Magnetic Quasiparticles Research Articles

Chiral Magnetic Quasiparticles with Zero Topological Charge in 2D Lattice

AbstractChiral quasiparticles, such as magnetic skyrmions, are fundamental phenomena in the field of condensed‐matter physics. So far, several chiral quasiparticles with nontrivial topology have been theoretically predicted and experimentally confirmed, most characterized by carrying a nonzero topological charge. Here, through model analysis, a mechanism of realizing chiral magnetic quasiparticles with zero topological charge in the 2D lattice is proposed. The physics is related to the marriage of layer‐dependent staggered Dzyaloshinskii‐Moriya interaction and interlayer exchange interaction, and the nontrivial topology is protected indirectly. By means of first‐principles calculations and atomic spin model simulations, this mechanism in a real material of bilayer Tl2NO2 is further demonstrated. The realized chiral quasiparticles with zero topological charge can be considered as the superposition of vortex and anti‐vortex. Even though the topological charge is zero, their topologies are nontrivial, indirectly protected by the correlated magnetic bimerons. The finding opens up a new direction for the research on topological spin structures in 2D lattice.

Chiral and flat-band magnetic quasiparticles in ferromagnetic and metallic kagome layers.

Magnetic kagome metals are a promising platform to develop unique quantum transport and optical phenomena caused by the interplay between topological electronic bands, strong correlations, and magnetic order. This interplay may result in exotic quasiparticles that describe the coupled electronic and spin excitations on the frustrated kagome lattice. Here, we observe novel elementary magnetic excitations within the ferromagnetic Mn kagome layers in TbMn6Sn6 using inelastic neutron scattering. We observe sharp, collective acoustic magnons and identify flat-band magnons that are localized to a hexagonal plaquette due to the special geometry of the kagome layer. Surprisingly, we observe another type of elementary magnetic excitation; a chiral magnetic quasiparticle that is also localized on a hexagonal plaquette. The short lifetime of localized flat-band and chiral quasiparticles suggest that they are hybrid excitations that decay into electronic states.

Coarse-graining collective skyrmion dynamics in confined geometries

Magnetic skyrmions are magnetic quasi-particles with enhanced stability and different manipulation mechanisms using external fields and currents, making them promising candidates for future applications such as neuromorphic computing. Recently, several measurements and simulations have shown that thermally activated skyrmions in confined geometries, as they are necessary for device applications, arrange themselves predominantly based on commensurability effects. In this simulational study, based on the Thiele model, we investigate the enhanced dynamics and degenerate non-equilibrium steady state of a system in which the intrinsic skyrmion–skyrmion and skyrmion–boundary interaction compete with thermal fluctuations as well as current-induced spin–orbit torques. The investigated system is a triangular-shaped confinement geometry hosting four skyrmions, where we inject spin-polarized currents between two corners of the structure. We coarse grain the skyrmion states in the system to analyze the intricacies of arrangements of the skyrmion ensemble. In the context of neuromorphic computing, such methods address the key challenge of optimizing readout positions in confined geometries and form the basis for understanding collective skyrmion dynamics in systems with competing interactions on different scales. The findings are corroborated by experimental measurements.

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.

Stability of hopfions in bulk magnets with competing exchange interactions

Magnetic hopfions are string-like three-dimensional topological solitons, characterised by the Hopf number. They serve as a fundamental prototype for three-dimensional magnetic quasiparticles and are an inspiration for novel device concepts in the field of spintronics. Based on a micromagnetic model and without considering temperature, the existence of such hopfions has been predicted in certain magnets with competing exchange interactions. However, physical realisation of freely moving hopfions in bulk magnets have so far been elusive. Here, we consider an effective Heisenberg model with competing exchange interactions and study the stability of small toroidal hopfions with Hopf number ${Q}_{\text{H}}=1$ by finding first-order saddle points on the energy surface representing the transition state for the decay of hopfions via the formation of two coupled Bloch points. We combine the geodesic nudged elastic band method and an adapted implementation of the dimer method to resolve the sharp energy profile of the reaction path near the saddle point. Our analysis reveals that the energy barrier can reach substantial height and is largely determined by the size of the hopfion relative to the lattice constant.

