Easy Magnetization Plane Research Articles

Anisotropic magnon frequency comb based on antiferromagnetic bimerons

The interaction between propagating magnons and topological spin textures is attracting a lot of recent attention from the magnonic community. It has been shown that the three-wave mixing between magnons and breathing skyrmion can induce the magnon frequency comb (MFC) with equidistant coherent peaks. However, a magnetic bimeron is a nontrivial spin texture and is regarded as the counterpart of the skyrmion in easy-plane magnets with Dzyaloshinskii–Moriya interaction, which allows anisotropic magnon propagations. This raises the question of whether the nonlinear interaction between magnons and bimerons can generate an MFC. If so, how does the direction of magnon propagation affect the characteristics of the MFC? In this Letter, we demonstrate that the three-wave mixing between propagating magnons and locally breathing bimerons can induce a terahertz MFC in easy-plane antiferromagnets. Micromagnetic simulations reveal that the three-wave coupling strength weakly depends on the driving frequency, but it strongly relies on the propagation direction of incident magnons. Our findings uncover the anisotropic nature of MFC in bimeron structures, which may have potential applications for ultrafast magnonic devices with spectroscopy, metrology, and sensing functionalities.

Strain-induced giant enhancement and transformation of magnetocrystalline anisotropy in 1T-FeSe2 monolayer

The development of magnetic materials with large magnetic anisotropy energy (MAE) is crucial for magnetic storage and spintronics applications. The electronic structure and magnetic properties of 1T-FeSe2 monolayer have been systematically investigated by first-principles computational methods. The monolayer 1T-FeSe2 is confirmed to be a ferromagnetic semimetal with 100% spin polarization and exhibits pronounced magnetic anisotropy. It shows an easy magnetization plane in the two-dimensional plane and its MAE is as high as 8.134 meV. The MAE mainly originates from the contributions of transitions between Fe dyz and dz2, dxy and dx2−y2 orbitals, while the contributions of transitions between p x and p y, p x and p z orbitals of Se are also obvious. The biaxial strain significantly regulates the magnetic properties and electronic structures of the 1T-FeSe2 monolayer. Interestingly, the MAE increases significantly to 77.680 meV when the tensile strain increases to 3.5%, we can expect to achieve a reasonable value of MAE up to 98.951 meV by applying 5% tensile strain. In addition, the easy magnetization plane changes to perpendicular easy magnetization axis direction at −6% compressive strain or 7% tensile strain, which leads to a significant change in the band structure near the Fermi level. The 100% spin-polarized ferromagnetic semi-metallic behavior and the huge strain-induced magnetic anisotropy make the 1T-FeSe2 monolayer material promising for spintronics and magnetic data storage.

"Polymerization" of Bimerons in Quasi-Two-Dimensional Chiral Magnets with Easy-Plane Anisotropy.

We re-examine the internal structure of bimerons, which are stabilized in easy-plane chiral magnets and represent coupled states of two merons with the same topological charge |1/2| but with opposite vorticity and the polarity. We find that, in addition to the vortices and antivortices, bimerons feature circular regions which are located behind the anti-vortices and bear the rotational sense opposite to the rotational sense chosen by the Dzyaloshinskii-Moriya interaction. In an attempt to eliminate these wrong-twist regions with an excess of positive energy density, bimerons assemble into chains, and as such exhibit an attracting interaction potential. As an alternative to chains, we demonstrate the existence of ring-shaped bimeron clusters of several varieties. In some rings, bimeron dipoles are oriented along the circle and swirl clockwise and/or counterclockwise (dubbed "roundabouts"). Moreover, a central meron encircled by the outer bimerons may possess either positive or negative polarity. In other rings, the bimeron dipoles point towards the center of a ring and consequently couple to the central meron (dubbed "crossings"). We point out that the ringlike solutions for baryons obtained within the Skyrme model of pions, although driven by the same tendency of the energy reduction, yield only one type of bimeron rings. The conditions of stability applied to the described bimeron rings are additionally extended to bimeron networks when bimerons fill the whole space of two-dimensional samples and exhibit combinations of rings and chains dispersed with different spatial density (dubbed bimeron "polymers"). In particular, bimeron crystals with hexagonal and the square bimeron orderings are possible when the sides of the unit cells represent chains of bimerons joined in intersections with three or four bimerons, respectively; otherwise, bimeron networks represent disordered bimeron structures. Moreover, we scrutinize the inter-transformations between hexagonal Skyrmion lattices and disordered bimeron polymers occuring via nucleation and mutual annihilation of merons within the cell boundaries. Our theory provides clear directions for experimental studies of bimeron orderings in different condensed-matter systems with quasi-two-dimensional geometries.

