arXiv subjects

T. J. Sato

Publications and source records attributed to T. J. Sato.

At least 19 recordsLinked to original sources

arXiv2026

Coupled dimerized alternating-bond quantum spin chains in the distorted honeycomb-lattice magnet Cu$_5$SbO$_6$

We analyze powder-averaged inelastic neutron scattering and magnetization data for the distorted honeycomb compound Cu$_5$SbO$_6$ using a first-order dimer expansion calculation and quantum Monte Carlo simulations. We show that, in contrast to the previously proposed honeycomb lattice model, Cu$_5$SbO$_6$ accommodates interacting dimerized spin chains with alternating ferromagnetic-antiferromagnetic couplings along the chain. Moreover, unlike the typical couplings observed in other Cu$^{2+}$-based distorted honeycomb magnets, the spin chains in Cu$_5$SbO$_6$ primarily couple through an antiferromagnetic coupling that arises between the honeycomb layers, rather than the expected interchain coupling in the layers. This finding reveals a different magnetic coupling scheme for Cu$_5$SbO$_6$. In addition, utilizing x-ray spectroscopy and transmission electron microscopy, we also refine the crystal structure and stacking-fault model of the compound.

cond-mat.str-el↗

arXiv2024

Multiple metamagnetic transitions in helical antiferromagnet CeVGe$_3$

We report on neutron diffraction, magnetoresistance, magnetization, and magnetic torque measurements under high magnetic field in the helical antiferromagnet CeVGe$_3$. This compound exhibits Kondo lattice coherence and helical antiferromagnetic (AFM) ordering at ambient pressure, similar to the well-studied CeRhIn$_5$. Our measurements reveal that CeVGe$_3$ undergoes a magnetic transition from an incommensurate (ICM) AFM state to an up-up-down-down commensurate (CM) AFM structure, followed by a transition to a novel phase at higher fields. A quantum phase transition occurs around 21.3 T. This rich magnetic field phase diagram closely resembles that of CeRhIn$_5$. Furthermore, angle-dependent magnetoresistance measurements reveal that all transitions in CeVGe$_3$ occur from the field component along the $ab$ plane. These findings highlight the intricate interplay among exchange interactions, crystal field effects, ground state properties, and crystalline symmetries.

cond-mat.str-el↗

arXiv2024

Detailed dynamics of a moving magnetic skyrmion lattice in MnSi observed using a small-angle neutron scattering under an alternating electric current flow

Lattice formation of swirling textures is ubiquitous in solid-state materials, such as a magnetic skyrmion lattice in chiral magnets. In the magnetic skyrmion lattices, their moving states and dynamics under external perturbations are still unrevealed, although a detailed understanding of the dynamics is crucial to realizing spintronic applications, such as magnetic domain-wall racetrack memory. Here, we report in detail on the transient state of a moving magnetic skyrmion lattice in bulk single-crystalline MnSi under alternating current (AC) using small-angle neutron scattering. A rotation and concomitant broadening of the spot width in the azimuthal direction of the magnetic skyrmion reflections originating from the plastic deformation of the magnetic skyrmion lattice were found only at low AC frequencies, whereas above the threshold AC frequency (ft ~ 0.12 Hz) the rotation was not observed, and the spot width becomes sharper. The observed complex response of the magnetic skyrmion reflections can be explained by the change in dislocation density in the magnetic skyrmion lattice. At frequencies higher than ft, the magnetic skyrmions oscillate removing the dislocations, indicating that the dislocation density is controlled by the AC frequency.

cond-mat.str-el↗

arXiv2023

Magnetic structure and Kondo lattice behavior in CeVGe$_3$: an NMR and neutron scattering study

