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Honeycomb-lattice antiferromagnets have attracted wide attention for exploration of exotic heat transport and their interplay with magnetic excitations. In this work, we have revealed a contrasting behavior in the magneto-thermal 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 ~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 implicit impacts of non-magnetic ions on thermal transport, and hints at the potential for broad heat-transport tunability while preserving magnetism and lattice structures., Comment: 7 pages, 4 figures, doi.org/10.1103/PhysRevB.110.L041116

Frustrated rare-earth-based intermetallics provide a promising platform for emergent magnetotransport properties through exchange coupling between conduction electrons and localized rare-earth magnetic moments. Metamagnetism, the abrupt change of magnetization under an external magnetic field, is a signature of first-order magnetic phase transitions; recently, metamagnetic transitions in frustrated rare earths intermetallics have attracted interest for their accompanying nontrivial spin structures (e.g. skyrmions) and associated non-linear and topological Hall effects. Here, we present metamagnetism-induced Hall anomalies in single-crystalline ErGa$_2$, which recalls features arising from the topological Hall effect but wherein the strong Ising type anisotropy of Er moments prohibit noncoplanar spin structures. We show that the observed anomalies are neither due to anomalous Hall effect nor topological Hall effect, instead, can be accounted for via 4f-5d interactions which produce a band-dependent mobility modulation. This leads to a pronounced multiband Hall response across the magnetization process -- a metamagnetic multiband Hall effect that resembles a topological-Hall-like response but without nontrivial origins. The present findings may be of general relevance in itinerant metamagnetic systems regardless of coplanar/non-coplanar nature of spins and are important for the accurate identification of Hall signals due to emergent magnetic fields., Comment: Main 15 pages, 4 figures, Supple 22 pages, 9 figures, 1 Table

We report the magnetic, magnetoelastic, and magnetotransport properties of single crystals of polar magnets $R$AuGe ($R=$ Dy, Ho, and Gd), grown by Au-Ge self-flux. Magnetization and magnetostriction measurements reveal multi-step metamagnetic transitions for the $c$-axis magnetic field ($H\parallel c$) 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 analogue 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 ($\sim$ 345\%, at 1.8 K and 9 T) due to higher hole-type carrier mobility ($\sim$ 6400 cm$^2$/Vs). Using the two-band model analysis considering the mobility change during the magnetization process, we extract the anomalous Hall conductivity: $\sim 1200$ S/cm and $\sim 530$ S/cm for $R=$ Dy and Ho, respectively, at 1.8 K with 9 T for $H\parallel c$. 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., Comment: 11 pages, 9 figures

Zintl phase EuIn$_{2}$As$_{2}$ has garnered growing attention as an axion insulator candidate, triggered by the identification of a commensurate double-${\mathbf Q}$ broken-helix state in previous studies, however, its periodicity and symmetry remain subjects of debate. Here, we perform resonant x-ray scattering experiments on EuIn$_{2}$As$_{2}$, 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 broken-fanlike state preserving the double-${\mathbf Q}$ nature, which could be attributed to multiple-spin interactions in momentum space. We propose that the itinerant character of EuIn$_{2}$As$_{2}$, possibly 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., Comment: 6 pages, 5 figures, SM: 9 pages, 6 figures

Exploration of nontrivial magnetic states induced by strong spin-orbit interaction is a central topic of frustrated magnetism. Extensive studies are concentrated on rare-earth-based magnets and 4d/5d transition metal compounds, which are mostly described by an effective spin $S_{\text{eff}} = 1/2$ for the Kramers doublet of the lowest crystal-electric-field levels. Variety of magnetic orderings may 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 with $S_{\text{eff}} > 1/2$. Here, our synchrotron x-ray diffraction near the Dy $L_3$ edge has unveiled a canted antiferromagnetic ground state arising from a quasi-quartet ($S_{\text{eff}} = 3/2$) of 4f electrons in a triangular-lattice (TL) rare-earth intermetallics DyAuGe. Magnetic moment and electric-quadrupole moment are closely interlocked and noncollinear magnetic-dipole alignment is induced by antiferroic electric-quadrupole (AFQ) ordering in the TL layers. The correlation between the AFQ and canted magnetic structures is further confirmed by phase transitions in an in-plane magnetic field. These findings offer insights into the emergence of nontrivial magnetic states in frustrated TL systems described beyond the $S_{\text{eff}} = 1/2$., Comment: 12 pages, 5 figures

