD and those via the surface-sensitive NRR are identical to each other, revealing that the AFM domains in ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ are uniform in the thickness direction. The domain observation methods provided here will be widely applied to various ME antiferromagnets, not only insulators but also metals, with the diagonal components of linear ME tensor.

Crystal Lattice Witness vs Actor Roles in Correlated Electronic Materials

The line which separates a witness from an actor is very thin, and strongly correlated microscopic mechanisms in complex materials can be either or both at once. In some cases, such as the geometrically-frustrated quantum magnet, Shastry–Sutherland compound SrCu2(BO)3, the crystal lattice appears to stabilize field-induced magnetic superstructures. In others, such as the antiferromagnetic electrically insulating nuclear fuel UO2, or the metallic URu2Si2 the lattice is arguably a silent witness for phase transitions, the symmetry of ordered states, unusual antiferromagnetic domains flip, and standing magneto-elastic waves driven by a still unidentified mechanism. Understanding the roles assumed by different mechanisms when using external tuning parameters such as temperature, magnetic/electric fields, pressure, strain, or others is crucial to success in tuning desired functionality. Here we discuss experimental studies of the crystalline lattice in complex metallic and electrically insulating magnetic materials under extreme conditions of pulsed magnetic fields at cryogenic temperatures. We summarize findings from FBG-based dilatometry studies developed for and completed in static-fields to 45 T, non-destructive pulsed-fields to 100 T, and single-turn destructive pulsed magnetic fields to 150 T.

Direct visualization and control of antiferromagnetic domains and spin reorientation in a parent cuprate

We report magnetic optical second-harmonic generation (SHG) polarimetry and imaging on Sr$_2$Cu$_3$O$_4$Cl$_2$, which allows direct visualization of the mesoscopic antiferromagnetic (AFM) structure of a parent cuprate. Temperature- and magnetic-field-dependent SHG reveals large domains with 90$^{\circ}$ relative orientations that are stabilized by a combination of uniaxial magnetic anisotropy and the Earth's magnetic field. Below a temperature $T_R$ $\sim$ 97 K, we observe an unusual 90$^{\circ}$ spin reorientation transition, possibly driven by competing magnetic anisotropies of the two copper sublattices, which swaps the AFM domain states while preserving the domain structure. This allows deterministic switching of the AFM states by thermal or laser heating. Near $T_R$, the domain walls become exceptionally responsive to an applied magnetic field, with the Earth's field sufficient to completely expel them from the crystal. Our findings unlock opportunities to study the mesoscopic AFM behavior of parent cuprates and explore their potential for AFM technologies.

Strain-induced shape anisotropy in antiferromagnetic structures

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

Atomic-scale spin-wave polarizer based on a sharp antiferromagnetic domain wall

We theoretically study the scattering of spin waves from a sharp domain wall in an antiferromagnetic spin chain. While the continuum model for an antiferromagnetic material yields the well-known result that spin waves can pass through a wide domain wall with no reflection, here we show that, based on the discrete spin Hamiltonian, spin waves are generally reflected by a domain wall with a reflection coefficient that increases as the domain-wall width decreases. Remarkably, we find that, in the interesting case of an atomically sharp domain wall, the reflection of spin waves exhibits strong dependence on the state of circular polarization of the spin waves, leading to mainly reflection for one polarization while permitting partial transmission for the other, thus realizing an atomic-scale spin-wave polarizer. The polarization of the transmitted spin wave depends on the orientation of the spin in the sharp domain wall, which can be controlled by an external field or spin torque. Our utilization of a sharp antiferromagnetic domain wall as an atomic-scale spin-wave polarizer leads us to envision that ultra-small magnetic solitons such as domain walls and skyrmions may enable realizations of atomic-scale spin-wave scatterers with useful functionalities.

Antiferromagnet thickness dependence and rotatable spins in exchange biased CoO/Fe films

The emergence of exchange bias and coercivity enhancement has been investigated in epitaxial CoO/Fe films with varied antiferromagnet (AF) thicknesses, even smaller than the critical value where the frozen CoO spins are detectable. Vector magnetometry and first-order reversal curve (FORC) measurements reveal different CoO thickness dependence of the exchange bias and coercivity enhancement, including the evolution of magnetization reversal from a high coercivity, low bias phase due to rotatable CoO moments to a high bias, low coercivity phase due to frozen CoO moments. The AF domain state is found to be metastable, which can be reoriented by external and exchange fields prior to the appearance of frozen spins, pointing to a generic origin of the training effect. Monte Carlo simulations show that the AF anisotropy energy barrier and the rotatable spins induced by magnetic field and exchange interaction at the interface are responsible for the observed effects.

