Zintl phase EuIn2As2 has garnered growing attention as an axion insulator candidate, triggered by the identification of a commensurate double-Q broken-helix state in previous studies, however, its periodicity and symmetry remain subjects of debate. Here, we perform resonant x-ray scattering experiments on EuIn2As2, revealing an incommensurate nature of the broken-helix state, where both the wave number and the amplitude of the helical modulation exhibit systematic sample dependence. Furthermore, the application of an in-plane magnetic field brings about a fanlike state that appears to preserve the double-Q nature, which might be attributed to multiple-spin interactions in momentum space. We propose that the itinerant character of EuIn2As2, most likely induced by Eu deficiency, gives rise to the helical modulation and impedes the realization of a theoretically predicted axion state with the collinear antiferromagnetic order.

The two-dimensional triangular lattice (TAL) is a model system of magnetic frustration and competing interactions, where skyrmion spin vortices can be induced by a vertical magnetic field B. We target the binary compound GdGa2 with an undistorted TAL of Gd3+ Heisenberg moments. At higher temperature (T > 5 K, B = 0, phase II), we reveal the cycloidal spin textures in GdGa2 via resonant elastic X-ray scattering (REXS). Further, a transition with strong magneto- elastic response occurs when cooling into the zero-field ground state (T < 5 K, phase I). We also report the thermodynamic phase boundaries of B-induced magnetic A-phases, which are suppressed by an in-plane magnetic field and which have enhanced resistivity due to the partial opening of a charge gap. In analogy to Gd2PdSi3 and GdRu2Si2, these phases may represent a superposition of various cycloids, possibly a N & eacute;el skyrmion lattice. Our work lays the basis for further studies of the magnetic phase diagram of GdGa2.

Nodal-line semimetals are a class of topological materials hosting one dimensional lines of band degeneracy. Kramers nodal-line (KNL) metals/semimetals have recently been theoretically recognized as a class of topological states inherent to all non-centrosymmetric achiral crystal lattices. The electronic structure of candidate KNL semimetal YAuGe is investigated by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillations as well as by density functional theory (DFT) calculations. DFT has revealed that YAuGe hosts KNLs on the Γ-A-L-M plane of the Brillouin zone, that are protected by the time reversal and mirror-inversion symmetries. Through ARPES and quantum oscillations, signatures of hole bands enclosing the Γ point are identified, and the observed splitting of quantum oscillation frequency with angle is attributed to spin-orbit-coupling-induced band splitting away from the KNLs. Furthermore, it is shown that the degeneracy of the nodal lines along the Γ-A line is lifted by the time-reversal-symmetry breaking when the Y is substituted by magnetic R ions (R = rare earth). This becomes a source of Berry curvature and contributes to the anomalous Hall effect in magnetic RAuGe. These findings establish RAuGe as a new class of KNL semimetals offering significant potential for engineering of anomalous magnetotransport properties via magnetic rare-earth substitution.

We report on a comprehensive thermodynamic study of a quasi-two-dimensional (quasi-2D) quantum magnet Cu-2(OH)(3)Br which in the 2D layer can be viewed as strongly coupled alternating antiferromagnetic and ferromagnetic chains. In an applied magnetic field transverse to the ordered spins below TN = 9.3 K, a field- induced phase transition from the 3D ordered to a disordered phase occurs at Bc = 16.3 T for the lowest temperature, which is featured by an onset of a one-half plateaulike magnetization. By performing quantum Monte Carlo simulations of the relevant 2D model, we find that the plateaulike magnetization corresponds to a partial symmetry restoration and the full polarization in the ferromagnetic chains. Our numerical simulations also show that the magnetization saturation occurs with full symmetry restoration at a much higher field of Bs similar or equal to 95 T, corresponding to a 1D quantum phase transition in the antiferromagnetic chains. We argue that the experimentally observed field-induced phase transition at Bc follows from the partial symmetry restoration and the concomitant dimensional reduction.