We present nuclear magnetic resonance (NMR), neutron diffraction, magnetization, and transport measurements on a single crystal and powder of CeVGe$_3$. This material exhibits heavy fermion behavior at low temperature, accompanied by antiferromagnetic (AFM) order below 5.8 K. We find that the magnetic structure is incommensurate with AFM helical structure, characterized by a magnetic modulated propagation vector of $(0, 0, 0.49)$ with in-plane moments rotating around the $c$-axis. The NMR Knight shift and spin-lattice relaxation rate reveal a coherence temperature $T^*\sim 15$ K, and the presence of significant antiferromagnetic fluctuations reminiscent of the archetypical heavy fermion compound CeRhIn$_5$. We further identify a metamagnetic transition above $H_m\sim 2.5$ T for magnetic fields perpendicular to $c$. We speculate that the magnetic structure in this field-induced phase consists of a superposition with both ferromagnetic and antiferromagnetic components, which is consistent with the NMR spectrum in this region of the phase diagram. Our results thus indicate that CeVGe$_3$ is a hexagonal structure analog to tetragonal CeRhIn$_5$.

cond-mat.str-el↗

arXiv2022

Magnetic structure and spin dynamics of the quasi-2D antiferromagnet Zn-doped copper pyrovanadate

Magnetic properties of the antiferromagnet Zn$_{0.15}$Cu$_{1.85}$V$_2$O$_7$ (ZnCVO) have been thoroughly investigated on powder and single-crystal samples. The crystal structure determination using powder x-ray and neutron diffraction confirms that ZnCVO with Zn = 0.15 is isostructural with $β$-Cu$_{2}$V$_2$O$_7$ ($β$-CVO) with small deviation in the lattice parameters. Macroscopic magnetic properties measurements also confirm the similarity between the two compounds. The Cu$^{2+}$ spins were found to align along the crystallographic $c$-axis, antiparallel to their nearest neighbors connected by the leading exchange interaction $J_1$. Spin dynamics reveals a typical symmetric spin-wave dispersion with strong interactions in the $bc$-plane and weak interplane coupling. The exchange interaction analysis indicates that the spin network of ZnCVO is topologically consistent with the previous DFT prediction but the values of leading exchange interactions are contradictory. Furthermore, rather than the predicted 2D honeycomb structure, the spin network in ZnCVO could be better described by the anisotropic 2D spin network composing of $J_1$, $J_5$, and $J_6$ interactions, four bonds per one spin site, coupled by weak interplane interactions.

cond-mat.str-el↗

arXiv2022

Breakdown of Linear Spin-wave Theory and Existence of Spinon Bound States in the Frustrated Kagome Lattice Antiferromagnet

Spin dynamics of the spin-1/2 kagome lattice antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$ was studied using high-resolution, time-of-flight inelastic neutron scattering. The flat mode, a characteristic of the frustrated kagome antiferromagnet, and the low-energy dispersive mode, which is dominated by magnons, can be well described by the linear spin-wave theory. However, the theory fails to describe three weakly dispersive modes between 9 and 14 meV. These modes could be attributed to two-spinon bound states, which decay into free spinons away from the zone center and at a high temperature, giving rise to continuum scattering.

cond-mat.str-el↗

arXiv2021

Magnetic structure and high-field magnetization of the distorted kagome lattice antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$

High-resolution time-of-flight powder neutron diffraction and high-field magnetization were measured to investigate the magnetic structure and existence of a field-induced magnetic phase transition in the distorted kagome antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$. Upon cooling from room temperature, the compound undergoes a structural phase transition at $T_\textrm{t}=185$ K from the rhombohedral space group $R\bar{3}m$ with the perfect kagome spin network to the monoclinic space group $P2_1/n$ with the distorted kagome planes. The distortion results in three inequivalent exchange interactions among the $S=1/2$ Cu$^{2+}$ spins that magnetically order below $T_\textrm{N}=20.2$ K. Magnetization measured with a magnetic field applied within the kagome plane reveals small in-plane ferromagnetism resulting from spin canting. On the other hand, the out-of-plane magnetization does not show a clear hysteresis loop of the ferromagnetic component nor a prominent anomaly up to 170 T, with the exception of the subtle knee-like bend around 90 T, which could indicate the 1/3 magnetization plateau. The combined analysis using the irreducible representations of the magnetic space groups and magnetic structure refinement on the neutron powder diffraction data suggests that the magnetic moments order in the magnetic space group $P2_1'/n'$ with the all-in-all-out spin structure, which by symmetry allows for the in-plane canting, consistent with the in-plane ferromagnetism observed in the magnetization.