Voltage-induced terahertz magnon excitation associated with antiferromagnetic domain wall precession

Exploring a low-power method for exciting terahertz (THz) signals is of great interest in developing devices with ultrafast signal processing. We numerically investigated the THz magnon excitation from a moving antiferromagnetic (AFM) domain wall (DW) driven by a voltage-induced magnetic anisotropy energy gradient (dEa/dx). This magnon excitation originates from the DW precession induced by dEa/dx. Unlike the AFM DW precession triggered by DW acceleration as predicted in previous investigation, the dEa/dx-induced DW precession is possible when the DW acceleration is negligible. When the frequency of DW precession is greater than the frequency gap of spin-wave propagation in the AFM medium, THz magnons are resonantly excited. Because dEa/dx can be generated under a moderate DC voltage, our work provides a potential approach for developing THz spintronic devices with a low power and dissipation.

Direct observation of antiferromagnetic parity violation in the electronic structure of Mn2Au

Using momentum microscopy with sub-µm spatial resolution, allowing momentum resolved photoemission on individual antiferromagnetic domains, we observe an asymmetry in the electronic band structure, , in Mn2Au. This broken band structure parity originates from the combined time and parity symmetry, , of the antiferromagnetic order of the Mn moments, in connection with spin–orbit coupling. The spin–orbit interaction couples the broken parity to the Néel order parameter direction. We demonstrate a novel tool to image the Néel vector direction, N, by combining spatially resolved momentum microscopy with ab-initio calculations that correlate the broken parity with the vector N.

Magnetoresistance enhancement in a perpendicular (Co/Pt)4/Co/IrMn/(Co/Pt)2/Co structure

We investigate the magnetoresistance (MR) effect and magnetic properties in (Co/Pt)4/Co/IrMn (SFM) and (Co/Pt)4/Co/IrMn/(Co/Pt)2/Co (DFM) structures with (Co/Pt)4/Co and (Co/Pt)2/Co multilayers having a perpendicular magnetic anisotropy. Despite the exchange bias field, the antiferromagnetic IrMn layer itself influences the coercivity (Hc) and MR differently for both types of structures when the IrMn layer is thin. A suppressed Hc and an enhanced MR in the DFM samples are obtained compared with those in the SFM samples. The maximum MR reaches up to (0.6 ± 0.1)% when the IrMn thickness (tIrMn) of the DFM samples varies from 1.5 to ∼5 nm, but the MR value of the SFM samples remains (0.1 ± 0.05)% with the same tIrMn range. The suppressed Hc and the enhanced MR in the DFM samples may be due to the formation of an antiferromagnetic-type contact when large antiferromagnetic domains in the IrMn layer are sandwiched by (Co/Pt)4/Co and (Co/Pt)2/Co multilayers.

Imaging mesoscopic antiferromagnetic spin textures in the dilute limit from single-geometry resonant coherent x-ray diffraction.

The detection and manipulation of antiferromagnetic domains and topological antiferromagnetic textures are of central interest to solid-state physics. A fundamental step is identifying tools to probe the mesoscopic texture of an antiferromagnetic order parameter. In this work, we demonstrate that Bragg coherent diffractive imaging can be extended to study the mesoscopic texture of an antiferromagnetic order parameter using resonant magnetic x-ray scattering. We study the onset of the antiferromagnet transition in PrNiO3, focusing on a temperature regime in which the antiferromagnetic domains are dilute in the beam spot and the coherent diffraction pattern modulating the antiferromagnetic peak is greatly simplified. We demonstrate that it is possible to extract the arrangements and sizes of these domains from single diffraction patterns and show that the approach could be extended to a time-structured light source to study the motion of dilute domains or the motion of topological defects in an antiferromagnetic spin texture.