Phase Diagram Mapping out the Complex Magnetic Structure of Single Crystals of (Gd, Er)B4 Solid Solutions

Measurements of specific heat and magnetization in single crystals were used to map out the magnetic phase diagram of Gd1−xErxB4 (x = 0.2 and 0.4) solid solutions along the c-axis. While GdB4 orders antiferromagnetically (AF) at 41.7 K, with the easy plane of magnetization oriented perpendicularly to the c-axis, ErB4 displays AF ordering below 15.4 K, with the easy axis along c. Therefore, in solid solutions, the competition between the different spin anisotropies, as well as frustration, lead to a complex spin configuration. These measurements reveal that a 40% substitution of Er for Gd is sufficient for generating a phase diagram similar to the one for the ErB4 system, characterized by the occurrence of plateau phases and other exotic features attributed to the interplay of competing magnetic anisotropies.

Emergence of Stable Meron Quartets in Twisted Magnets.

The investigation of twist engineering in easy-axis magnetic systems has revealed remarkable potential for generating topological spin textures. Implementing twist engineering in easy-plane magnets, we introduce a novel approach to achieving fractional topological spin textures, such as merons. Through atomistic spin simulations on twisted bilayer magnets, we demonstrate the formation of a stable double Meron pair, which we refer to as the "Meron Quartet" (MQ). Unlike a single pair, the merons within the MQ exhibit exceptional stability against pair annihilation due to the protective localization mechanism induced by the twist that prevents collision of the Meron cores. Furthermore, we showcase that the stability of the MQ can be enhanced by adjusting the twist angle, resulting in an increased resistance to external perturbations such as external magnetic fields. Our findings highlight the twisted magnet as a promising platform for achieving merons as stable magnetic quasiparticles in van der Waals magnets.

Magneto-electric coupling beyond van der Waals interaction in two-dimensional multiferroic heterostructures

Exploring two-dimensional (2D) multiferroic systems with strong magneto-electric coupling properties holds significant application value in nanoscale spintronics devices. However, due to the weak interlayer van der Waals interactions, strong magneto-electric coupling in 2D heterostructures is relatively rare. By using first-principles simulations, we demonstrate that in the NiPS3/Sc2CO2 heterostructure, the ferroelectric polarization switching of the Sc2CO2 layer induces a transition in the magnetic ground state of the NiPS3 layer from the ferromagnetic state to antiferromagnetic ordering, accompanied by a transformation from a semiconductor to a half-metallic state. This magnetic phase transition is caused by a novel magneto-electric coupling mechanism: the polarization switching changes the band alignment between the two materials and then induces a significant interlayer charge transfer, leading to the emergence of Stoner itinerant ferromagnetism. In addition, the polarization switching can also change the magnetic anisotropy from an easy magnetization plane to an easy magnetization axis. These results not only offer a promising multiferroic heterostructure for nonvolatile memory devices and magnetic sensors but also provide a feasible approach for designing multiferroic system with strong magneto-electric coupling.