The exploration of nontrivial magnetic states induced by strong spin-orbit interaction is a central topic of frustrated magnetism. Numerous studies have been conducted on rare-earth-based magnets and 4d/5d transition metal compounds. These are mostly described by an effective spin Seff = 1/2 for the Kramers doublet of the lowest crystal-electric-field levels. The variety of magnetic orderings can be greatly enhanced when magnetic dipolar moments intertwined with multipolar degrees of freedom, which are described by higher-rank tensors and often require the magnetic ions to have Seff > 1/2. Here, using synchrotron x-ray diffraction near the Dy L3 edge, we unveil a canted antiferromagnetic ground state arising from a quasi-quartet (Seff = 3/2) of 4f electrons in a triangular-lattice (TL) rare-earth intermetallics DyAuGe. The magnetic moment and electric-quadrupole moment are closely interlocked and a noncollinear magnetic-dipole alignment is induced by antiferroic electric-quadrupole (AFQ) ordering in the TL layers. The AFQ order is suppressed by an in-plane magnetic field, leading to the metamagnetic transition to a collinear up-up-down magnetic state. These findings offer insights into the emergence of nontrivial magnetic states in frustrated TL systems with Seff > 1/2.

We report low-temperature powder X-ray diffraction and neutron scattering studies on breathing pyrochlore magnets CuMCr4S8 (M = Al, Ga), which undergo a magnetic transition at T N 21 and 31 K for M = Al and Ga, respectively. X-ray diffraction reveals that the magnetic transition accompanies a structural transition from cubic F43m to polar orthorhombic Imm2 symmetry for both compounds, with larger distortion observed for M = Ga at low temperatures. Neutron scattering reveals incommensurate magnetic modulation Q = ( q IC , 0.5, 0) in the orthorhombic setting, where q IC 0.39 and 0.31 for M = Al and Ga, respectively. We propose that a cycloid-type order, rather than a proper screw- type one, is one of the candidate magnetic structures for both compounds. The spin-lattice coupling as well as magnetic frustration should play an important role in determining the ground state, as suggested by the correlation between the local spin configuration and the Cr-Cr bond lengths. CuMCr4S8 potentially offers a platform to explore magnetoelectric effects arising from the helimagnet driven electric polarity.

We report hard x-ray photoelectron spectroscopy on SrFeO3 which is one of the classical conducting transition-metal oxides with a noncollinear magnetic structure. The obtained spectra show a detailed charge-transfer (CT) satellite structure, the Fe 2p3/2 main peak exhibits multiplet splitting, and the deterioration signs present in previous reports are absent here, allowing for a better determination of its intrinsic electronic structure. The results are well described by a FeO6 cluster model with a charge-transfer energy of about −1.0 eV, confirming the values obtained in the previous works. The negative CT energy indicates that the electronic configuration of the tetravalent Fe is d5L rather than d4 where L represents an O 2p hole. The small spectral weight observed at the Fermi level indicates the correlated metallic state with localized Fe 3d electrons and mobile O 2p holes which are governed by a large d−d Coulomb interaction and negative CT energy. Published by the American Physical Society 2024

Honeycomb-lattice antiferromagnets have attracted wide attention for the exploration of exotic heat transport and their interplay with magnetic excitations. In this work, we have revealed a contrasting behavior in the magnetothermal conductivity (MTC) between two Co-based honeycomb-lattice magnets Co4M2O9 (M = Nb, Ta), despite their identical lattice structures and quite similar magnetism. Co4Ta2O9 exhibits enhanced MTC of about 550 % at 9 T of an in-plane magnetic field, comparable to other honeycomb magnets, while MTC for Co4Nb2O9 reaches only similar to 30 %. This marked difference is ascribed to distinct features in the field-induced evolution of magnetic excitations that resonantly scatter phonons. This finding sheds light on the implicit impacts of nonmagnetic ions on thermal transport and hints at the potential for broad heat-transport tunability while preserving magnetism and lattice structures.