Magnetic-monopole-induced polarons in atomic superlattices

Magnetic monopoles have been realized as emergent quasiparticles in both condensed matter and ultracold atomic platforms, with growing interest in the coupling effects between the monopole and different magnetic quasiparticles. In this work, interaction effects between monopoles and magnons are investigated for an atomic pseudospin chain. We reveal that the monopole can excite a virtual magnon cloud in the paramagnetic chain, thereby giving rise to an unconventional type of polaron, the monopole-cored polaron (MCP). The MCP is composed of the monopole as the impurity core and the virtual magnon excitation as the dressing cloud. The magnon dressing facilitates the Dirac string excitation and impacts the monopole hopping. This induces an antitrapping effect of the MCP, which refers to the fact that the dressing enhances the mobility of the MCP, in contrast to the self-trapping of the common polarons. Moreover, heterogeneous bipolarons are shown to exist under the simultaneous doping of a north and a south monopole. The heterogeneous bipolaron possesses an inner degree of freedom composed of two identical impurities. Our investigation sheds light on the understanding of how the coupling between the impurity core and the dressing cloud can engineer the property of the polaron.

Multiple Magnetic Phases and Anomalous Hall Effect in Sb1.9Fe0.1Te2.85S0.15 Topological Insulators

Due to fundamental and technological concerns, investigating materials with topological magnetic structures has always been a focus of significant research. We explored Sb1.9Fe0.1Te2.85S0.15 where a unique combination of disordered glassy phases, competitive FM–AFM interactions, and nontrivial surface state coexisted at the same time. We have discussed the impact of those complicated magnetic phases upon the observed AHE in Sb1.9Fe0.1Te2.85S0.15 with magneto-transport studies. The AC susceptibility results demonstrate a shift in the freezing temperature with excitation frequency, the comprehensive analysis verifies the slower dynamics, and a nonzero Vogel–Fulcher temperature T0 suggests cluster spin glass. This, together with an intermediate value of the Mydosh parameter, provides an evidence for the formation of a cluster spin glass state in the present system. Topological frustrated magnets, which can host both magnetic frustrations and Dirac quasi-particles, are highly sought after class of compounds. Furthermore, as seen by the de Haas–van Alphen (dHvA) oscillation study, the fermiology deviates with doping and produces multiple Fermi pockets, revealing a rich complexity in the underlying electronic structure.

Ferroelectricity controlled chiral spin textures and anomalous valley Hall effect in the Janus magnet-based multiferroic heterostructure

Realizing effective manipulation and explicit identification of topological spin textures are two crucial ingredients to make them as information carrier in spintronic devices with high storage density, high data handling speed and low energy consumption. Electric-field manipulation of magnetism has been achieved as a dissipationless method compared with traditional regulations. However, the magnetization is normally insensitive to the electric field since it does not break time-reversal symmetry directly, and distribution of topological magnetic quasiparticles is difficult to maintain due to the drift arising from external fluctuation, which could result in ambiguous recognition between quasiparticles and uniform magnetic background. Here, we demonstrate that electric polarization-driven skyrmionic and uniform ferromagnetic states can be easily and explicitly distinguished by transverse voltage arising from anomalous valley Hall effect in the Janus magnet-based multiferroic heterostructure LaClBr/In2Se3. Our work provides an alternative approach for data encoding, in which data are encoded by combing topological spin textures with detectable electronic transport.

Voltage-Controlled Skyrmionic Interconnect with Multiple Magnetic Information Carriers.

Magnetic skyrmions have been in the spotlight since they were observed in technologically relevant systems at room temperature. More recently, there has been increasing interest in additional quasiparticles that may exist as stable/metastable spin textures in magnets, such as the skyrmionium and the antiskyrmionite (i.e., a skyrmion bag with two skyrmions inside) that have distinct topological characteristics. The next challenge and opportunity, at the same time, is to investigate the use of multiple magnetic quasiparticles as information carriers in a single device for next-generation nanocomputing. In this paper, we propose a spintronic interconnect device where multiple sequences of information signals are encoded and transmitted simultaneously by skyrmions, skyrmioniums, and antiskyrmionites. The proposed spintronic interconnect device can be pipelined via voltage-controlled magnetic anisotropy (VCMA) gated synchronizers that behave as intermediate registers. We demonstrate theoretically that the interconnect throughput and transmission energy can be effectively tuned by the VCMA gate voltage and appropriate electric current pulses. By carefully adjusting the device structure characteristics, our spintronic interconnect device exhibits comparable energy efficiency with copper interconnects in mainstream CMOS technologies. This study provides fresh insight into the possibilities of skyrmionic devices in future spintronic applications.

SU(N) toric code and non-Abelian anyons

We construct SU($N$) toric code model describing the dynamics of SU($N$) electric and magnetic fluxes on a two dimensional torus. We show that the model has $N^2$ topologically distinct ground states $|\psi_0\rangle_{({\mathsf p},{\mathsf q})}$ which are loop states characterized by $Z_N \otimes Z_N$ centre charges $({\mathsf p},{\mathsf q} =0,1,2,\cdots, N-1)$. We explicitly construct them in terms of coherent superpositions of all possible spin network states on torus with Wigner coefficients as their amplitudes. All excited quasiparticle states with SU($N$) electric charges and magnetic fluxes are constructed. We show that the braiding statistics of these SU(N) electric, magnetic quasiparticles or nonabelian anyons is encoded in the Wigner rotation matrices.