Reduction of power loss in easy-plane FeSi soft magnetic composite under a weak bias magnetic field

This paper investigates the impact of a weak bias magnetic field (Hbias) along the easy magnetization plane direction on the power loss of easy-plane Fe-6.5 wt%Si (FeSi) soft magnetic composite (SMC). The results demonstrate that the power loss (Pcv) decreases by 18 % when the Hbias is 11.6 kA/m. Furthermore, it is found that the reduction in Pcv is not only attributed to decreases in hysteresis loss but also involves a decrease in excess loss. The causes of hysteresis loss and excess loss reduction are further investigated and it is found that the effect of the Hbias on the magnetization state plays an important role in the reduction of hysteresis loss. Additionally, the decrease in excess loss can be attributed to the reduction in local eddy current losses caused by non-uniform domain wall motion. These findings provide valuable insights for achieving high-frequency (MHz), high-efficiency, and miniaturized power electronic devices.

Exploitable magnetic anisotropy and half-metallicity controls in multiferroic van der Waals heterostructure

Two-dimensional (2D) XY ferromagnets have drawn pronounced interest in recent years, but the characteristic of easy-plane magnetization restricts their application in spintronics to some extent. Here, we propose a general strategy for constructing multiferroic van der Waals heterostructures, aiming to achieve electrical control over the magnetic anisotropy in 2D XY ferromagnets. The validity of this strategy is verified by the heterostructure composed of ferromagnetic VBi2Te4 and ferroelectric In2Se3 monolayers. By manipulating the polarized states of In2Se3, the VBi2Te4 can be reversibly transformed between 2D XY and Heisenberg ferromagnets, characterized by the switching of easy magnetization axis between in-plane and out-of-plane directions. More interestingly, accompanied by the changes in magnetic anisotropy, the VBi2Te4 also demonstrates a phase transition from a semiconductor to a half-metal state, which can be ascribed to the band alignment and interfacial charge transfer. The switchable magnetic and electronic properties enable the heterostructure to be utilized in nonvolatile memory and logic devices. Additionally, the half-metallicity and magnetocrystalline anisotropy energy of the heterostructure can be effectively tuned by biaxial strain. These findings not only pave the way for electrically nonvolatile control of 2D XY ferromagnet, but also facilitate the development of interfacial magnetoelectric physics and applications.

Quantum spin helices more stable than the ground state: Onset of helical protection

Topological magnetic structures are promising candidates for resilient information storage. An elementary example is spin helices in one-dimensional easy-plane quantum magnets. To quantify their stability, we numerically implement the stochastic Schr\"odinger equation and time-dependent perturbation theory for spin chains with fluctuating local magnetic fields. We find two classes of quantum spin helices that can reach and even exceed ground-state stability: spin-current-maximizing helices and, for fine-tuned boundary conditions, the recently discovered ``phantom helices.'' Beyond that, we show that the helicity itself (left or right rotating) is even more stable. We explain these findings by separated helical sectors and connect them to topological sectors in continuous spin systems. The resulting helical protection mechanism is a promising phenomenon for stabilizing helical quantum structures, e.g., in ultracold atoms and solid-state systems. We also identify a third type of phantom helix in the system.

Dielectric tunability of magnetic properties in orthorhombic ferromagnetic monolayer CrSBr

Monolayer CrSBr is a recently discovered semiconducting spin-3/2 ferromagnet with a Curie temperature of around 146 K. In contrast to many other known 2D magnets, the orthorhombic lattice of CrSBr gives rise to spatial anisotropy of magnetic excitations within the 2D plane. Triaxial magnetic anisotropy and considerable magnetic dipolar interactions in CrSBr challenge its theoretical description in terms of spin Hamiltonians. Here, we employ a Green’s function formalism combined with first-principles calculations to study the magnetic properties of monolayer CrSBr in different regimes of surrounding dielectric screening. In the free-standing limit, the system is close to an easy-plane magnet, whose long-range ordering is partially suppressed. On the contrary, in the regime of large external screening, monolayer CrSBr behaves like an easy-axis ferromagnet with more stable magnetic ordering. Our findings suggest that anisotropic layered magnets form a potentially promising platform for studying the effects of substrate screening on magnetic ordering in 2D.

