Search Results (35)

Chiral magnetic orderings in itinerant magnets have recently attracted considerable attention as a source of emergent electromagnetic phenomena such as topological Hall effects and magnetoelectric couplings. In this study, we investigate emergent multipole degrees of freedom arising from chiral magnetic orderings on a two-dimensional triangular lattice. Focusing on a four-sublattice spin configuration characterized by noncoplanar spin textures, we demonstrate that various types of multipoles, such as magnetic dipoles and electric toroidal dipoles, naturally emerge even in the absence of relativistic spin–orbit coupling. By employing a microscopic tight-binding model, we classify the resulting multipole moments and clarify their relationships to the underlying chiral spin texture. We further explore how spin–orbit coupling modifies these multipole characters, leading to additional uniform responses. The results provide a unified framework connecting noncollinear magnetic orderings and emergent multipole phenomena, offering insights into unconventional cross-correlation phenomena in itinerant chiral magnets. Full article

We theoretically investigate multiple-Q instabilities in centrosymmetric hexagonal magnets, formulated as superpositions of independent six ordering wave vectors related by sixfold rotational and mirror symmetries. By employing a spin model that incorporates biquadratic interactions and an external magnetic field, we establish a comprehensive low-temperature phase diagram hosting single-Q, double-Q, triple-Q, and sextuple-Q states, as well as skyrmion crystals with topological charges of one and two. The field evolution of the magnetization, scalar spin chirality, and finite wave-vector magnetic amplitudes reveals a hierarchical buildup of multiple-Q order, accompanied by first-order transitions between topologically distinct and trivial phases. At large biquadratic coupling, all six symmetry-related ordering wave vectors coherently participate, giving rise to two sextuple-Q states under magnetic fields and to another spontaneous sextuple-Q state even at zero field. The latter zero-field sextuple-Q state represents a fully developed sixfold interference pattern stabilized solely by the biquadratic interaction, characterized by alternating skyrmion- and antiskyrmion-like cores with vanishing uniform scalar spin chirality. These findings establish a unified framework for understanding hierarchical multiple-Q ordering and demonstrate that the interplay between bilinear and biquadratic interactions under hexagonal symmetry provides a generic route to complex noncoplanar magnetism in centrosymmetric itinerant systems. Full article

We theoretically investigate topological transitions between coplanar and noncoplanar magnetic states in centrosymmetric itinerant magnets on a square lattice. A canonical effective spin model incorporating bilinear and biquadratic exchange interactions at finite wave vectors is analyzed to elucidate the emergence of multiple-Q magnetic orders. By taking into account high-harmonic wave-vector interactions, we demonstrate that a coplanar double-Q spin texture continuously evolves into a noncoplanar triple-Q state carrying a finite scalar spin chirality. The stability of these multiple-Q states is examined using simulated annealing as a function of the relative strengths of the high-harmonic coupling, the biquadratic interaction, and the external magnetic field. The resulting phase diagrams reveal a competition between double-Q and triple-Q states, where the noncoplanar triple-Q phase is stabilized through the cooperative effect of the high-harmonic and biquadratic interactions. Real-space spin textures, spin structure factors, and scalar spin chirality distributions are analyzed to characterize the distinct magnetic phases and the topological transitions connecting them. These findings provide a microscopic framework for understanding the emergence of noncoplanar magnetic textures driven by the interplay between two- and four-spin interactions in centrosymmetric itinerant magnets. Full article

Multiple-Q magnetic states encompass a broad class of noncollinear and noncoplanar spin textures generated by the superposition of spin density waves. In this study, we theoretically explore the emergence of vortex crystals formed by multiple-Q spin density waves on a two-dimensional triangular lattice with $D_{3\mathrm{h}}$ point group symmetry. Using simulated annealing applied to an effective spin model, we demonstrate that the synergy among the easy-plane single-ion anisotropy, the biquadratic interaction, and the out-of-plane Dzyaloshinsky–Moriya interaction defined in momentum space can give rise to a variety of double-Q and triple-Q vortex crystals. We further examine the role of easy-plane single-ion anisotropy in triple-Q vortex crystals and show that weakening the anisotropy drives topological transitions into skyrmion crystals with skyrmion numbers $\pm 1$ and $\pm 2$. The influence of an external magnetic field is also analyzed, revealing a field-induced phase transition from vortex crystals to single-Q conical spirals. These findings highlight the crucial role of out-of-plane Dzyaloshinskii–Moriya interactions in stabilizing unconventional vortex crystals, which cannot be realized in systems with purely polar or chiral symmetries. Full article

The full Heusler alloys Ni₂MnZ (Z = In, Sn, Sb) exhibit ferromagnetic properties with a Curie temperature (T_C) above room temperature. The magnetic properties of Ni₂MnZ (Z = In, Sn, Sb) were studied through a combination of experiments and band calculations under ambient and elevated pressures. The main results of this study open up further prospects for controlling the magnetic properties of the multifunctional Heusler alloys Ni₂Mn_1+xZ_1−x (Z = In, Sn, Sb) and their practical application. Full article