A-type antiferromagnetic order in semiconducting EuMg2Sb2 single crystals

Eu-based Zintl-phase materials EuA$_2$Pn$_2$ (A = Mg, In, Cd, Zn; Pn = Bi, Sb, As, P) have generated significant recent interest owing to the complex interplay of magnetism and band topology. Here, we investigated the electronic, magnetic, and electronic properties of the layered Zintl-phase single crystals of EuMg$_2$Sb$_2$ with the trigonal CaAl$_2$Si$_2$ crystal structure (space group $P\bar{3}m1$). Electrical resistivity measurements complemented with angle-resolved photoemission spectroscopy (ARPES) studies find an activated behavior with the intrinsic conductivity at high temperatures indicating a semiconducting electronic ground state with a narrow energy gap of 370 meV. Magnetic susceptibility and zero-field heat-capacity measurements indicate that the compound undergoes antiferromagnetic (AFM) ordering at the Neel temperature $T_{\rm N}$ = 8.0(2) K. Zero-field neutron-diffraction measurements reveal that the AFM ordering is A-type where the Eu ordered moments (Eu$^{2+}$, S= 7/2) arranged in ab-plane layers are aligned ferromagnetically in the ab plane with the Eu moments in adjacent layers aligned antiferromagnetically. We also find that Eu-moment reorientation in the trigonal AFM domains within the ab planes occurs below $T_{\rm N}$ at low fields < 0.05 T due to very small in-plane anisotropy. Although isostructural semimetallic EuMg$_2$Bi$_2$ is reported to host Dirac surface states, the observation of narrow-gap semiconducting behavior in EuMg$_2$Sb$_2$ implies a strong role of spin-orbit coupling in tuning the electronic states of these materials.

Setting of the magnetic structure of chiral kagome antiferromagnets by a seeded spin-orbit torque.

The current-induced spin-orbit torque switching of ferromagnets has had huge impact in spintronics. However, short spin-diffusion lengths limit the thickness of switchable ferromagnetic layers, thereby limiting their thermal stability. Here, we report a previously unobserved seeded spin-orbit torque (SSOT) by which current can set the magnetic states of even thick layers of the chiral kagome antiferromagnet Mn3Sn. The mechanism involves setting the orientation of the antiferromagnetic domains in a thin region at the interface with spin currents arising from an adjacent heavy metal while also heating the layer above its magnetic ordering temperature. This interface region seeds the resulting spin texture of the entire layer as it cools down and, thereby, overcomes the thickness limitation of conventional spin-orbit torques. SSOT switching in Mn3Sn can be extended beyond chiral antiferromagnets to diverse magnetic systems and provides a path toward the development of highly efficient, high-speed, and thermally stable spintronic devices.

Ferromagnetic diagonal stripe states in the two-dimensional Hubbard model with U≲∞

We have performed a variational Monte Carlo simulation to study the ground state of a two-dimensional Hubbard model on a square lattice in the strong coupling region. The energy gain of possible inhomogeneous electron states are computed as a function of U when the hole density ϵ=1/8 and next nearest-neighbor hopping t′/t=−0.30. The bond-centered ferromagnetic diagonal stripe state is stabilized in the strong coupling region (U/t≥16), which is due to the gain of both kinetic energy and on-site Coulomb interaction energy due to the holon moving over the ferromagnetic domain and the gain of kinetic-exchange-interaction energy at the antiferromagnetic domain wall.

Inertial domain wall characterization in layered multisublattice antiferromagnets

The motion of a Néel-like 180∘ domain wall induced by a time-dependent staggered spin–orbit field in the layered collinear antiferromagnet Mn2Au is explored. Through an effective version of the two sublattice nonlinear σ-model which does not take into account the antiferromagnetic exchange interaction directed along the tetragonal c-axis, it is possible to replicate accurately the relativistic and inertial traces intrinsic to the magnetic texture dynamics obtained through atomistic spin dynamics simulations for quasistatic processes. In the case in which the steady-state magnetic soliton motion is extinguished due to the abrupt shutdown of the external stimulus, its stored relativistic exchange energy is transformed into a complex translational mobility, being the rigid domain wall profile approximation no longer suitable. Although it is not feasible to carry out a detailed follow-up of its temporal evolution in this case, it is possible to predict the inertial-based distance travelled by the domain wall in relation to its steady-state relativistic mass. This exhaustive dynamical characterization for different time-dependent regimes of the driving force is of potential interest in antiferromagnetic domain wall-based device applications.