A topological lattice of plasmonic merons

Topology is an intrinsic property of the orbital symmetry and elemental spin–orbit interaction, but also, intriguingly, designed vectorial optical fields can break existing symmetries, to impose (dress) topology through coherent interactions with trivial materials. Through photonic spin–orbit interaction, light can transiently turn on topological interactions, such as chiral chemistry, or induce non-Abelian physics in matter. Employing electromagnetic simulations and ultrafast, time-resolved photoemission electron microscopy, we describe the geometric transformation of a normally incident plane wave circularly polarized light carrying a defined spin into surface plasmon polariton field carrying orbital angular momentum which converges into an array of plasmonic vortices with defined spin textures. Numerical simulations show how within each vortex domain, the photonic spin–orbit interaction molds the plasmonic orbital angular momentum into quantum chiral spin angular momentum textures resembling those of a magnetic meron quasiparticles. We experimentally examine the dynamics of such meron plasmonic spin texture lattice by recording the ultrafast nanofemto plasmonic field evolution with deep subwavelength resolution and sub-optical cycle time accuracy from which we extract the linear polarization, L-line singularity distribution, that defines the periodic lattice boundaries. Our results reveal how vectorial optical fields can impress their topologically nontrivial spin textures by coherent dressing or chiral excitations of matter.

Various facets of magnetic charge correlation: Micromagnetic and distorted-wave Born approximation simulations study

The emergent concept of magnetic charge quasi-particle provides a new realm to study the evolution of magnetic properties in two-dimensional artificially frustrated magnets. We report on the exploration of magnetic phases due to various magnetic charge correlation using the complementary numerical techniques of micromagnetic and distorted wave Born approximation simulations in artificial permalloy honeycomb lattice. The honeycomb element length varies between 10 nm and 100 nm, while the width and thickness are kept within the single domain limit. In addition to the charge ordered loop state, we observe disordered charge arrangement, characterized by the random distribution of $\pm$Q charges, in single domain size honeycomb lattice. As the length of honeycomb element increases, low multiplicity magnetic charges tend to form contiguous bands in thinner lattice. Thin honeycomb lattice with 100 nm element length exhibits a perfect spin ice pattern, which remains unaffected to the modest increase in the width of element size. We simulate scattering profiles under the pretext of distorted wave Born approximation formalism for the micromagnetic phases. The results are expected to provide useful guidance in the experimental investigation of magnetic phases in artificial honeycomb magnet.

Non-Abelian statistics in light-scattering processes across interacting Haldane chains

The $S=1$ Haldane state is constructed from a product of local singlet dimers in the bulk and topological states at the edges of a chain. It is a fundamental representative of topological quantum matter. Its well-known representative, the quasi-one-dimensional SrNi$_2$V$_2$O$_8$ shows both conventional as well as unconventional magnetic Raman scattering. The former is observed as one- and two-triplet excitations with small linewidths and energies corresponding to the Haldane gap $\Delta_H$ and the exchange coupling $J_c$ along the chain, respectively. Well-defined magnetic quasiparticles are assumed to be stabilized by interchain interactions and uniaxial single-ion anisotropy. Unconventional scattering exists as broad continua of scattering with an intensity $I(T)$ that shows a mixed bosonic / fermionic statistic. Such a mixed statistic has also been observed in Kitaev spin liquids and could point to a non-Abelian symmetry. As the ground state in the bulk of SrNi$_2$V$_2$O$_8$ is topologically trivial, we suggest its fractionalization to be due to light-induced interchain exchange processes. These processes are supposed to be enhanced due to a proximity to an Ising ordered state with a quantum critical point. A comparison with SrCo$_2$V$_2$O$_8$, the $S=1/2$ analogue to our title compound, supports these statements.

Magnetic Skyrmions Could Act as Qubits

Predictions suggest that magnetic quasiparticles known as skyrmions could provide the basis for a novel quantum-computing platform.

Vortex-mediated relaxation of magnon BEC into light Higgs quasiparticles

A magnon Bose-Einstein condensate in superfluid $^3$He is a fine instrument for studying the surrounding macroscopic quantum system. At zero temperature, the BEC is subject to a few, distinct forms of decay into other collective excitations, owing to momentum and energy conservation in a quantum vacuum. We study the vortex-Higgs mechanism: the vortices relax the requirement for momentum conservation, allowing the optical magnons of the BEC to transform into light Higgs quasiparticles. This observation expands the spectrum of possible interactions between magnetic quasiparticles in $^3$He-B, opens pathways for hunting down elusive phenomena such as the Kelvin wave cascade or bound Majorana fermions, and lays groundwork for building magnon-based quantum devices.

