Dzyaloshinskii-Moriya Interaction Research Articles

"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.

Spin–orbit torque true random number generator with thermal stability

Interfacial Dzyaloshinskii–Moriya interaction (DMI) plays a pivotal role in spin–orbit torque (SOT)-induced magnetization switching, notably seen in deterministic switching even in the absence of an external magnetic field at 0 K. However, in SOT devices operating at room temperature, thermal fluctuations contribute significantly to magnetization switching due to the altered energy profile caused by DMI. In this work, we unveil that unlike the deterministic magnetization switching observed at 0 K, SOT-induced magnetization switching assisted by DMI is highly stochastic. Following the SOT-induced nucleation of a domain wall (DW), thermal fluctuations can induce rapid back-and-forth DW motion under the influence of a current pulse, resulting in stochastic switching. Furthermore, our findings indicate that the switching probability remains stable as the temperature increases. These results illustrate that SOT-induced magnetization switching assisted by DMI is well-suited for a true random number generator with robust thermal stability.

Electric manipulation of the magnetization in heterostructure Pt/Co/Bi2Se3

Spin–orbit torque (SOT) can provide efficient electrical manipulation of magnetism via applying electrical current to breaking the symmetry of damping-like torque. In the heterojunction of heavy and ferromagnetic metal, Dzyaloshinskii–Moriya interaction (DMI) is one of the key ingredients for stabilizing chiral spin structures, like chiral domain walls. Meanwhile, materials with larger charge-spin conversion rates are also highly expected for the efficient SOT. In this paper, spin–orbit torque magnetic switching is observed in the perpendicularly magnetized Pt/Co/Bi2Se3 and shows relatively high efficiency with low critical switching current density of about 5 × 105 A cm−2. The SOT efficiency and DMI in perpendicularly magnetized Pt/Co/Bi2Se3 were quantitatively investigated by electrical detection of the effective spin Hall field. The DMI constant is about 2.6 mJ m−2, and the effective spin Hall angle of Pt/Co/Bi2Se3 is about 0.14. The work also demonstrates that the Bi2Se3 layer takes the main responsibility for SOT, and the Pt/Co interface is the main source of DMI in Pt/Co/Bi2Se3 structures, which makes it possible to achieve independent optimization of DMI and SOT in the Pt/Co/Bi2Se3 structure at room temperature for the advanced application of spintronic devices.

Renormalization of net steered coherence and average coherence in the XXZ model with Dzyaloshinskii–Moriya interaction

The critical behaviors of the net steered coherence (NSC) and average coherence (AC) in the XXZ model with Dzyaloshinskii–Moriya interaction are investigated by adopting the quantum renormalization group method. It is found that they both develop different saturated values in different phases of the model after enough iterations of the renormalization of the coupling constants. In addition, their susceptibilities at the phase transition point are divergent in the thermodynamic limit, and the scaling exponent is determined by the exponent governing the divergence of the correlation length close to the phase transition point. This exemplifies that both the NSC and AC can serve as reliable indicators of quantum phase transitions.

Atomistic Origin of Diverse Charge Density Wave States in CsV_{3}Sb_{5}.

Kagome metals AV_{3}Sb_{5} (A=K, Rb, or Cs) exhibit intriguing charge density wave (CDW) instabilities, which interplay with superconductivity and band topology. However, despite firm observations, the atomistic origins of the CDW phases, as well as hidden instabilities, remain elusive. Here, we adopt our newly developed symmetry-adapted cluster expansion method to construct a first-principles-based effective Hamiltonian of CsV_{3}Sb_{5}, which not only reproduces the established inverse star of David (ISD) phase, but also predict a series of D_{3h}-n states under mild tensile strains. With such atomistic Hamiltonians, the microscopic origins of different CDW states are revealed as the competition of the second-nearest neighbor V-V pairs versus the first-nearest neighbor V-V and V-Sb couplings. Interestingly, the effective Hamiltonians also reveal the existence of ionic Dzyaloshinskii-Moriya interaction in the high-symmetry phase of CsV_{3}Sb_{5} and drives the formation of noncollinear CDW patterns. Our work thus not only deepens the understanding of the CDW formation in AV_{3}Sb_{5}, but also demonstrates that the effective Hamiltonian is a suitable approach for investigating CDW mechanisms, which can be extended to various CDW systems.

Anisotropic creep velocity of Dzyaloshinskii domain walls

We have measured the field-driven velocity of chiral Néel domain walls (DWs) stabilized by the Dzyaloshinskii–Moriya interaction (DMI) in a Pt/Co/Ta/Pt film with perpendicular magnetic anisotropy. A simple model based on the universal creep theory allows us to describe the anisotropic propagation of a DW along the contour of a bubble domain, driven by an out-of-plane field in the presence of a static in-plane field. This model is used to obtain the DMI constant from the measurement of the DW propagation with only one value of the in-plane field, simplifying the existing method relying on several measurements. The DMI constant extracted from the model is in good agreement with independent measurements.

