Search Results (40)

We theoretically investigate the influence of uniaxial distortion on the stability of square skyrmion crystals, which are described as double-Q spin textures composed of two orthogonal spiral modulations, in noncentrosymmetric magnets. An effective spin model incorporating momentum-resolved frustrated exchange interactions and Dzyaloshinskii–Moriya (DM) interactions is analyzed using simulated-annealing calculations at low temperatures. The results reveal that uniaxial distortion drives a transformation from the double-Q square skyrmion crystal to a single-Q tilted conical spiral or vertical spiral state. The low-temperature phase diagrams further show that the stability region of the skyrmion crystal expands with increasing the magnitude of the DM interaction, making the phase more robust against the uniaxial anisotropy between exchange interactions parallel and perpendicular to the distortion axis. This study provides insight into how uniaxial strain and DM interactions cooperatively influence the formation and stability of skyrmion crystal phases in noncentrosymmetric magnetic systems. 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

Geometrically frustrated magnets exhibit exotic excitations due to competing interactions between spins. The spinel compound MgCr₂O₄, a three-dimensional Heisenberg antiferromagnet, hosts both spin-wave and spin-resonance modes, but the origin of its resonant excitations remains debated. Suppressing magnetic order via non-magnetic doping can help isolate these modes in neutron scattering studies. We synthesized Ga³⁺ and Cd²⁺-doped MgCr₂O₄ via solid-state reaction and analyzed their structure and magnetism. Ga³⁺ doping (0–20%) causes anomalous lattice shrinkage due to site disorder from Ga³⁺ occupying both Mg²⁺ and Cr³⁺ sites. Magnetically, Ga³⁺ doping drives the system from the antiferromagnetic order to a spin-glass state, fully suppressing magnetic ordering at 20% doping. In contrast, Cd²⁺ replaces only Mg²⁺, expanding the lattice and meantime inducing strong spin-glass behavior. At 10% Cd²⁺, long-range antiferromagnetic order is entirely suppressed. Thus, 10% Cd-doped MgCr₂O₄ offers an ideal platform to study the resonant magnetic excitations without any spin-wave interference. Full article

We investigate magnetic instabilities toward multiple-Q states in centrosymmetric bilayer triangular-lattice systems. By focusing on the interplay between the layer-dependent Dzyaloshinskii–Moriya interaction and layer-independent bond-dependent anisotropic interaction, both of which originate from the relativistic spin-orbit coupling, we construct a low-temperature phase diagram based on an effective spin model that also includes frustrated isotropic exchange interactions. Employing simulated annealing, we reveal the stabilization of three distinct double-Q phases in the absence of an external magnetic field, each characterized by noncoplanar spin textures with spatially modulated local scalar spin chirality. Under applied magnetic fields, we identify field-induced phase transitions among single-Q, double-Q, and triple-Q states, some of which exhibit a finite net scalar spin chirality indicative of topologically nontrivial order. These findings highlight centrosymmetric systems with sublattice-dependent Dzyaloshinskii–Moriya interactions as promising platforms for realizing a variety of multiple-Q spin textures. Full article

In this study, we investigate the presence of the Griffiths-like anomaly in the geometrically frustrated antiferromagnet ${\mathrm{HoBaCo}}_4$${\mathrm{O}}_{7+\delta }$ and globally its absence in ${\mathrm{ErBaCo}}_4$${\mathrm{O}}_{7+\delta }$, despite only small differences in the ionic radii, f-electron occupancy, and the corresponding crystal structures of the ${\mathrm{Ho}}^{3+}$ and ${\mathrm{Er}}^{3+}$-members. Previous studies have identified the Griffiths phase in the Dy-analog, ${\mathrm{DyBaCo}}_4$${\mathrm{O}}_{7+\delta }$, suggesting certain inherent features of this class of materials that regularly give rise to such anomalies. To explore the curious disappearance of such an anomalous feature in ${\mathrm{ErBaCo}}_4$${\mathrm{O}}_{7+\delta }$, we prepared a series of compounds with varying compositions ${\mathrm{Ho}}_{1-x}$${\mathrm{Er}}_x$${\mathrm{BaCo}}_4$${\mathrm{O}}_{7+\delta }$ ($0\le x\le 1$) and systematically studied the evolution of various physical properties as a function of Er-doping. Our experimental studies, including X-ray diffraction (XRD), magnetic, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), heat capacity, and muon spin relaxation spectroscopy ($\mu$SR spectroscopy), revealed that while the Griffiths-like anomaly indeed disappears with doping at the macroscopic level, signatures of inhomogeneity are retained in ${\mathrm{ErBaCo}}_4$${\mathrm{O}}_{7+\delta }$ too, at least at the local level. Overall, our results highlight the significant role of ionic radius and local structural distortions in stabilizing the Griffiths phase in this class of systems. Full article