Competing magnetic orders and multipolar Weyl fermions in 227 pyrochlore iridates

Owing to comparably strong spin-orbit coupling and Hubbard interaction, iridium based 227 pyrochlore oxides harbor a rich confluence of competing magnetic orders and emergent multipolar Weyl quasiparticles. Here we show that this family of materials is predominantly susceptible toward the nucleation of electronic noncoplanar all-in all-out (AIAO) and three-in one-out (3I1O) orders, respectively transforming under the singlet $A_{2u}$ and triplet $T_{1u}$ representations, supporting octupolar and dipolar Weyl fermions, and favored by strong on-site Hubbard and nearest-neighbor ferromagnetic interaction. Furthermore, a coplanar magnetic order generically appears as an intermediate phase between them. This order transforms under the triplet $T_{2u}$ representation and also hosts octupolar Weyl fermions. With the AIAO and 3I1O phases possibly being realized in (Nd$_{1-x}$Pr$_{x}$)$_2$Ir$_2$O$_7$ when $x=0$ and 1, respectively, the intervening $T_{2u}$ order can in principle be found at an intermediate doping ($0<x<1$) or in pressured (hydrostatic) Nd$_2$Ir$_2$O$_7$.

Dipolar-stabilized first and second-order antiskyrmions in ferrimagnetic multilayers

Skyrmions and antiskyrmions are topologically protected spin structures with opposite vorticities. Particularly in coexisting phases, these two types of magnetic quasi-particles may show fascinating physics and potential for spintronic devices. While skyrmions are observed in a wide range of materials, until now antiskyrmions were exclusive to materials with D2d symmetry. In this work, we show first and second-order antiskyrmions stabilized by magnetic dipole–dipole interaction in Fe/Gd-based multilayers. We modify the magnetic properties of the multilayers by Ir insertion layers. Using Lorentz transmission electron microscopy imaging, we observe coexisting antiskyrmions, Bloch skyrmions, and type-2 bubbles and determine the range of material properties and magnetic fields where the different spin objects form and dissipate. We perform micromagnetic simulations to obtain more insight into the studied system and conclude that the reduction of saturation magnetization and uniaxial magnetic anisotropy leads to the existence of this zoo of different spin objects and that they are primarily stabilized by dipolar interaction.

Extreme anti-reflection enhanced magneto-optic Kerr effect microscopy

Magnetic and spintronic media have offered fundamental scientific subjects and technological applications. Magneto-optic Kerr effect (MOKE) microscopy provides the most accessible platform to study the dynamics of spins, magnetic quasi-particles, and domain walls. However, in the research of nanoscale spin textures and state-of-the-art spintronic devices, optical techniques are generally restricted by the extremely weak magneto-optical activity and diffraction limit. Highly sophisticated, expensive electron microscopy and scanning probe methods thus have come to the forefront. Here, we show that extreme anti-reflection (EAR) dramatically improves the performance and functionality of MOKE microscopy. For 1-nm-thin Co film, we demonstrate a Kerr amplitude as large as 20° and magnetic domain imaging visibility of 0.47. Especially, EAR-enhanced MOKE microscopy enables real-time detection and statistical analysis of sub-wavelength magnetic domain reversals. Furthermore, we exploit enhanced magneto-optic birefringence and demonstrate analyser-free MOKE microscopy. The EAR technique is promising for optical investigations and applications of nanomagnetic systems.

A New Way to Measure Magnons

Magnetic quasiparticles called magnons are predicted to exhibit a measurable “orbital magnetic moment” in addition to their spin magnetic moment.

Coexistence and Interaction of Spinons and Magnons in an Antiferromagnet with Alternating Antiferromagnetic and Ferromagnetic Quantum Spin Chains.

In conventional quasi-one-dimensional antiferromagnets with quantum spins, magnetic excitations are carried by either magnons or spinons in different energy regimes: they do not coexist independently, nor could they interact with each other. In this Letter, by combining inelastic neutron scattering, quantum MonteCarlo simulations, and random phase approximation calculations, we report the discovery and discuss the physics of the coexistence of magnons and spinons and their interactions in Botallackite-Cu_{2}(OH)_{3}Br. This is a unique quantum antiferromagnet consisting of alternating ferromagnetic and antiferromagnetic spin-1/2 chains with weak interchain couplings. Our study presents a new paradigm where one can study the interaction between two different types of magnetic quasiparticles: magnons and spinons.