We report the magnetic, magnetoelastic, and magnetotransport properties of single crystals of polar magnets reveal multistep metamagnetic transitions for the c-axis magnetic field (H H c) for DyAuGe and HoAuGe, suggesting magnetic frustration in the triangular lattice of R ions. The magnetic phase diagrams have clarified a close connection between the magnetoelastic property and the emergence of the intermediate metamagentic phase. The magnetic-field dependence of the resistivity and Hall resistivity reveal the semimetallic transport dominated by hole-type carriers, consistent with the behavior in a nonmagnetic analog YAuGe. We also identify a signature of an anomalous Hall effect (AHE) proportional to the field-induced magnetization in R = Dy, Ho, and Gd. GdAuGe shows magnetic and transport behavior as reported in a previous study using Bi-flux grown single crystals, while the self-flux grown crystal shows larger magnetoresistance (<^>345%, at 1.8 K and 9 T) due to higher hole-type carrier mobility [<^>6400 cm2/(V s)]. Using the two-band model analysis considering the mobility change during the magnetization process, we extract the anomalous Hall conductivity: <^>1200 and <^>530 S/cm for R = Dy and Ho, respectively, at 1.8 K with 9 T for H H c. The magnitude of conductivity suggests a contribution of intrinsic origin, possibly related to the Berry curvature in the electron bands induced by the time-reversal symmetry breaking and the polar lattice.

Transition-metal atoms with d electrons sometimes form clusters in crystals, which significantly affect the physical properties. Such a cluster formation frequently accompanies a change in the crystal system, leading to the presence of domains with different crystal orientations. In particular, the cubic symmetry is rarely retained after the cluster formation. Here, we identify a cubic-to-cubic phase transition in lacunar spinel GaNb4Se8, where the change in the lattice parameter is less than 0.0001%. Each Nb3.25+ tetramer with seven 4d electrons is distorted into an Nb3+ trimer and an Nb4+ monomer induced by charge disproportionation among Nb ions. While the Nb3+ trimer with six 4d electrons forms spin singlets in the sigma-bonding orbitals for three Nb-Nb bonds, a localized S = 1/2 spin remains on the Nb4+ ion. Furthermore, a local electric dipole moment is induced along the 3-fold rotation axis of each distorted tetramer by the cluster rearrangement. The electric dipole moments are regularly arranged to maintain cubic symmetry, giving rise to chiral order.

We present a comprehensive study of the magnetoelastic properties of the Ising pyrochlore oxide Ho2Ti2O7, known as spin ice, by means of high-field magnetostriction measurements and numerical calculations. When a magnetic field is applied along the crystallographic ( 111 ) axis, the longitudinal magnetostriction exhibits a broad maximum in the low-field regime around 30 T, followed by a dramatic lattice contraction due to crystal-field (CF) level crossing at B cf 65 T. The transverse magnetostriction exhibits a contrasting behavior, highlighting the anisotropic nature of the CF striction. By applying a magnetic field at varying sweep rates, we identify distinct timescales of spin dynamics that are relevant to monopole formation and annihilation, as well as CF-phonon dynamics. Our mean-field calculations, based on a point-charge model, successfully reproduce the overall magnetostriction behavior, revealing the competition between the exchange striction and CF striction. A signature of the CF level crossing is also observed through adiabatic magnetocaloric-effect measurements, consistent with our magnetostriction data.

The spin-1/2 Heisenberg antiferromagnet on an anisotropic triangular lattice (ATL) is an archetypal spin system hosting exotic quantum magnetism and dimensional crossover. However, the progress in experimental research on this field has been limited due to the scarcity of ideal model materials. Here, we show that rhenium oxyhalides $A_{3}$ReO$_{5}X_{2}$, where spin-1/2 Re$^{6+}$ ions form a layered structure of ATLs, allow for flexible chemical substitution in both cation $A^{2+}$ ($A$ = Ca, Sr, Ba, Pb) and anion $X^{-}$ ($X$ = Cl, Br) sites, leading to seven synthesizable compounds. By combining magnetic susceptibility and high-field magnetization measurements with theoretical calculations using the orthogonalized finite-temperature Lanczos method, we find that the anisotropy $J'/J$ ranges from 0.25 to 0.45 depending on the chemical composition. Our findings demonstrate that $A_{3}$ReO$_{5}X_{2}$ is an excellent platform for realizing diverse effective spin Hamiltonians that differ in the strength of the anisotropy $J'/J$ as well as the relevance of perturbation terms such as the Dzyaloshinskii-Moriya interaction and interlayer exchange coupling.