The magnetization and electric polarization of a polar antiferromagnet Fe$_2$Mo$_3$O$_8$ are studied up to 66 T for spin-saturation magnetic fields applied along the polar axis. The magnetization process at 1.4 K exhibited multistep structures below the saturation field of 65 T. The electric polarization along the polar axis exhibits a similar multistep behavior with a total change of 1.2 $\rm{\mu}C/cm^{2}$. A combined triangular-lattice antiferromagnetic model with strong Ising-type spin anisotropy reproduces this multistep magnetoelectric (ME) effect. The exchange striction mechanism explains the remarkable ME response in the two sub-lattice type-I multiferroic materials. These results and interpretation demonstrate a method for realizing multistage magnetoelectric effects in hybrid spin systems., Comment: 10 pages, 5 figures

Hysteretic behaviour accompanies any first-order phase transition, forming a basis for many applications. However, its quantitative understanding remains challenging, and even a qualitative understanding of pronounced hysteresis broadening at low temperature, which is often observed in magnetic-field-induced first-order phase transition materials, is unclear. Here, we show that such pronounced hysteresis broadening emerges if the phase-front velocity during the first-order phase transition exhibits an activated behaviour as a function of both temperature and magnetic field. This is demonstrated by using real-space magnetic imaging techniques, for the magnetic-field-induced first-order phase transition between antiferromagnetic and ferrimagnetic phases in (Fe$_{0.95}$Zn$_{0.05}$)$_{2}$Mo$_{3}$O$_{8}$. When combined with the Kolmogorov-Avrami-Ishibashi model, the observed activated temperature- and field-dependences of the growth velocity of the emerging antiferromagnetic domain quantitatively reproduce the pronounced hysteresis broadening. Furthermore, the same approach also reproduces the field-sweep-rate dependence of the transition field observed in the experiment. Our findings thus provide a quantitative and comprehensive understanding of pronounced hysteresis broadening from the microscopic perspective of domain growth., Comment: 32 pages, 8 figures

The in-plane (thermal) Hall effect is an unconventional transverse response when the applied magnetic field is in the (heat) current plane. In contrast to the normal Hall effect, the in-plane Hall effect requires the absence of certain crystal symmetries, and possibly manifests a non-trivial topology of quantum materials. An accurate estimation of the intrinsic in-plane (thermal) Hall conductivity is crucial to identify the underlying mechanisms as in the case of the Kitaev spin-liquid candidate alpha-RuCl3. Here, we give the symmetry conditions for the in-plane Hall effect and discuss the implications that may impede the experimental evaluation of the in-plane (thermal) Hall conductivity within the single-device measurement. First, the lack of symmetry in crystals can create merohedral twin domains that cancel the total Hall signal. Second, even in a twin-free crystal, the intrinsic response is potentially contaminated by the out-of-plane conduction in three-dimensional systems, which is systematically unavoidable in the in-plane Hall systems. Third, even in a quasi-two-dimensional system, the conversion of (thermal) resistivity, rho (lambda), to (thermal) conductivity, sigma (kappa) requires protocols beyond the widely-used simplified formula due to the lack of in-plane-rotational symmetry. In principle, two independent sample devices are necessary to accurately estimate the s_xy (k_xy). As a case study, we discuss the half-integer quantization of the in-plane thermal Hall effect in the spin-disordered state of alpha-RuCl3. For an accurate measurement of the thermal Hall effect, it is necessary to avoid crystals with the merohedral twins contributing oppositely to k_xy, while the out-of-plane transport may have a negligible effect. To deal with the field-induced rotational-symmetry breaking, we propose two symmetry-based protocols, improved single-device and two-device methods., Comment: Main: 9 pages, 6 figs; Supple: 7 pages, 5 figs

We report an experimental demonstration of the magnetoelectrocaloric effect (MECE) in multiferroic $\mathrm{GdFeO_3}$. The temperature of the magnetic material changes when an external electric field is suddenly changed. MECE is the largest below the ordering temperature of Gd moments and modifiable by a magnetic field, suggesting that the ferroelectric transition affects MECE. The observed MECE shows a higher energy efficiency than typical magnetocaloric effects and adiabatic nuclear demagnetization. The present findings provide a proof of concept of MECE in multiferroics.