In this study, we investigate the stability of a magnetic skyrmion crystal with short-period magnetic modulations in a centrosymmetric body-centered tetragonal system. By performing the simulated annealing for the spin model, incorporating the effects of the biquadratic interaction and high-harmonic wave–vector interaction in momentum space, we find that the double-Q square skyrmion crystal consisting of two spin density waves is stabilized in an external magnetic field. We also show that double-Q states appear in both low- and high-field regions; the low-field spin configuration is characterized by an anisotropic double-Q modulation consisting of a superposition of the spiral wave and sinusoidal wave, while the high-field spin configuration is characterized by an isotropic double-Q modulation consisting of a superposition of two sinusoidal waves. Furthermore, we show that the obtained multiple-Q instabilities can be realized for various ordering wave vectors. The results provide the possibility of realizing the short-period skyrmion crystals under the body-centered tetragonal lattice structure. Full article

We investigate the emergence of magnetic skyrmion crystals with swirling topological spin textures in itinerant magnets with an emphasis on momentum-resolved multi-spin interactions. By performing the simulated annealing for the effective spin model with the two-spin and four-spin interactions on a two-dimensional triangular lattice, we show that various types of four-spin interactions become the microscopic origin of the magnetic skyrmion crystal with the skyrmion numbers of one and two. We find that the four-spin interactions between the different wave vectors lead to the skyrmion crystal with the skyrmion number of one, whereas those at the same wave vectors lead to the skyrmion crystals with the skyrmion number of one and two. Our results indicate that the multi-spin interactions arising from the itinerant nature of electrons provide rich topological spin textures in magnetic metals. Full article

Employing the spin-wave formalism within and beyond the harmonic-oscillator approx-imation, we study the dynamic structure factors of spin-${\displaystyle \frac{1}{2}}$ nearest-neighbor quantum Heisenberg antiferromagnets on two-dimensional quasiperiodic lattices with particular emphasis on a mag-netic analog to the well-known confined states of a hopping Hamiltonian for independent electrons on a two-dimensional Penrose lattice. We present comprehensive calculations on the C_5v Penrose tiling in comparison with the C_8v Ammann–Beenker tiling, revealing their decagonal and octagonal antiferromagnetic microstructures. Their dynamic spin structure factors both exhibit linear soft modes emergent at magnetic Bragg wavevectors and have nearly or fairly flat scattering bands, signifying magnetic excitations localized in some way, at several different energies in a self-similar manner. In particular, the lowest-lying highly flat mode is distinctive of the Penrose lattice, which is mediated by its unique antiferromagnons confined within tricoordinated sites only, unlike their itinerant electron counterparts involving pentacoordinated, as well as tricoordinated, sites. Bringing harmonic antiferromagnons into higher-order quantum interaction splits, the lowest-lying nearly flat scattering band in two, each mediated by further confined antiferromagnons, which is fully demonstrated and throughly visualized in the perpendicular as well as real spaces. We disclose superconfined antiferromagnons on the two-dimensional Penrose lattice. Full article

In contrast to the observed high-temperature superconductivity in monolayer $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ films, akin to ${\mathrm{CoS}\mathrm{b}/\mathrm{SrTiO}}_3$, the bulk counterpart, FeSe, does not exhibit superconductivity even under elevated pressure, and its magnetic characteristics remain subject to debate. This investigation delves into the electrical and magnetic attributes, alongside X-ray photoelectron spectroscopy (XPS) analysis, of FeSe mono-crystal. Magnetic and electrical transport assessments indicate that FeSe demonstrates characteristics of a Pauli paramagnetic metal within non-Fermi liquid traits. XPS analysis further reveals that the Fe and Se pair in FeSe exist in a zero-valence state, forming a predominantly metallic-bonded alloy. The Pauli paramagnetism observed in FeSe is ascribed to its itinerant electrons. The comprehension of the electronic states in FeSe mono-crystal not only clarifies its lack of magnetic characteristics but also paves the way for exploring potential high-temperature superconductivity. Full article

We report the continuous argon ions irradiation of itinerant Fe₃GeTe₂, a two-dimensional ferromagnetic metal, with the modification to its transport properties measured in situ. Our results show that defects generated by argon ions irradiation can significantly weaken the magnetization (M) and coercive field ($H_c$) of Fe₃GeTe₂, demonstrating the tunable magnetism of this material. Specifically, at base temperature, we observed a reduction of M and $H_c$ by up to 40% and 62.4%, respectively. After separating the contribution from different mechanisms based on the Tian-Ye-Jin (TYJ) scaling relation, it’s the skew scattering that dominates the contribution to anomalous Hall effect in argon ions irradiated Fe₃GeTe₂. These findings highlight the potential of in situ transport modification as an effective method for tailoring the magnetic properties of two-dimensional magnetic materials, and provides new insights into the mechanisms underlying the tunable magnetism in Fe₃GeTe₂. Full article