cond-mat.str-el↗

arXiv2020

Mössbauer studies of spin- and charge-modulations in BaFe2(As1-xPx)2

The BaFe2(As1-xPx)2 compounds with x = 0 (parent), x = 0.10 (under-doped), x = 0.31, 0.33, 0.53 (superconductors with Tc = 27.3 K, 27.6 K, 13.9 K, respectively) and x = 0.70, 0.77 (over-doped) have been investigated versus temperature using 57Fe Mössbauer spectroscopy. Special attention was paid to regions of the spin-density-wave (SDW) antiferromagnetic order, spin-nematic phase, and superconducting transition. The BaFe2(As0.90P0.10)2 compound exhibits a reduced amplitude of SDW as compared to the parent compound and preserved universality class of two-dimensional magnetic planes with one-dimensional spins. The spin-nematic phase region for x = 0.10 is characterized by an incoherent magnetic order. BaFe2(As0.69P0.31)2 shows coexistence of a weak magnetic order and superconductivity due to the vicinity of the quantum critical point. The charge density modulations in the BaFe2(As0.67P0.33)2 and BaFe2(As0.47P0.53)2 superconductors are perturbed near Tc. Pronounced hump of the average quadrupole splitting across superconducting transition is observed for the system with x = 0.33. The phosphorus substitution increases the Debye temperature of the BaFe2(As1-xPx)2 compound. Moreover, experimental electron charge densities at Fe nuclei in this material conclusively show that it should be recognized as a hole-doped system. The measured Mössbauer spectral shift and spectral area are not affected by transition to the superconducting state. This indicates that neither the average electron density at Fe nuclei nor the dynamical properties of the Fe-sublattice in BaFe2(As1-xPx)2 are sensitive to the superconducting transition. Theoretical calculations of hyperfine parameters determining the patterns of Mössbauer spectra of BaFe2(As1-xPx)2 with x = 0, 0.31, 0.5, and 1.0 are performed within the framework of the density functional theory.

cond-mat.supr-con↗

arXiv2020

Magnetic properties and magnetic structure of the frustrated quasi-one-dimensional antiferromagnet SrCuTe$_2$O$_6$

Magnetization measurements on single-crystal cubic SrCuTe$_2$O$_6$ with an applied magnetic field of along three inequivalent high symmetry directions $[100]$, $[110]$, and $[111]$ reveal weak magnetic anisotropy. The fits of the magnetic susceptibility to the result from a quantum Monte Carlo simulation on the Heisenberg spin-chain model, where the chain is formed via the dominant third-nearest-neighbor exchange interaction $J_3$, yield the intra-chain interaction $(J_3/k_B)$ between 50.12(7) K for the applied field along $[110]$ and 52.5(2) K along $[100]$ with about the same $g$-factor of 2.2. Single-crystal neutron diffraction unveils the transition to the magnetic ordered state as evidenced by the onset of the magnetic Bragg intensity at $T_\textrm{N1}=5.25(9)$ K with no anomaly of the second transition at $T_\textrm{N2}$ reported previously. Based on irreducible representation theory and magnetic space group analysis of powder and single-crystal neutron diffraction data, the magnetic structure in the Shubnikov space group $P4_132$, where the Cu$^{2+}$~$S=1/2$ spins antiferromagnetically align in the direction perpendicular to the spin chain, is proposed. The measured ordered moment of $0.52(6)~μ_B$, which represents 48% reduction from the expected value of $1~μ_B$, suggests the remaining influence of frustration resulting from the $J_1$ and $J_2$ bonds.