Frustrated magnetic systems arising in geometrically constrained lattices represent rich platforms for exploring unconventional phases of matter, including fractional magnetization plateaus, incommensurate orders and complex domain dynamics. However, determining the microscopic spin configurations that stabilize such phases is a key challenge, especially when in-plane and out-of-plane spin components coexist and compete. Here, we combine neutron scattering and magnetic susceptibility experiments with simulations to investigate the emergence of field-induced fractional plateaus and the related criticality in a frustrated magnet holmium tetraboride (HoB₄) that represents the family of rare earth tetraborides that crystalize in a Shastry–Sutherland lattice in the $ab$ plane. We focus on the interplay between classical and quantum criticality near phase boundaries, as well as the role of material defects in the stabilization of the ordered phases. We find that simulations using classical annealing can explain certain observed features in the experimental Laue diffraction and the origin of multiple magnetization plateaus. Our results show that defects and out-of-plane interactions play an important role and can guide the route towards resolving microscopic spin textures in highly frustrated magnets. Full article

Layered chalcogenides are interesting from the point of view of the formation of two-dimensional magnetic systems for relevant applications in spintronics. High-spin Mn²⁺ or Fe³⁺ cations with five unpaired electrons are promising in the search for compounds with interesting magnetic properties. In this study, a new layered modification of the Mn₂In₂Se₅ compound from the A₂B₂X₅ family (“225”) was synthesized and investigated. A phase transition to the polymorph with primitive trigonal lattice was recorded at a temperature of 711 °C, which was confirmed by simultaneous thermal analysis, X-ray powder diffraction at elevated temperatures, and sample annealing and quenching. The stability of Mn₂In₂Se₅ in air at high temperatures was investigated by thermal gravimetric analysis and powder X-ray diffraction. The new polymorph of Mn₂In₂Se₅ crystallizes in the Mg₂Al₂Se₅ structure type, as revealed by the Rietveld refinement against powder X-ray diffraction data. The crystal structure can be viewed as a close-packing of Se anions, in which indium and manganese cations are enclosed inside tetrahedral and octahedral voids, respectively, according to the A_MnB_InCB_InC_MnA… sequence. Magnetization measurements reveal an antiferromagnetic-like transition at a temperature of 6.3 K. The same magnetic properties are reported in the literature for the low-temperature R-centered trigonal polymorph. An approximation by the modified Curie–Weiss law yields a significant ratio of |θ|/T_N = 28, which indicates strong magnetic frustration. Full article

We present first-principles results on the electronic and magnetic properties of the cubic bulk $\beta$-phase of ${\mathrm{Fe}}_2{\mathrm{O}}_3$. Given that all Fe–Fe magnetic couplings are expected to be antiferromagnetic within this high-symmetry crystal structure, the system may exhibit some signature of magnetic frustration, making it challenging to identify its magnetic ground state. We have analyzed the possible magnetic phases of the $\beta$-phase, among which there are ferrimagnets, altermagnets, and Kramers antiferromagnets. While the $\alpha$-phase is an altermagnet and the $\gamma$-phase is a ferrimagnet, we conclude that the magnetic ground state for the bulk $\beta$-phase of ${\mathrm{Fe}}_2{\mathrm{O}}_3$ is a Kramers antiferromagnet. Moreover, we find that close in energy, there is a bulk d-wave altermagnetic phase. We report the density of states and the evolution band gap as a function of the electronic correlations. For suitable values of the Coulomb repulsion, the system is a charge-transfer insulator with an indirect band gap of 1.5 eV. More in detail, the unit cell of the $\beta$-phase is composed of $8{\mathrm{Fe}}_a$ atoms and $24{\mathrm{Fe}}_b$ atoms. The $8{\mathrm{Fe}}_a$ atoms lie on the corner of a cube, and their magnetic ground state is a G-type. This structural phase is composed of zig-zag chains ${\mathrm{Fe}}_a‐{\mathrm{Fe}}_b‐{\mathrm{Fe}}_a‐{\mathrm{Fe}}_b$ with spin configuration ↑-↑-↓-↓ along the 3 directions such that for every ${\mathrm{Fe}}_a$ atoms there are $3{\mathrm{Fe}}_b$ atoms. As the opposite to the $\gamma$-phase, the magnetic configuration between the first neighbor of the same kind is always antiferromagnetic while the magnetic configuration between ${\mathrm{Fe}}_a$ and ${\mathrm{Fe}}_b$ is ferro or antiferro. In this magnetic arrangement, first-neighbor interactions cancel out in the mean-field estimation of the Néel temperature, leaving second-neighbor magnetic exchanges as the primary contributors, resulting in a Néel temperature lower than that of other phases. Our work paves the way toward the ab initio study of nanoparticles and alloys for the $\beta$-phase of ${\mathrm{Fe}}_2{\mathrm{O}}_3$. Full article