Investigation of the intrinsic magnetic properties of GdCo4B single crystal: determination of the magnetocrystalline anisotropy from the first-order magnetization processes

We report on the intrinsic magnetic properties of a GdCo4B single crystal as derived from magnetization measurements. The occurrence of a first-order magnetization process (FOMP) in a magnetic field applied perpendicularly to the easy magnetization direction provides an opportunity to determine the anisotropy parameters K1 and K2. To this end, the theoretical approach proposed previously for easy-plane magnets has been adapted for the case of easy-axis compounds exhibiting a FOMP. The obtained anisotropy parameters of GdCo4B are compared with the values deduced by other classical techniques. The presence of a compensation point in the thermal dependence of the spontaneous magnetization has enabled the determination of the exchange field on Gd, Bex = 131 T, which is in good agreement with inelastic neutron scattering results published earlier. Influence of the applied pressure on the anisotropy parameters is quantified using the pressure dependence of the FOMP, revealing a significant sensitivity of the anisotropy parameters to pressure.

Superfluidlike spin transport in the dynamic states of easy-axis magnets

The existing proposals for superfluidlike spin transport have been based on easy-plane magnets where the U(1) spin-rotational symmetry is spontaneously broken in equilibrium, and this has been limiting material choices for realizing superfluidlike spin transport to restricted classes of magnets. In this work, we lift this limitation by showing that superfluidlike spin transport can also be realized based on easy-axis magnets, where the U(1) spin-rotational symmetry is intact in equilibrium but can be broken in nonequilibrium. Specifically, we find the condition to engender a nonequilibrium easy-cone state by applying a spin torque to easy-axis magnets, which dynamically induces the spontaneous breaking of the U(1) spin-rotational symmetry and thereby can support superfluidlike spin transport. By exploiting this dynamic easy-cone state, we show theoretically that superfluidlike spin transport can be achieved in easy-axis magnets under suitable conditions and confirm the prediction by micromagnetic simulations. We envision that our work broadens the material library for realizing superfluidlike spin transport, showing the potential utility of dynamic states of magnets as venues to look for spin-transport phenomena that do not occur in static magnetic backgrounds.

Magnetic and absorption properties in hundreds of megahertz band for a FeSi@SiO2 composite with brick stacking structure and crystallographic orientation

Electromagnetic (EM) pollution in the 0.1–1 GHz band is increasing with the rapid advancement in 5G communication technology. However, there are barely any absorption materials in the said band. Herein, we report a high-performance EM wave absorption composite with brick stacking structure and crystallographic orientation prepared using flake-like FeSi@SiO2 particles. The permeability of this composite with such novel structure is increased by more than ten times compared with conventional composites, reaching 42 at 1 MHz. The reflection loss (RL) can reach −40 dB around 0.3 GHz and the effective absorption bandwidth (RL 〈−10) can cover 0.2–1 GHz, which is significantly better than previous reports. Such a high permeability is ascribed to easy magnetization plane and low demagnetization factor parallel to the flake-like particles plane. The excellent absorption is attributed to the intrinsic loss introduced by the enhanced domain wall resonance, and the interference loss generated by quarter-wavelength cancellation. Particularly, the composite proposed in this study has great business and application value in the field of EM wave absorption, and the preparation method can be extended to other EM wave absorption composites, which provides a considerable guidance for the preparation of magnetic metallic absorption composites.