Phase diagrams form the basis for the study of material science, and the profiles of phase-transition lines separating different thermodynamic phases include comprehensive information about thermodynamic quantities, such as latent heat. However, in some materials exhibiting field-induced first-order transitions (FOTs), the equilibrium phase-transition line is hidden by the hysteresis region associated with the FOT; thus, it cannot be directly determined from measurements of resistivity, magnetization, etc. Here, we demonstrate a thermodynamics-based method for determining the hidden equilibrium FOT line. This method is verified for the FOT between antiferromagnetic and ferrimagnetic states in magneto-electric compounds (Fe$_{0.95}$Zn$_{0.05}$)$_{2}$Mo$_{3}$O$_{8}$. The equilibrium FOT line determined based on the Clausius-Clapeyron equation exhibits a reasonable profile in terms of the third law of thermodynamics, and it shows marked differences from the midpoints of the hysteresis region. Our findings highlight that care should be taken for referring to the hysteresis midpoint line when discussing field-induced latent heat or magnetocaloric effects., Comment: 22 pages, 7 figures

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., Comment: 10 pages, 7 figures, 1 table

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., Comment: 8 pages, 5 figures, SM: 4 pages, 3 figures. To be published in Physical Review B as a Letter

We have investigated magnetic structures of the centrosymmetrric skyrmion material Gd2PdSi3 by means of polarized neutron scattering near zero field with an isotope-160Gd-enriched single crystal. In a previous study, magnetic structures in Gd2PdSi3 at low temperatures were studied by resonant X-ray scattering measurements [T. Kurumaji et al. Science 365, 914 (2019)]. The present polarized neutron results confirm that the magnetic structure in zero field has elliptic screw-type magnetic modulation with a propagation vector of (q, 0, 0) with q ~ 0.14 and its equivalents. As the temperature increases, the system undergoes a magnetic phase transition while keeping the incommensurate q-vector of (q,0,0). We found that the thermally-induced phase has sinusoidal magnetic modulations with magnetic moments perpendicular both to the c axis and to the q-vector. We also investigate the spin-helicity degree of freedom in the ground state by polarized neutrons, revealing that the system contains equal fractions of the left-handed and right-handed screw-type orders as expected from the centrosymmetric crystal structure., Comment: 6 pages, 4 figures

We investigate the thermodynamic, magnetic, and electrical transport properties of a triangular-lattice antiferromagnet EuZnGe using single crystals grown from Eu-Zn flux in sealed tantalum tubes. Magnetic properties are found to be isotropic in the paramagnetic state while we observe an enhancement of in-plane magnetic susceptibility at the temperature near T* =11.3 K, suggesting an easy-plane anisotropy at low temperatures. Magnetic transition temperature is lower than T* as specific heat shows a peak at TN =7.6 K. We reveal the magnetic modulation along the c axis by resonant x-ray scattering at Eu L2 edge, which suggests competing magnetic interaction among Eu triangular-lattice layers. We observe a double-peak structure in the intensity profile along (0, 0, L) below TN, which is mainly composed of a dominant helical modulation with q ~ (0, 0, 0.4) coexisting with a secondary contribution from q ~ (0, 0, 0.5). We reproduce the intensity profile with a random mixture of five- and four-sublattice helices with spin rotation skipping due to hexagonal in-plane anisotropy. The metallic conductivity is highly anisotropic with the ratio rho_zz/rho_xx exceeding 10 over the entire temperature range and additionally exhibits a sharp enhancement of rho_zz at TN giving rise to rho_zz/rho_xx ~ 50, suggesting a coupling between out-of-plane electron conduction and the spiral magnetic modulations. In-plane magnetic field induces a spin-flop like transition, where the q = 0.4 peak disappears and an incommensurate peak of approximately qICM ~ 0.47 emerges, while the q = 0.5 modulation retains a finite intensity. This transition correlates with non-monotonic magnetoresistance and Hall resistivity, suggesting a significant interplay between electrons and spin structures through Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction., Comment: 9 pages, 6 figures

For the skyrmion-hosting intermetallic Gd$_2$PdSi$_3$ with centrosymmetric hexagonal lattice and triangular net of rare earth sites, we report a thorough investigation of the magnetic phase diagram. Our work reveals a new magnetic phase with isotropic value of the critical field for all orientations, where the magnetic ordering vector $\mathbf{q}$ is depinned from its preferred directions in the basal plane. This is in contrast to the highly anisotropic behavior of the low field phases, such as the skyrmion lattice (SkL), which are easily destroyed by in-plane magnetic field. The bulk nature of the SkL and of other magnetic phases was evidenced by specific-heat measurements. Resistivity anisotropy, likely originating from partial gapping of the density of states along $\mathbf{q}$ in this RKKY magnet, is picked up via the planar Hall effect (PHE). The PHE confirms the single-$\mathbf{q}$ nature of the magnetic order when the field is in the hexagonal plane, and allows to detect the preferred directions of $\mathbf{q}$. For field aligned perpendicular to the basal plane, several scenarios for the depinned phase (DP), such as tilted conical order, are discussed on the basis of the data., Comment: 27 pages, 12 figures (including SI)