A binuclear cobalt–radical complex formed by the reaction of Co(hfac)₂·2H₂O (hfac = hexafluoroacetylacetonate) with the 2,2-bis(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolinyl) biradical (BR) has been synthesized. The complex {(hfac)Co^II(BN)Co^II(hfac)} crystallizes in the triclinic space group P$\overline{1}$ :  C₃₄H₂₈Co₂F₂₄N₄O₁₂, a = 11.1513(5) Å, b = 12.8362(7) Å, c = 18.2903(8) Å, α = 103.061(1)°, β = 100.898(2)°, γ = 102.250(1)°, Z = 2. The compound consists of two non-equivalent pseudo-octahedral Co^II ions, each bearing two hfac ancillary ligands bridged by the tetradentate bis-nitroxide (BN). The temperature dependence of the magnetic susceptibility indicates a strong antiferromagnetic exchange between each of the Co²⁺ ions and the nitroxyl biradical, as well as between the spins within the bridging ligand, forming a spin-frustrated system. Micro-squid investigations, performed on a single crystal of {(hfac)Co^II(BN)Co^II(hfac)}, reveal a peculiarity of the M(H) graph at temperatures below 0.4 K displaying a step that is a result of ground and first excited levels mixing by the applied magnetic field due to a small energy gap between them, as inferred from ab initio calculation. The latter was also carried out for two models of mononuclear Co²⁺ complexes in order to obtain a set of initial parameters for fitting the experimental magnetic curves using the Phi program. Moreover, direct CAS(12,10)/def2-TZVP calculations of the magnetic dependences χT(T) and M(H) were performed, which satisfactorily reproduced the experimental ones. Full article

Spin 1/2 quantum spin chains represent the prototypical model for coupled two-level systems. Consequently, they offer a fertile playground for both fundamental and technological applications ranging from the theory of thermalization to quantum computation. Recently, it has been shown that interesting phenomena are associated to the boundary conditions imposed on the quantum spin chains via the so-called topological frustration. In this work, we analyze the effects of such frustration on a few-spin system, with a particular focus on the strong even–odd effects induced in the ground-state energy. We then implement a topologically frustrated quantum spin chain on a quantum computer to show that our predictions are visible on current quantum hardware platforms. Full article

Layered chalcogenides containing 3d transition metals are promising for the development of two-dimensional nanomaterials with interesting magnetic properties. Both mechanical and solution-based exfoliation of atomically thin layers is possible due to the low-energy van der Waals bonds. In this paper, we present the synthesis and crystal structures of the Mn₂Ga₂S₅ and Mn₂Al₂Se₅ layered chalcogenides. For Mn₂Ga₂S₅, we report magnetic properties, as well as the exfoliation of nanofilms and nanoscrolls. The synthesis of both polycrystalline phases and single crystals is described, and their chemical stability in air is studied. Crystal structures are probed via powder X-ray diffraction and high-resolution transmission electron microscopy. The new compound Mn₂Al₂Se₅ is isomorphous with Mn₂Ga₂S₅ crystallizing in the Mg₂Al₂Se₅ structure type. The crystal structure is built by the ABCBCA sequence of hexagonal close-packing layers of chalcogen atoms, where Mn²⁺ and Al³⁺/Ga³⁺ species preferentially occupy octahedral and tetrahedral voids, respectively. Mn₂Ga₂S₅ exhibits an antiferromagnetic-like transition at 13 K accompanied by the ferromagnetic hysteresis of magnetization. Significant frustration of the magnetic system may yield spin-glass behavior at low temperatures. The exfoliation of Mn₂Ga₂S₅ layers was performed in a non-polar solvent. Nanolayers and nanoscrolls were observed using high-resolution transmission electron microscopy. Fragments of micron-sized crystallites with a thickness of 70–100 nanometers were deposited on a glass surface, as evidenced by atomic force microscopy. Full article

