arXiv subjects

Y. Ishii

Publications and source records attributed to Y. Ishii.

At least 19 recordsLinked to original sources

arXiv2026

Compact nondestructive generator for microsecond-class ultrafast pulsed magnetic fields

The sweep rate of pulsed magnetic fields is a key factor in exploring of nonequilibrium phenomena in matter. However, the nondestructive generation of ultrafast pulsed magnetic fields is challenging because of the large time constants associated with the large inductance and capacitance of conventional pulsed magnetic field generators, as well as the residual inductance of the power devices and the system itself. We have developed a new compact pulsed magnetic field generator using commercially available state-of-the-art ultrafast power devices, enabling the generation of ultrafast pulsed magnetic fields up to approximately 1 T with a pulse duration of 3.8 $μ$s. We present the performance of the newly developed system and compare the experimentally obtained magnetic field waveforms with the results of a simple circuit simulation. In addition, we demonstrate magnetization measurements on cobalt powder and Faraday rotation measurements on CdS single crystal using the developed system.

physics.ins-det↗

arXiv2026

Spectral Indicators of Piezomagnetically Induced Symmetry Breaking in Altermagnets

Recent developments in the multipole reformulation of X-ray absorption spectroscopy (XAS) have provided a unified framework to describe magnetic and orbital responses in terms of ferroic multipole order parameters. X-ray magnetic circular dichroism (XMCD) is known to probe spin, orbital, and anisotropic magnetic dipole (AMD) moments. Its applications to altermagnets and noncollinear antiferromagnets have revealed that the XMCD response is often governed by the ferroic states of the AMD in the photo-excited states rather than by conventional magnetic dipoles in the ground states. In this work, we extend the multipole-based analysis to X-ray magnetic linear dichroism (XMLD) and demonstrate that XMLD in altermagnets can be understood as a manifestation of piezomagnetic effects: linear couplings between magnetic dipole and electric quadrupole moments. Using symmetry analysis combined with exact diagonalization calculations of $L_{2,3}$-edge XAS, we systematically investigate representative altermagnets, including $α$-MnTe, MnF$_2$, and CrSb. We show that the ferroic ordering of higher-rank magnetic multipoles, particularly spinful magnetic octupoles, gives rise to characteristic field-odd XMLD signals that directly reflect the underlying piezomagnetic response tensors allowed by magnetic point-group symmetry. Furthermore, we discuss XMCD signals induced by piezomagnetic effects, in which strain generates magnetic dipole moments. Our results establish XMLD and XMCD as element-specific probes of magnetoelastic multipole order in altermagnets and provide a general symmetry-based pathway to identify hidden ferroic multipoles and strain-controllable spin phenomena beyond conventional ferromagnetism.

cond-mat.mtrl-sci↗

arXiv2026

Altermagnetic XMCD in Hematite Distinct from Weak Ferromagnetic Contributions

Altermagnets are compensated collinear magnets that break time-reversal symmetry without net magnetization, enabling unconventional magneto-optical responses. Here, altermagnetic X-ray magnetic circular dichroism (XMCD) is experimentally demonstrated in hematite $α$-Fe$_2$O$_3$. By employing a symmetry-selective geometry in which the x-ray propagation vector is orthogonal to the Dzyaloshinskii-Moriya-induced weak ferromagnetic moment, we isolate a finite XMCD signal that cannot be attributed to conventional weak ferromagnetism. Moreover, we demonstrate that distinct altermagnetic states characterized by different magnetic symmetries can be reversibly switched through the application of an in-plane external magnetic field. Full-multiplet calculations reveal that the signal originates from an anisotropic magnetic dipole moment realized in the $2p^53d^6$ excited states, despite the isotropic $2p^63d^5$ ground state. Our results establish XMCD as a direct probe of excited-state magnetic multipoles and provide a general route for the optical detection of altermagnetic order in compensated magnets.

cond-mat.str-el↗

arXiv2026

Quintuplet condensation in the skyrmionic insulator Cu2OSeO3 at ultrahigh magnetic fields

We report ultrahigh magnetic field Faraday rotation results on the chiral helimagnet Cu2OSeO3, the first Mott insulator showing skyrmion lattice phases and a linear magnetoelectric effect. Between 180 and 300 T, we find signatures of a Bose-Einstein condensation (BEC) of magnons, which can be described as a canted XY ferrimagnet. Due to the magnetoelectric coupling, the transverse magnetic order of the indivual Cu2+ spins is accompanied by a characteristic dome-like electric polarization which is crucial for the observation of the condensate via the Faraday rotation effect.

cond-mat.str-el↗

arXiv2025

Sum rules for x-ray circular and linear dichroism based on complete magnetic multipole basis