The topological Hall effect (THE) and its thermoelectric counterpart, the topological Nernst effect (TNE), are hallmarks of the skyrmion lattice phase (SkL). We observed the giant TNE of the SkL in centrosymmetric Gd$_2$PdSi$_3$, comparable in magnitude to the largest anomalous Nernst signals in ferromagnets. Significant enhancement (suppression) of the THE occurs when doping electrons (holes) to Gd$_2$PdSi$_3$. On the electron-doped side, the topological Hall conductivity approaches the characteristic threshold $\sim 1000\,\left(\mathrm{\Omega cm}\right)^{-1}$ for the intrinsic regime. We use the filling-controlled samples to confirm Mott's relation between TNE and THE and discuss the importance of Gd-5d orbitals for transport in this compound., Comment: Main text: 9 pages, 4 figures; with SI: 36 pages, 15 figures

The wavefuntion of conduction electrons moving in the background of a non-coplanar spin structure can gain a quantal phase - Berry phase - as if the electrons were moving in a strong fictitious magnetic field. Such an emergent magnetic field effect is approximately proportional to the solid angle subtended by the spin moments on three neighbouring spin sites, termed the scalar spin chirality. The entire spin chirality of the crystal, unless macroscopically canceled, causes the geometrical Hall effect of real-space Berry-phase origin, whereas the intrinsic anomalous Hall effect (AHE) in a conventional metallic ferromagnet is of the momentum-space Berry-phase origin induced by relativistic spin-orbit coupling (SOC). Here, we report the ordering phenomena of the spin-trimer scalar spin chirality and the consequent large geometrical Hall effect in the breathing kagom\'e lattice compound Dy$_3$Ru$_4$Al$_{12}$, where the Dy$^{3+}$ moments form non-coplanar spin trimers with local spin chirality. Using neutron diffraction, we show that the local spin chirality of the spin trimers as well as its ferroic/antiferroic orders can be switched by an external magnetic field, accompanying large changes in the geometrical Hall effect. Our finding reveals that systems composed of tunable spin trimers can be a fertile field to explore large emergent electromagnetic responses arising from real-space topological magnetic orders., Comment: 15 pages, 12 figures

Magnetic skyrmion textures are realized mainly in non-centrosymmetric, e.g. chiral or polar, magnets. Extending the field to centrosymmetric bulk materials is a rewarding challenge, where the released helicity / vorticity degree of freedom and higher skyrmion density result in intriguing new properties and enhanced functionality. We report here on the experimental observation of a skyrmion lattice (SkL) phase with large topological Hall effect and an incommensurate helical pitch as small as 2.8 nm in metallic Gd3Ru4Al12, which materializes a breathing kagom\'e lattice of Gadolinium moments. The magnetic structure of several ordered phases, including the SkL, is determined by resonant x-ray diffraction as well as small angle neutron scattering. The SkL and helical phases are also observed directly using Lorentz transmission electron microscopy. Among several competing phases, the SkL is promoted over a low-temperature transverse conical state by thermal fluctuations in an intermediate range of magnetic fields., Comment: 35 pages, 4 figures

Geometrically frustrated magnets provide abundant opportunities for discovering complex spin textures, which sometimes yield unconventional electromagnetic responses in correlated electron systems. It is theoretically predicted that magnetic frustration may also promote a topologically nontrivial spin state, i.e., magnetic skyrmions, which are nanometric spin vortices. Empirically, however, skyrmions are essentially concomitant with noncentrosymmetric lattice structures or interfacial-symmetry-breaking heterostructures. Here, we report the emergence of a Bloch-type skyrmion state in the frustrated centrosymmetric triangular-lattice magnet Gd2PdSi3. We identified the field-induced skyrmion phase via a giant topological Hall response, which is further corroborated by the observation of in-plane spin modulation probed by resonant x-ray scattering. Our results exemplify a new gold mine of magnetic frustration for producing topological spin textures endowed with emergent electrodynamics in centrosymmetric magnets., Comment: 23 pages, 3 figures

Formation of the triangular skyrmion-lattice is found in a tetragonal polar magnet VOSe$_2$O$_5$. By magnetization and small-angle neutron scattering measurements on the single crystals, we identify a cycloidal spin state at zero field and a N\'eel-type skyrmion-lattice phase under a magnetic field along the polar axis. Adjacent to this phase, another magnetic phase of an incommensurate spin texture is identified at lower temperatures, tentatively assigned to a square skyrmion-lattice phase. These findings exemplify the versatile features of N\'eel-type skyrmions in bulk materials, and provide a unique occasion to explore the physics of topological spin textures in polar magnets., Comment: 11 pages, 4 figures, supplemental material (7 pages)