Magnons in a two-dimensional Weyl magnet

The discovery of topological states of matter has led to a revolution in condensed-matter science. While a non-trivial band topology in a material is often associated with intriguing transport properties, much less attention has been given to the impact on spin dynamics and non-equilibrium magnetization states. Here, we provide evidence that a chiral asymmetric magnon dispersion in the two-dimensional Weyl magnet Fe/W(110) is related to the presence of Weyl fermions close to the Fermi energy and surface Fermi arcs. We find that the large anomalous Hall conductivity and the Dzyaloshinskii–Moriya interaction are attributed to the non-trivial band topology in the composite momentum-magnetization space. Our results show the direct impact of Weyl fermions on both the charge and spin dynamics in a two-dimensional magnet. Unveiling these principles can promote innovative technologies in magnonics by utilizing topological materials, where magnons and non-trivial topological electronic states can be manipulated through magnetization.

Homochiral antiferromagnetic merons, antimerons and bimerons realized in synthetic antiferromagnets

The ever-growing demand for device miniaturization and energy efficiency in data storage and computing technology has prompted a shift towards antiferromagnetic topological spin textures as information carriers. This shift is primarily owing to their negligible stray fields, leading to higher possible device density and potentially ultrafast dynamics. We realize in this work such chiral in-plane topological antiferromagnetic spin textures namely merons, antimerons, and bimerons in synthetic antiferromagnets by concurrently engineering the effective perpendicular magnetic anisotropy, the interlayer exchange coupling, and the magnetic compensation ratio. We demonstrate multimodal vector imaging of the three-dimensional Néel order parameter, revealing the topology of those spin textures and a globally well-defined chirality, which is a crucial requirement for controlled current-induced dynamics. Our analysis reveals that the interplay between interlayer exchange and interlayer magnetic dipolar interactions plays a key role to significantly reduce the critical strength of the Dzyaloshinskii-Moriya interaction required to stabilize topological spin textures, such as antiferromagnetic merons, in synthetic antiferromagnets, making them a promising platform for next-generation spintronics applications.

Field-Free Switching of Perpendicular Magnetization in an Ultrathin Epitaxial Magnetic Insulator.

For energy-efficient magnetic memories, switching of perpendicular magnetization by spin-orbit torque (SOT) appears to be a promising solution. This SOT switching requires the assistance of an in-plane magnetic field to break the symmetry. Here, we demonstrate the field-free SOT switching of a perpendicularly magnetized thulium iron garnet (Tm3Fe5O12, TmIG). The polarity of the switching loops, clockwise or counterclockwise, is determined by the direction of the initial current pulses, in contrast with field-assisted switching where the polarity is controlled by the direction of the magnetic field. From Brillouin light scattering, we determined the Dzyaloshinskii-Moriya interaction (DMI) induced by the Pt-TmIG interface. We will discuss the possible origins of field-free switching and the roles of the interfacial DMI and cubic magnetic anisotropy of TmIG. This discussion is substantiated by magnetotransport, Kerr microscopy, and micromagnetic simulations. Our observation of field-free electrical switching of a magnetic insulator is an important milestone for low-power spintronic devices.

Blended ferron solitary wave emerging from electron–phonon–magnon interaction in magnetic clusters: Ferrons vs skyrmions

In this paper, the solitonic behaviour of a blended ferron in a ferromagnetic material is studied and the different spin configurations obtained are discussed. It is found that the electron–magnon–phonon coupling significantly affects the exchange between neighbouring spins and thus both the lattice and the spin are polarized. The dynamics of the model is well described by a set of nonlinear discrete self-trapped equations with stable resonant solutions. In terms of topology, the ferron configurations obtained mimic a skyrmion, while their induced polarization and magnetization are Bell-shape soliton-like. We observe that a significant presence of the Dzyaloshinsky–Moriya interaction topologically favours a vortex skyrmion configuration (Bloch’s skyrmion), while its absence restores a Neel’s skyrmion shape. Therefore, the blended ferron is manifested in the magnetic material by its localization and the formation of self-trapped states leading to skyrmion topology. This wonderful combination is a huge boost in skyrmion formation and thus the probable application to superconductivity are opened through skyrmion pairing.