cond-mat.str-el↗

arXiv2020

Ferroelectric atomic displacement in multiferroic tetragonal perovskite Sr$_{1/2}$Ba$_{1/2}$MnO$_3$

We investigate the crystal structure in multiferroic tetragonal perovskite Sr$_{1/2}$Ba$_{1/2}$MnO$_3$ with high accuracy of the order of 10$^{-3}$ Angstrom for an atomic displacement. The large atomic displacement for Mn ion from the centerosymmetric position, comparable with the off-centering distortion in the tetragonal ferroelectric BaTiO$_3$, is observed in the ferroelectric phase ($T_\mathrm{N}$ $\leq$ $T$ $\leq$ $T_\mathrm{C}$). In stark contrast, in the multiferroic phase ($T$ $\leq$ $T_\mathrm{N}$), the atomic displacement for Mn ion is suppressed, but those for O ions are enlarged. The atomic displacements in the polar crystal structures are also analyzed in terms of the ferroelectric modes. In the ferroelectric phase, the atomic displacements are decomposed into dominant positive Slater, negative Last, and small positive Axe modes. The suppression of Slater and Last modes, the sign change of Last mode, and the enlargement of Axe mode are found in the multiferroic phase. The ferroelectric distortion is well reproduced by a first-principles calculation based on Berry phase method, providing an additional information on competing mechanisms to induce the polarization, electronic $p$-$d$ hybridization vs. magnetic exchange-striction.

cond-mat.mtrl-sci↗

arXiv2019

Controlling the stoichiometry of the triangular lattice antiferromagnet Li$_{1+x}$Zn$_{2-y}$Mo$_3$O$_8$

The control of the stoichiometry of Li$_{1+x}$Zn$_{2-y}$Mo$_3$O$_8$ was achieved by the solid-state-reaction. We found that the best sample that has the chemical composition Li$_{0.95(4)}$Zn$_{1.92(8)}$Mo$_3$O$_8$ was obtained from the starting nominal composition with Li : Zn : Mo : O = $(1+w)$ : $(2.8-w)$ : $3$ : $8.6$ with $w = -0.1$, indicating that the stoichiometry is greatly improved compared to those in the earlier reports. For larger $w$ detailed structural analysis indicates that the mixed sites of Li and Zn are preferentially occupied by Li atoms, as well as the fraction of the non-magnetic secondary phase Zn$_2$Mo$_3$O$_8$ decreases. Magnetic susceptibility of the improved stoichiometry powder samples shows a broad hump in the temperature range of 100 $< T <$ 200 K. This suggests that the development of antiferromagnetic correlations at the high temperatures is inherent to the ideal stoichiometric LiZn$_2$Mo$_3$O$_8$.

cond-mat.str-el↗

arXiv2018

Deformation of the Magnetic Skyrmion Lattice in MnSi under Electric Current Flow

Using small-angle neutron scattering (SANS), we investigate the deformation of the magnetic skyrmion lattice in bulk single-crystalline MnSi under electric current flow. A significant broadening of the skyrmion-lattice-reflection peaks was observed in the SANS pattern for current densities greater than a threshold value j_t ~ 1 MA/m^2 (10^6 A/m^2). We show this peak broadening to originate from a spatially inhomogeneous rotation of the skyrmion lattice, with an inverse rotation sense observed for opposite sample edges aligned with the direction of current flow. The peak broadening (and the corresponding skyrmion lattice rotations) remain finite even after switching off the electric current. These results indicate that skyrmion lattices under current flow experience significant friction near the sample edges, and plastic deformation due to pinning effects, these being important factors that must be considered for the anticipated skyrmion-based applications in chiral magnets at the nanoscale.