Inspired in pyrochlore materials presenting residual entropy and featuring collective excitation behaving like emergent monopoles, geometrically frustrated arrays of nanomagnets, denominated artificial spin ices (ASIs), were proposed as an interesting platform to investigate such excitation at room temperature. However, in such artificial systems, emergent magnetic monopoles lack the same freedom present their natural counterpart, once energetic strings connecting opposite magnetic charges arise. In this work, we aim to experimentally investigate the proposed degeneracy obtained in connected square arrays of ASIs, a characteristic that allows a reduction in the string connecting monopole–antimonopole pairs in regular non-connected ASIs and could represent an important development for technological applications of connected nanomagnets. As in general those systems are developed by nanofabrication processes involving expensive and time-consuming physical vapour deposition techniques, we also present a new nanofabrication route using an electrodeposition technique for permalloy growth in different lattice geometries as an alternative for fast and low-cost ASI system production. Full article

Measurements of specific heat and magnetization in single crystals were used to map out the magnetic phase diagram of Gd_1−xEr_xB₄ (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. Full article

One-dimensional (1D) magnetic systems offer rich phenomena in the quantum limit, proving more chemically accessible than zero-dimensional or higher-dimensional frameworks. Single-walled carbon nanotubes (SWCNT) have recently been used to encapsulate trimetric nickel(II) acetylacetonate [Nanoscale, 2019, 11, 10615–10621]. Here, we investigate the magnetization on spin chains based on nickel trimers by Matrix Product State (MPS) simulations. Our findings reveal plateaus in the exchange/magnetic-field phase diagram for three coupling configurations, showcasing effective dimeric and trimeric spin-ordering with similar or staggered entanglement across chains. These ordered states allow the qubit-like tuning of specific local magnetic moments, exhibiting disengagement or uniform coupling in entanglement plateaus. This behavior is consistent with the experimental transition from frustrated (3D) to non-frustrated (1D) molecules, corresponding to large and smaller SWCNT diameters. Our study offers insights into the potential of 1D-confined trimers for quantum computation, extending beyond the confinement of trimetric nickel-based molecules in one dimension. Full article

One-dimensional spin crossover (SCO) solids that convert between the low spin (LS) and the high spin (HS) states are widely studied in the literature due to their diverse thermal and optical characteristics which allow obtaining many original behaviors, such as large thermal hysteresis, incomplete spin transitions, as multi-step spin transitions with self-organized states. In the present work, we investigate the thermal behaviors of a system of two elastically coupled 1D mononuclear chains, using the electro-elastic model, by including an elastic frustration in the nearest neighbors (nn) bond length distances of each chain. The chains are made of SCO sites that are coupled elastically through springs with their nn and next-nearest neighbors. The elastic interchain coupling includes diagonal springs, while the nn inter-chain distance is fixed to that of the high spin state. The model is solved using MC simulations, performed on the spin states and the lattice distortions. When we only frustrate the first chain, we found a strong effect on the thermal dependence of the HS fraction of the second chain, which displays an incomplete spin transition with a significantly lowered transition temperature. In the second step, we frustrate both chains by imposing different frustration rates. Here, we demonstrate that for high frustration values, the thermal dependence of the total HS fraction exhibits multi-step spin transitions. The careful examination of the spin state structures in the plateau regions showed the coexistence of special dimerized ferro–antiferro patterns of type LL-HH-LL-HH along the first chain and HH-LL-HH-LL (H=HS and L=LS) along the second one, revealing that the two chains are antiferro-elastically coupled. This type of spatial modulation of the spin state and bond length distances is very attractive because it anticipates the possible existence of periodic structures in 2D lattices, made of alternate 1D SCO strings with HLHLHL structures, coupled in the ferro-like fashion along the interchain direction. Full article