Spin Seebeck effect in W-type and Z-type hexagonal ferrite thin films

The spin Seebeck effect was studied in thin films of hexagonal ferrites of W-type SrCo2−xZnxFe16O27 with x = 0, 1, and 2 and Z-type with the composition Sr3Co2Fe24O41. The present data were compared with the previously obtained data on Y-type Ba2Zn2−xCoxFe12O22 with x = 0 and 2. Our work showed that the SSE signal can also be generated in materials with complicated crystal and magnetic structures. Hexaferrites with the magnetic easy plane were selected except for uniaxial W-type with x = 2, for which the highest saturated SSE 0.144 μV/K was observed; however, magnetic field above 1 T must be applied to achieve saturation. For low field application, hexaferrites with easy magnetization in ab-plane are more suitable, namely, Y-type with x = 2 or W-type with x = 1. SSE is suppressed in Co-substituted hexaferrites due to the random distribution of Co among various sites. Therefore, the Co cations interfere with the long-range magnetic ordering and diminish the spin-wave propagation. Additional suppression of SSE might be invoked by the higher magnetic anisotropy of the Co-substituted hexaferrites, which opens a gap in the magnon spectrum and, thus, reduces the contribution of lower energy magnons with longer diffusion length.

In‐Plane Quantum Anomalous Hall Effect in Ferromagnetic Type‐I Nodal Line Semimetal of Monolayer YbN

The monolayer YbN is predicted to be a 2D nodal line semimetal with full spin polarization in the absence of spin–orbit coupling as well as an intrinsic ferromagnet with an easy magnetic xy plane and T c = 85 K. Considering the spin–orbit coupling with magnetic moments along z direction, the band crossings near Fermi level around Γ points are still preserved and robust against spin–orbit coupling. On the contrary, when the spin–orbit coupling is considered with magnetic moments oriented in‐plane along x and y directions, the band crossings are broken, and the bandgap of the magnetic moment along the y direction is larger than that in the x direction, which results in topologically nontrivial insulating property. The anomalous Hall conductivity calculations show the Chern number C = −1, implying the existence of one edge state in the finite‐width nanoribbon. The findings provide ideal candidates for investigating the design of electronic devices related to 2D nontrivial magnetic materials.

Easy-plane anisotropic-exchange magnets on a honeycomb lattice: Quantum effects and dealing with them

We provide analytical and numerical insights into the phase diagram and other properties of the extended Kitaev-Heisenberg model on the honeycomb lattice in the {\it easy-plane} limit, in which interactions are only between spin components that belong to the plane of magnetic ions. This parameter subspace allows for a much-needed systematic {\it quantitative} investigation of spin excitations in the ordered phases and of their generic features. Specifically, we demonstrate that in this limit one can consistently take into account magnon interactions in both zero-field zigzag and field-polarized phases. For the nominally polarized phase, we propose a regularization of the unphysical divergences that occur at the critical field and are plaguing the $1/S$-approximation in this class of models. For the explored parameter subspace, all symmetry-allowed terms of the standard parametrization of the extended Kitaev-Heisenberg model, such as $K$, $J$, and $\Gamma$, are significant, making the offered consideration relevant to a much wider parameter space. The dynamical structure factor near paramagnetic critical point illustrates this relevance by showing features that are reminiscent of the ones observed in $\alpha$-RuCl$_3$, underscoring that they are not unique and should be common to a wide range of parameters of the model and, by an extension, to other materials.

Spin Hall effect of vorticity

Using mapping between topological defects in an easy-plane magnet and electrical charges, we study interplay between vorticity and spin currents. We demonstrate that the flow of vorticity is accompanied by the transverse spin current generation -- an effect which can be termed as the spin Hall effect of vorticity. We study this effect across the BKT transition and establish the role of dissipation and spin non-conservation in the crossover from spin superfluidity to diffusive spin transport. Our results pave the way for low power computing devices relying on vorticity and spin flows.

Enhancement of magnetocaloric properties by control of magnetocrystalline anisotropy in Ge doped MnFeHfPSi alloy

This study investigated the improvement in magnetocaloric properties with magnetocrystalline anisotropy by Ge doping in MnFeHfPSi alloys. The maximum magnetic entropy change gets larger in alloys with higher magnetocrystalline anisotropy. This result is induced by the fractions of easy and hard magnetization planes. In addition, the magnetocaloric properties indicate large differences based on the direction between the easy magnetization plane and the applied magnetic field. Especially, the difference in magnetic entropy change under a low magnetic field is higher than that under a strong magnetic field. Besides, when magnetic transition occurs from a ferromagnetic to a paramagnetic state, the fraction of the easy magnetization plane decreases. Therefore, orientations with an easy magnetization plane play an important role in the enhancement of magnetocaloric properties.