cond-mat.str-el↗

arXiv2017

Spin Seebeck effect in a polar antiferromagnet $α$-Cu$_{2}$V$_{2}$O$_{7}$

We have studied the longitudinal spin Seebeck effect in a polar antiferromagnet $α$-Cu$_{2}$V$_{2}$O$_{7}$ in contact with a Pt film. Below the antiferromagnetic transition temperature of $α$-Cu$_{2}$V$_{2}$O$_{7}$, spin Seebeck voltages whose magnetic field dependence is similar to that reported in antiferromagnetic MnF$_{2}$$\mid$Pt bilayers are observed. Though a small weak-ferromagnetic moment appears owing to the Dzyaloshinskii-Moriya interaction in $α$-Cu$_{2}$V$_{2}$O$_{7}$, the magnetic field dependence of spin Seebeck voltages is found to be irrelevant to the weak ferromagnetic moments. The dependences of the spin Seebeck voltages on magnetic fields and temperature are analyzed by a magnon spin current theory. The numerical calculation of spin Seebeck voltages using magnetic parameters of $α$-Cu$_{2}$V$_{2}$O$_{7}$ determined by previous neutron scattering studies reveals that the magnetic-field and temperature dependences of the spin Seebeck voltages for $α$-Cu$_{2}$V$_{2}$O$_{7}$$\mid$Pt are governed by the changes in magnon lifetimes with magnetic fields and temperature.

cond-mat.mtrl-sci↗

arXiv2017

Scaling of Memories and Crossover in Glassy Magnets

Glassiness is ubiquitous and diverse in characteristics in nature. Understanding their differences and classification remains a major scientific challenge. Here, we show that scaling of magnetic memories with time can be used to classify magnetic glassy materials into two distinct classes. The systems studied are high-temperature superconductor-related materials, spin-orbit Mott insulators, frustrated magnets, and dilute magnetic alloys. Our bulk magnetization measurements reveal that most densely populated magnets exhibit similar memory behavior characterized by a relaxation exponent of 1-n ~ 0.6(1). This exponent is different from 1-n ~ 1/3 of dilute magnetic alloys that was ascribed to their hierarchical and fractal energy landscape and is also different from 1-n=1 of the conventional Debye relaxation expected for a spin solid, a state with long range order. Furthermore, our systematic study on dilute magnetic alloys with varying magnetic concentration exhibits crossovers among the two glassy states and spin solid.

cond-mat.dis-nn↗

arXiv2017

Nonreciprocal magnons and symmetry-breaking in the noncentrosymmetric antiferromagnet

Magnons, the spin-wave quanta, are disturbances that embody a wave propagating through a background medium formed by ordered magnetic moments. In an isotropic Heisenberg system, these disturbances vary in a continuous manner around an ordered spin structure, thus requiring infinitesimal energy as a wavevector approaches a magnetic zone centre. However, competing anisotropic interactions arising from broken symmetry can favour a distinct static and dynamic spin state causing a shift of the minimum point of the magnon dispersion to a nonreciprocal wavevector. Here we report the first direct evidence of these nonreciprocal magnons in an antiferromagnet. In the antiferromagnet we investigated, namely, noncentrosymmetric $α$-Cu$_2$V$_2$O$_7$, they are caused by the incompatibility between anisotropic exchange and antisymmetric Dzyaloshinskii-Moriya interactions resulting in competing collinear and helical spin structures, respectively. The nonreciprocity introduces the difference in the phase velocity of the counter-rotating modes, causing the opposite spontaneous magnonic Faraday rotation of the left- and right-propagating spin-waves. The breaking of spatial inversion and time reversal symmetry is revealed as a magnetic-field-induced asymmetric energy shift, which provides a test for the detailed balance relation.

cond-mat.str-el↗

arXiv2017

High-field magnetization and magnetic phase diagram of $α$-Cu$_{2}$V$_{2}$O$_{7}$