An anomalously high valence state sometimes shows up in transition-metal oxide compounds. In such systems, holes tend to occupy mainly the ligand p orbitals, giving rise to interesting physical properties such as superconductivity in cuprates and rich magnetic phases in ferrates. However, no one has ever observed the distribution of ligand holes in real space. Here, a successful observation of the spatial distribution of valence electrons in cubic perovskite SrFeO3 by high-energy X-ray diffraction experiments and precise electron density analysis using a core differential Fourier synthesis method is reported. A real-space picture of ligand holes formed by the orbital hybridization of Fe 3d and O 2p is revealed. The anomalous valence state in Fe is attributed to the considerable contribution of the ligand hole, which is related to the metallic nature and the absence of Jahn-Teller distortions in this system.

We investigate the magnetoelectric property of a quasi-two-dimensional magnet Ba2FeSi2O7. An in-plane magnetic field induces a phase transition that is accompanied by only a small magnetization anomaly but apparent electric polarization change. Our numerical simulation using a mean-field Hamiltonian reveals that the transition is associated with a change in the magnetic wave vector from k = (1 0 1=2) to (1 0 0), which originates from the competition between the intralayer Dzyaloshinskii-Moriya (DM) interaction and the interlayer antiferromagnetic exchange interaction. We determine the sign of the DM interaction from the magnetoelectric behavior in the k = (1 0 0) state. We find that the sign of the DM interaction is opposite to that in the case of Ba2CoGe2O7, which is another well-studied isostructural multiferroic material belonging to the melilite family of compounds.

We investigate the crystallographic and magnetic properties of a chromium-based thiospinel CuGaCr$_{4}$S$_{8}$. From a synchrotron x-ray diffraction experiment and structural refinement, Cu and Ga atoms are found to occupy the tetrahedral $A$-sites in an alternate way, yielding breathing pyrochlore Cr network. CuGaCr$_{4}$S$_{8}$ undergoes a magnetic transition associated with a structural distortion at 31 K in zero magnetic field, indicating that the spin-lattice coupling is responsible for relieving the geometrical frustration. When applying a pulsed high magnetic field, a sharp metamagnetic transition takes place at 40 T, followed by a 1/2-magnetization plateau up to 103 T. These phase transitions accompany dielectric anomalies, suggesting the presence of helical spin correlations in low-field phases. The density-functional-theory calculation reveals that CuGaCr$_{4}$S$_{8}$ is dominated by antiferromagnetic and ferromagnetic exchange couplings within small and large tetrahedra, respectively, in analogy with CuInCr$_{4}$S$_{8}$. We argue that $A$-site-ordered Cr thiospinels serve as an excellent platform to explore diverse magnetic phases along with pronounced magnetoelastic and magnetodielectric responses.

We study structural and magnetic properties of rare-earth based semimetals RAuGe (R = Y, Gd-Tm, and Lu) using flux-grown single crystals. These compounds belong to the noncentrosymmetric polar space group P63mc. We confirm the systematic structural evolution at room temperature as a function of ionic radius of rare earths to clarify the isopointal crossover between two polar structures: three-dimensional LiGaGe-type and quasi-two-dimensional NdPtSb-type. Magnetism shows a characteristic anisotropy in reasonable agreement with the crystal electric field (CEF) theory; the easy-plane-type anisotropy for R = Tb and Dy turns into the Ising-type anisotropy for R = Er and Tm. We evaluate the CEF parameters based on the Stevens operators to reasonably reproduce the temperature dependence of magnetic susceptibilities and specific heat for RAuGe (R = Tb-Tm). The estimated energy scale of the Ising gap (~ 11 meV) in TmAuGe is consistent with an excitation observed in an inelastic neutron scattering experiment. These findings suggest an opportunity for interplay between conduction electrons and nontrivial spin structures in the family of magnetic polar semimetals RAuGe.