Potential high-performance magnet materials: Co- and Al-alloyedSm2Fe17

${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}$ has long been known as a potential high-performance magnet whose deficiencies---planar anisotropy and lower-than-optimal ${T}_{c}$---can be remedied by nitrogen addition, but which presents synthesis difficulties. Herein we apply first-principles calculations to search for alternative low-cost, high-performance permanent magnets in this family, by exploring simultaneous Fe and Al substitution. Specifically, the goal is to improve properties of ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{14}{\mathrm{Al}}_{3}$ easy-plane magnet at the stoichiometric composition. Density functional theory calculations were executed for three series of compounds, i.e., ${\mathrm{Sm}}_{2}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{14}{\mathrm{Al}}_{3}$, ${\mathrm{Sm}}_{2}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{15}{\mathrm{Al}}_{2}$, and ${\mathrm{Sm}}_{2}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{16}\mathrm{Al}$. We find that substitution of Fe with 12--18 of Co in $%\phantom{\rule{4pt}{0ex}}{\mathrm{Sm}}_{2}{\mathrm{Fe}}_{14}{\mathrm{Al}}_{3}$ modifies the magnetic anisotropy type from easy plane to easy axis with a substantial anisotropy of 7.1 MJ/${\mathrm{m}}^{3}$. We also demonstrate that the largest part of magnetic anisotropy is introduced by $4f$ Sm atom electrons. Thus the rotation of magnetic moment orientation from $\ensuremath{\langle}1\overline{1}0\ensuremath{\rangle}$ to $\ensuremath{\langle}111\ensuremath{\rangle}$ is followed by an increase of the occupied $4f$ state number and, as a result, the orbital part of the magnetic moment of one of the Sm atoms. This increase of the occupied $4f$ state number at an energy $\ensuremath{\sim}\ensuremath{-}$4.3 eV results in a significant reduction of band structure energy. The substitution of Fe by Co does not significantly reduce the magnetization of the compound and keeps it slightly above 1 T. This combination of magnetic anisotropy and magnetization makes the compound a promising candidate for a permanent magnet.

Atomic-scale structure clarification of the planar Z phase and its influence on the magnetic properties in Sm(CoFeCuZr)z permanent magnets

2:17-type SmCo-based permanent magnets with special cellular structure composed of 2:17R, 1:5H and planar Z phase have exhibited great potential for high temperature application. However, there are still disputes on the atomic structure of the planar phase and its influence on magnetic properties. Here using advanced aberration corrected high-angle annular dark field scanning transmission electron microscopy we found that most of the planar Z phases possess SmZr4Co13–H (P-6m2) or SmZr2Co9–2H (P-6m2) structure instead of SmZr2Co9–3R (R-3m) structure. This experimental observation is corroborated well with our calculation of the formation enthalpy as well as Gibbs energy that SmZr4Co13–H and SmZr2Co9–2H are more thermodynamically stable. Moreover, we found that the transition from the matrix to the planar phase includes both Sm-Co co-displacement along the basal plane as well as Zr replacement for Sm 1a and Co 2c sites. The calculated magnetocrystalline anisotropy of the planar phase under relaxed state exhibits easy axis of magnetization, while that of the planar phase under strain state inside the magnets transforms to easy plane of magnetization. Micromagnetic simulation indicates that the magnetocrystalline anisotropy of the planar phase determines the initial nucleation process of reversal magnetization inside the 2:17R cells, which influences significantly on the coercivity. Tuning the magnetocrystalline anisotropy of the ultrathin planar phase inside the magnets to easy axis will benefit the achievement of higher coercivity. Our investigation may provide insights for developing SmCo-based permanent magnets with higher performance from atomic scale perspective.