Structural, chemical, and magnetic investigation of a graphene/cobalt/platinum multilayer system on silicon carbide

We investigate the magnetic interlayer coupling and domain structure of ultra-thin ferromagnetic (FM) cobalt (Co) layers embedded between a graphene (G) layer and a platinum (Pt) layer on a silicon carbide (SiC) substrate (G/Co/Pt on SiC). Experimentally, a combination of x-ray photoemission electron microscopy with x-ray magnetic circular dichroism has been carried out at the Co L-edge. Furthermore, structural and chemical properties of the system have been investigated using low energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS). In situ LEED patterns revealed the crystalline structure of each layer within the system. Moreover, XPS confirmed the presence of quasi-freestanding graphene, the absence of cobalt silicide, and the appearance of two silicon carbide surface components due to Pt intercalation. Thus, the Pt-layer effectively functions as a diffusion barrier. The magnetic structure of the system was unaffected by the substrate’s step structure. Furthermore, numerous vortices and anti-vortices were found in all samples, distributed all over the surfaces, indicating Dzyaloshinskii–Moriya interaction. Only regions with a locally increased Co-layer thickness showed no vortices. Moreover, unlike in similar systems, the magnetization was predominantly in-plane, so no perpendicular magnetic anisotropy was found.

Role of isotropic and anisotropic Dzyaloshinskii–Moriya interaction on skyrmions, merons and antiskyrmions in the C nv symmetric system

The lattice Hamiltonian with the presence of a chiral magnetic isotropic Dzyaloshinskii–Moriya interaction (DMI) in a square and hexagonal lattice is numerically solved to give the full phase diagram consisting of skyrmions and merons in different parameter planes. The phase diagram provides the actual regions of analytically unresolved asymmetric skyrmions and merons, and it is found that these regions are substantially larger than those of symmetric skyrmions and merons. With magnetic field, a change from meron or spin spiral (SS) to skyrmion is seen. The complete phase diagram for the C nv symmetric system with anisotropic DMI is drawn and it is shown that this DMI helps to change the SS propagation direction. Finally, the well-defined region of a thermodynamically stable antiskyrmion phase in the C nv symmetric system is shown.

Ultrastrong magnon-magnon coupling in synthetic antiferromagnets induced by interlayer Dzyaloshinskii-Moriya interaction

Ultrastrong magnon-magnon coupling in synthetic antiferromagnets induced by interlayer Dzyaloshinskii-Moriya interaction

Magnetic tunability in tetragonal Mn–Rh–Ir–Sn inverse Heusler compounds

Gaining control over magnetic structure has been an ongoing challenge in materials that form complex, nanoscale, and non-collinear magnetic configurations. Recently, it was predicted that tuning the ratio of the Dzyaloshinskii–Moriya interaction to the uniaxial magnetic anisotropy in tetragonal inverse Heuslers through changes in composition could allow a range of interesting magnetic states to be accessed, from simple ferrimagnetism, to helical and antiskyrmionic phases. Here, we show tunability of the magnetic phase behavior in the Mn–Rh–Sn system through Ir substitution on the Rh substructure. Iridium substitution correlates to an increase in the strength of ferromagnetic exchange couplings, at the expense of antiferromagnetic exchange couplings. However, we do not observe the complex non-collinear magnetic phases proposed previously, likely due to the extremely narrow composition window where these phases are predicted to form in a bulk sample. This work highlights the sensitivity of complex magnetic structures to stoichiometry, which makes them difficult to discover empirically.

Chiral superconducting diode effect by Dzyaloshinsky-Moriya interaction

Chiral superconducting diode effect by Dzyaloshinsky-Moriya interaction

In-plane strain-induced structural phase transition and interlayer antiferromagnetic skyrmions in 2H-VSe2 bilayer

The sliding and manipulation of interlayer magnetism and magnetic topological textures in two-dimensional (2D) layered materials have recently received tremendous attention. In this work, using first-principles calculations, we report a structural phase transition induced by manipulating the interlayer distance using an in-plane biaxial strain in a 2H-VSe2 bilayer. This structural phase transition is accompanied by a semiconductor-to-metal transition, in-plane-to-out-of-plane magnetization switching, and a reversal in the chirality of the Dzyaloshinskii–Moriya interaction (DMI). The binding strength of the interlayer Se2–Se3 atoms and charge density difference can serve as indicators for this structural phase transition. Furthermore, the interlayer distance of Se2–Se3 atoms can be employed as a descriptor that perfectly characterizes the degree of symmetry breaking and the magnitude of the DMI resulting from the broken spatial symmetry due to sliding. In addition, using atomistic simulations, we identify magnetic topological textures such as interlayer antiferromagnetic (AFM) frustrated bimerons and interlayer AFM skyrmions with strain. These results are beneficial for understanding and manipulating the interlayer properties of 2D layered materials through in-plane biaxial strain. In addition, the interlayer AFM frustrated bimerons and skyrmions in bilayer VSe2, which can suppress the skyrmion Hall effect due to the canceled Magnus forces in the top and bottom layers, highlight the promising applications of VSe2 in next-generation information storage devices.