High-field magnetization of the spin-$1/2$ antiferromagnet $α$-Cu$_2$V$_2$O$_7$ was measured in pulsed magnetic fields of up to 56 T in order to study its magnetic phase diagram. When the field was applied along the easy axis (the $a$-axis), two distinct transitions were observed at $H_{c1}=6.5$~T and $H_{c2}=18.0$~T. The former is a spin-flop transition typical for a collinear antiferromagnet and the latter is believed to be a spin-flip transition of canted moments. The canted moments, which are induced by the Dzyaloshinskii-Moriya interactions, anti-align for $H_{c1}<H<H_{c2}$ due to the anisotropic exchange interaction that favors the antiferromagnetic arrangement along the $a$-axis. Above $H_{c2}$, the Zeeman energy of the applied field overcomes the antiferromagnetic anisotropic interaction and the canted moments are aligned along the field direction. Density functional theory was employed to compute the exchange interactions, which were used as inputs for quantum Monte Carlo calculations and then further refined by fitting to the magnetic susceptibility data. Contrary to our previous report in Phys. Rev. B {\bf 92}, 024423, the dominant exchange interaction is between the third nearest-neighbor spins, which form zigzag spin-chains that are coupled with one another through an intertwining network of the nonnegligible nearest and second nearest-neighbor interactions. In addition, elastic neutron scattering under the applied magnetic fields of up to 10 T reveals the incommensurate helical spin structure in the spin-flop state.

cond-mat.str-el↗

arXiv2015

Magnetic structure and Dzyaloshinskii-Moriya interaction in the $S = 1/2$ helical-honeycomb antiferromagnet $α$-Cu$_{2}$V$_{2}$O$_{7}$

Magnetic properties of the $S = 1/2$ antiferromagnet $α$-Cu$_{2}$V$_{2}$O$_{7}$ have been studied using magnetization, Quantum Monte Carlo (QMC) simulations, and neutron diffraction. Magnetic susceptibility shows a broad peak at $\sim50$~K followed by an abrupt increase indicative of a phase transition to a magnetically ordered state at $T_{N}$ = 33.4(1) K. Above $T_N$, a fit to the Curie-Weiss law gives a Curie-Weiss temperature of $Θ=-73(1)$~K suggesting the dominant antiferromagnetic coupling. The result of the QMC calculations on the helical-honeycomb spin network with two antiferromagnetic exchange interactions $J_1$ and $J_2$ provides a better fit to the susceptibility than the previously proposed spin-chain model. Two sets of the coupling parameters $J_1:J_2=1:0.45$ with $J_1=5.79(1)$~meV and $0.65:1$ with $J_2=6.31(1)$~meV yield equally good fits down to $\sim T_N$. Below $T_{N}$, weak ferromagnetism due to spin canting is observed. The canting is caused by the Dzyaloshinskii-Moriya interaction with an estimated $bc$-plane component $\left|D_p\right|$ $\simeq0.14J_1$. Neutron diffraction reveals that the $S=1/2$ Cu$^{2+}$ spins antiferromagnetically align in the $Fd'd'2$ magnetic space group. The ordered moment of 0.93(9)~$μ_B$ is predominantly along the crystallographic $a$-axis.

cond-mat.str-el↗

arXiv2015

Long-range order and spin-liquid states of polycrystalline Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$

Low-temperature states of polycrystalline samples of a frustrated pyrochlore oxide Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ have been investigated by specific heat, magnetic susceptibility, and neutron scattering experiments. We have found that this system can be tuned by a minute change of $x$ from a spin-liquid state ($x < x_{\text{c}}$) to a partly ordered state with a small antiferromagnetic ordering of the order of $0.1 μ_{\text{B}}$. Specific heat shows a sharp peak at a phase transition at $T_{\text{c}}= 0.5$ K for $x=0.005$. Magnetic excitation spectra for this sample change from a quasielastic to a gapped type through $T_{\text{c}}$. The possibility of a Jahn-Teller transition is discussed.

cond-mat.str-el↗