Nanoscale Antiferromagnetic Domain Imaging using Full-Field Resonant X-ray Magnetic Diffraction Microscopy.

The physical properties of magnetic materials frequently depend not only on the microscopic spin and electronic structures, but also on the structures of mesoscopic length scales that emerge, for instance, from domain formations, or chemical and/or electronic phase separations. However, experimental access to such mesoscopic structures is currently limited, especially for antiferromagnets with net zero magnetization. Here, full-field microscopy and resonant magnetic X-ray diffraction are combined to visualize antiferromagnetic (AF) domains of the spin-orbit Mott insulator Sr2 IrO4 with area over ≈0.1 mm2 and with spatial resolution as high as ≈150nm. With the unprecedented wide field of views and high spatial resolution, an intertwining of two AF domains on a length comparable to the measured average AF domain wall width of 545nm is revealed. This mesoscopic structure comprises a substantial portion of the sample surface, and thus can result in a macroscopic response unexpected from its microscopic magnetic structure. In particular, the symmetry analysis presented in this work shows that the inversion symmetry, which is preserved by the microscopic AF order, becomes ill-defined at the mesoscopic length scale. This result underscores the importance of this novel technique for a thorough understanding of the physical properties of antiferromagnets.

Identifying the octupole antiferromagnetic domain orientation in Mn3NiN by scanning anomalous Nernst effect microscopy

The intrinsic anomalous Nernst effect in a magnetic material is governed by the Berry curvature at the Fermi energy and can be realized in non-collinear antiferromagnets with vanishing magnetization. Thin films of (001)-oriented Mn3NiN have their chiral antiferromagnetic structure located in the (111) plane facilitating the anomalous Nernst effect unusually in two orthogonal in-plane directions. The sign of each component of the anomalous Nernst effect is determined by the local antiferromagnetic domain state. In this work, a temperature gradient is induced in a 50 nm thick Mn3NiN two micrometer-size Hall cross by a focused scanning laser beam, and the spatial distribution of the anomalous Nernst voltage is used to image and identify the octupole macrodomain arrangement. Although the focused laser beam width may span many individual domains, cooling from room temperature to the antiferromagnetic transition temperature in an in-plane magnetic field prepares the domain state, producing a checkerboard pattern resulting from the convolution of contributions from each domain. These images together with atomistic and micromagnetic simulations suggest an average macrodomain of the order of 1 μm2.

Real-space observation of fluctuating antiferromagnetic domains.

Magnetic domains play a fundamental role in physics of magnetism and its technological applications. Dynamics of antiferromagnetic domains is poorly understood, although antiferromagnets are expected to be extensively used in future electronic devices wherein it determines the stability and operational speed. Dynamics of antiferromagnets also features prominently in the studies of topological quantum matter. Real-space imaging of fluctuating antiferromagnetic domains is therefore highly desired but has never been demonstrated. We use coherent x-ray diffraction to obtain videos of fluctuating micrometer-scale antiferromagnetic domains in Ni2MnTeO6 on time scales from 10−1 to 103 s. In the collinear phase, thermally activated domain wall motion is observed in the vicinity of the Néel temperature. Unexpectedly, the fluctuations persist through the full range of the higher-temperature helical phase. These observations illustrate the high potential significance of the dynamic domain imaging in phase transition studies and in magnetic device research.

Terahertz bremsstrahlung and frequency comb induced by variable motion of an antiferromagnetic domain wall

We numerically investigate the emission of spin wave (SW) packets from an antiferromagnetic (AFM) domain wall (DW) moving at a varying velocity. This SW is analogous to the bremsstrahlung of a charge with variable motion. The SW packets are emitted towards both sides of DW in the DW acceleration/deceleration process. The frequency of this bremsstrahlung-type SW is at sub-terahertz (THz), which is one magnitude lower than that of SW emitted by the Lorentz contraction of DW. Based on the bremsstrahlung-type SW, we proposed a frequency comb with a sub-THz central frequency and a GHz frequency interval by exploiting the back-and-forth DW variable motion. Our work paves a way for developing a spin–orbit-torque -based AFM THz device with a tunable frequency band.