Observation of Topological Hall Effect in a Chemically Complex Alloy.

The topological Hall effect (THE) is the transport response of chiral spin textures and thus can serve as a powerful probe for detecting and understanding these unconventional magnetic orders. So far, the THE is only observed in either noncentrosymmetric systems where spin chirality is stabilized by Dzyaloshinskii-Moriya interactions, or triangular-lattice magnets with Ruderman-Kittel-Kasuya-Yosida-type interactions. Here, a pronounced THE is observed in a Fe-Co-Ni-Mn chemically complex alloy with a simple face-centered cubic (fcc) structure across a wide range of temperatures and magnetic fields. The alloy is shown to have a strong magnetic frustration owing to the random occupation of magnetic atoms on the close-packed fcc lattice and the direct Heisenberg exchange interaction among atoms, as evidenced by the appearance of a reentrant spin glass state in the low-temperature regime and the first principles calculations. Consequently, THE is attributed to the nonvanishing spin chirality created by strong spin frustration under the external magnetic field, which is distinct from the mechanism responsible for the skyrmion systems, as well as geometrically frustrated magnets.

Topological band engineering and tunable thermal Hall effect in trimerized Lieb lattice ferromagnets

We theoretically study the topological properties of magnons and the relevant magnon thermal Hall effect in trimerized Lieb lattice ferromagnets in the presence of next-nearest-neighbor Dzyaloshinskii-Moriya interactions. By calculating the magnon band structures and their Chern numbers with the linear spin-wave theory, we show that the system can undergo a phase transition between a magnonic topological insulator phase and a magnonic trivial insulator phase. The main results are presented in the form of topological phase diagrams, where the Chern numbers or magnon thermal Hall conductivity are shown as a function of the two lattice trimerization parameters. We find a sharp change of the thermal Hall conductivity across the critical point of phase transformations associated with topological nontrivial edge states. The behaviors reflect that the existence of trimerizations breaks the C4 rotational symmetry of the Lieb lattice. Finally, we show that our theoretical predictions could be experimentally realized in high-temperature cuprate superconductors or organic magnetic materials. Published by the American Physical Society 2024

Nonvolatile Multistate Manipulation of Topological Magnetism in Monolayer CrI3 through Quadruple-Well Ferroelectric Materials.

Nonvolatile multistate manipulation of two-dimensional (2D) magnetic materials holds promise for low dissipation, highly integrated, and versatile spintronic devices. Here, utilizing density functional theory calculations and Monte Carlo simulations, we report the realization of nonvolatile and multistate control of topological magnetism in monolayer CrI3 by constructing multiferroic heterojunctions with quadruple-well ferroelectric (FE) materials. The Pt2Sn2Te6/CrI3 heterojunction exhibits multiple magnetic phases upon modulating FE polarization states of FE layers and interlayer sliding. These magnetic phases include Bloch-type skyrmions and ferromagnetism, as well as a newly discovered topological magnetic structure. We reveal that the Dzyaloshinskii-Moriya interaction (DMI) induced by interfacial coupling plays a crucial role in magnetic skyrmion manipulation, which aligns with the Fert-Levy mechanism. Moreover, a regular magnetic skyrmion lattice survives when removing a magnetic field, demonstrating its robustness. The work sheds light on an effective approach to nonvolatile and multistate control of 2D magnetic materials.

Stable skyrmions in Co/Ni-based nanopillars with perpendicular magnetization anisotropy

Micromagnetic simulations have been conducted to explore the formation rules of stable skyrmions in Co/Ni-based perpendicular magnetization anisotropy nanopillars with Dzyaloshinskii–Moriya Interaction (DMI). The results show that an appropriate perpendicular magnetic field can generate a robust Néel-type skyrmion magnetic configuration in the free layer with a 5° tilted easy axis. The creation and stability of skyrmion states also depends on the strength of the DMI and the size of nano-disk. Furthermore, we observed intriguing behavior in the case of the free layer with a perfectly perpendicular easy axis. Skyrmion states appeared in two distinct regions of perpendicular magnetic field when both perpendicular and in-plane magnetic fields were applied. This discontinuity arises from the nucleation of initial Bloch-type skyrmions. These results contribute to comprehending the formation mechanism of stable skyrmions in perpendicular magnetic anisotropy multilayers and may promote the development of skyrmion-based nanodevices.