Quantum fluctuations inherent in electronic systems positioned close to magnetic instabilities can lead to novel collective phenomena. One such material, $\beta$-Ti${}_6$Sn${}_5$, sits close to ferromagnetic (FM) instability and can be pushed to an itinerant FM-ordered state with only minute magnetic or non-magnetic doping. The binary nature of this compound, however, limits the tuning variables that can be applied to study any emergent physics, which are likely to be sensitive to the introduction of chemical disorder.Accordingly, we grew high-quality single crystals of a new quaternary compound Zr${}_3$V${}_3$GeSn${}_4$ from a Sn-rich self flux, and determined the structure with single-crystal X-ray diffraction. Zr${}_3$V${}_3$GeSn${}_4$ forms in an ordered derivative of the hexagonal $\beta$-Ti${}_6$Sn${}_5$ structure with Zr and V atomic positions that show no indication of site interchange. Ge likewise occupies a single unique atomic position. The V site, which would be the one most likely to give rise to any magnetic character, is located at the center of a distorted octahedron of Sn, with such octahedra arranged in face-sharing chains along the crystallographic c axis, while the chains themselves are organized in a kagome geometry. Zr${}_3$V${}_3$GeSn${}_4$ represents the second known quaternary phase within this system, suggesting that other compounds with this structure type await discovery. Full article

The role of spin and orbital rotational symmetry on the laser-induced magnetization dynamics of itinerant-electron ferromagnets was theoretically investigated. The ultrafast demagnetization of transition metals is shown to be the direct consequence of the fundamental breaking of these conservation laws in the electronic system, an effect that is inherent to the nature of spin-orbit and electron-lattice interactions. A comprehensive symmetry analysis is complemented by exact numerical calculations of the time evolution of optically excited ferromagnetic ground states in the framework of a many-body electronic Hamiltonian. Thus, quantitative relations are established between the strength of the interactions that break the rotational symmetries and the time scales that are relevant for the magnetization dynamics. Full article

Polarons are quasiparticles relevant across many fields in physics: from condensed matter to atomic physics. Here, we study the quasiparticle properties of two-dimensional strongly interacting Bose polarons in atomic Bose–Einstein condensates and polariton gases. Our studies are based on the non-self consistent T-matrix approximation adapted to these physical systems. For the atomic case, we study the spectral and quasiparticle properties of the polaron in the presence of a magnetic Feshbach resonance. We show the presence of two polaron branches: an attractive polaron, a low-lying state that appears as a well-defined quasiparticle for weak attractive interactions, and a repulsive polaron, a metastable state that becomes the dominant branch at weak repulsive interactions. In addition, we study a polaron arising from the dressing of a single itinerant electron by a quantum fluid of polaritons in a semiconductor microcavity. We demonstrate the persistence of the two polaron branches whose properties can be controlled over a wide range of parameters by tuning the cavity mode. Full article

The effects of element substitution and element doping on the magnetic properties and magnetocaloric effect of the LaFe_11.5Si_1.5 compound were investigated. The crystals of the LaFe_11.5Si_1.5, La_0.8Nd_0.2Fe_11.5Si_1.5, and LaFe_11.5Si_1.5C_0.2 compounds all showed cubic NaZn₁₃-type structures, but the lattice of the La_0.8Nd_0.2Fe_11.5Si_1.5 shrank and the lattice of the LaFe_11.5Si_1.5C_0.2 expanded. All three compounds had the characteristic of first-order magnetic transition due to the obvious itinerant-electron metamagnetic (IEM) transition occurring above Curie temperature (T_C). For the LaFe_11.5Si_1.5, La_0.8Nd_0.2Fe_11.5Si_1.5, and LaFe_11.5Si_1.5C_0.2 compounds, the T_C were approximately 194 K, 188 K, and 232 K, respectively. Meanwhile, the maximum magnetic entropy changes (−ΔS_M) under a magnetic field change of 0–3 T were approximately 18.7 J/kg·K, 22.8 J/kg·K, and 16.4 J/kg·K, respectively. The T_C was mainly affected by the lattice constant. Furthermore, the −ΔS_M was mainly affected by the latent heat of the first-order magnetic transition. Full article

In 1991 the layered organic compound $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$ with a triangular lattice was synthesized for the first time. Although, originally, the focus was on the superconducting properties under pressure, this frustrated Mott insulator has been the most promising quantum-spin-liquid candidate for almost two decades, widely believed to host gapless spin excitations down to $T\to 0$. The recent observation of a spin gap rules out a gapless spin liquid with itinerant spinons and puts severe constraints on the magnetic ground state. This review evaluates magnetic, thermal transport, and structural anomalies around $T^{\star }=6$ K. The opening of a spin gap yields a rapid drop of spin susceptibility, NMR Knight shift, spin-lattice relaxation rate, and $\mu$-SR spin fluctuation rate, but is often concealed by impurity spins. The concomitant structural transition at $T^{\star }$ manifests in thermal expansion, THz phonons and ${}^{63}$Cu NQR relaxation. Based on the field dependence of $T^{\star }$, a critical field of 30–60 T is estimated for the underlying spin-singlet state. Overall, the physical properties are remarkably similar to those of spin-Peierls compounds. Thus, a strong case is made that the ‘6K anomaly’ in $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$ is the transition to a valence-bond-solid state and it is suggested that such a scenario is rather the rule than the exception in materials with strong magnetic frustration. Full article