The mutual coupling of spin and lattice degrees of freedom is ubiquitous in magnetic materials and potentially creates exotic magnetic states in response to the external magnetic field. Particularly, geometrically frustrated magnets serve as a fertile playground for realizing magnetic superstructure phases. Here, we observe an unconventional two-step magnetostructural transition prior to a half-magnetization plateau in a breathing pyrochlore chromium spinel by means of state-of-the-art magnetization and magnetostriction measurements in ultrahigh magnetic fields available up to 600 T. Considering a microscopic magnetoelastic theory, the intermediate-field phase can be assigned to a magnetic superstructure with a three-dimensional periodic array of 3-up-1-down and canted 2-up-2-down spin molecules. We attribute the emergence of the magnetic superstructure to a unique combination of the strong spin-lattice coupling and large breathing anisotropy.

The centrosymmetric tetragonal itinerant magnet EuAl$_{4}$ exhibits an intricate magnetic phase diagram including rhombic and square skyrmion-lattice (SkL) phases in the external magnetic field. Here, we report a multi-axis dilatometric investigation of EuAl$_{4}$ by means of a newly designed fiber-Bragg-grating technique complemented by a resonant x-ray scattering experiment, revealing anisotropic magnetostriction and magnetovolume effect associated with successive phase transitions. The rhombic and square SkL phases are found to possess $\sim$0.10% and $\sim$0.03% orthorhombic structural distortion within the $ab$ plane, respectively. We propose that the coupling between the spin system and the lattice deformation should be essential for the structural instability in EuAl$_{4}$, yielding a rich variety of topological spin textures with spontaneous rotational-symmetry breaking as well as a potential controllability of the SkL phases by uniaxial stress or pressure.

Bose–Einstein condensation (BEC) in quantum magnets, where bosonic spin excitations condense into ordered ground states, is a realization of BEC in a thermodynamic limit. Although previous magnetic BEC studies have focused on magnets with small spins of S ≤ 1, larger spin systems potentially possess richer physics because of the multiple excitations on a single site level. Here, we show the evolution of the magnetic phase diagram of S = 3/2 quantum magnet Ba 2 CoGe 2 O 7 when the averaged interaction J is controlled by a dilution of magnetic sites. By partial substitution of Co with nonmagnetic Zn, the magnetic order dome transforms into a double dome structure, which can be explained by three kinds of magnetic BECs with distinct excitations. Furthermore, we show the importance of the randomness effects induced by the quenched disorder: we discuss the relevance of geometrical percolation and Bose/Mott glass physics near the BEC quantum critical point.

The link between the metamagnetic transition and novel spin-triplet superconductivity of UTe2 was discussed thermodynamically through magnetostriction measurements in a pulsed-magnetic field. We revealed a discontinuous magnetostriction across the metamagnetic transition at p0Hm 35 T for the applied magnetic fields along the crystallographic b axis in the orthorhombic structure. The resultant volume magnetostriction of AV/V ???5.9 ?? 10-4 gives the initial pressure dependence of Hm by employing the Clausius???Clapeyron???s equation, which agrees with previous pressure experiments. Further, significant anisotropic magnetostriction (AMS), derived by subtracting the averaged linear magnetostriction, was revealed. Contrary to the weakly field-dependent AMS along the a axis, those along the b and c axes show strong field dependences with a similar magnitude but with opposite signs, indicating its lattice instability. The relationship between characteristic energy scales of magnetic fields and temperatures was discussed in terms of the Gr??neisen parameters compared to the other f-electron systems. The volume shrinkage in UTe2 at Hm, contrary to the volume expansion in typical heavy fermion metamagnets, pushes to invoke the link with the valence instability related to the itinerant-localized dual nature of the U magnetism.