X-ray magnetic circular dichroism (XMCD) and X-ray magnetic linear dichroism (XMLD) are powerful spectroscopic techniques for probing magnetic properties in solids. In this study, we revisit the XMCD and XMLD sum rules within a complete magnetic multipole basis that incorporates both spinless and spinful multipoles. We demonstrate that these multipoles can be clearly distinguished and individually detected through the sum-rule formalism. Within this framework, the anisotropic magnetic dipole term is naturally derived in XMCD, offering a microscopic origin for ferromagnetic-like behavior in antiferromagnets. Furthermore, we derive the sum rules for out-of-plane and in-plane XMLD regarding electric quadrupole contributions defined based on the complete multipole basis. Our theoretical approach provides a unified, symmetry-consistent framework for analyzing dichroic signals in various magnetic materials. These findings deepen the understanding of XMCD and XMLD and open pathways to exploring complex magnetic structures and spin-orbit coupling effects in emergent magnetic materials.

cond-mat.mtrl-sci↗

arXiv2025

High-Throughput Mapping of Magnetic Properties via the on-the-fly XMCD spectroscopy in a Combinatorial Fe-Co-Ni Film

High-throughput X-ray magnetic circular dichroism (XMCD) spectroscopy was conducted on Fe-Co-Ni compositionally graded films to systematically analyze the variation of magnetic properties as a function of composition. On-the-fly XMCD measurement enabled rapid spectral acquisition. Measurement time was reduced approximately tenfold compared to conventional stepwise methods, while maintaining high precision. The obtained XMCD spectra were processed using the Savitzky-Golay denoising technique, and element-specific magnetic properties were extracted using XMCD sum rules. By mapping the orbital and spin magnetic moments across the composition gradient, we identified key regions exhibiting enhanced soft magnetic properties. This study demonstrates the effectiveness of high-throughput synchrotron-based spectroscopy for accelerating materials discovery and optimizing functional magnetic materials.

cond-mat.mtrl-sci↗

arXiv2025

Gradual charge order melting in Bi0.5Ca0.5MnO3 induced by ultrahigh magnetic field

We have investigated the magnetic properties of Bi0.5Ca0.5MnO3 under ultrahigh magnetic fields by magnetization measurements. This compound shows the metamagnetic transition which indicates the magnetic field induced charge order melting at the room temperature T = 293 K. We reveled that the temperature dependence of the critical field, where the highest critical field is higher than 100 T. At low temperatures, it has been found that the metamagnetic transitions become broad and vanish in the field ascending process below 70 K. On the other hand, in the field descending process, a clear metamagnetic transition is observed even below 70 K. This unusual behavior can be understood as the gradual charge order melting by magnetic field, and its simultaneous reformation. A coexistence of multiple order parameters can account for the observed characteristic dynamics of the first order phase transition. Finally, we constructed the B-T phase diagram with the unusual critical feature at ultrahigh magnetic fields.

cond-mat.str-el↗

arXiv2025

XRD study of the magnetization plateau above 40 T in the frustrated helimagnet CuGaCr$_{4}$S$_{8}$

CuGaCr$_{4}$S$_{8}$, which contains a chromium breathing pyrochlore network, exhibits diverse magnetic phases, including an incommensurate helical state below 31 K and a 1/2-magnetization plateau above 40 T, owing to the interplay between magnetic frustration and spin-lattice coupling. Here, we perform a single-shot powder x-ray diffraction experiment on CuGaCr$_{4}$S$_{8}$ in a pulsed high magnetic field of 55 T, revealing an orthorhombic-to-cubic (or pseudocubic) structural transition upon entering the 1/2-magnetization plateau phase at low temperatures. This observation suggests the emergence of a commensurate ferrimagnetic order, where a 3-up-1-down spin configuration is realized in each small tetrahedron, and the all-up or all-down in each large tetrahedron. We propose two types of 16-sublattice magnetic structures, which are degenerate within exchange interactions between the first, second, and third nearest neighbors.

cond-mat.mtrl-sci↗

arXiv2023

Signatures of a magnetic superstructure phase induced by ultrahigh magnetic fields in a breathing pyrochlore antiferromagnet

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.

cond-mat.mtrl-sci↗

arXiv2023

Photodisintegration Cross Section of $^4$He in the Giant Dipole Resonance Energy Region

We simultaneously measured the $^4$He($γ, n$)$^3$He and $^4$He($γ, p$)$^3$H reactions in the energy range around the giant dipole resonance. A quasi-monoenergetic photon beam produced via the laser Compton scattering technique was irradiated on the active-target time-projection chamber filled with helium gas, and trajectories of charged decay particles emitted from $^4$He were measured. Our data suggest that the $^4$He($γ, n$)$^3$He and $^4$He($γ, p$)$^3$H cross sections peak around 26 MeV. This result contradicts the previous experimental data reported by Shima et al. but is consistent with other experimental results.

nucl-ex↗

arXiv2023

Unveiling new quantum phases in the Shastry-Sutherland compound SrCu2(BO3)2up to the saturation magnetic field

Under magnetic fields, quantum magnets often undergo exotic phase transitions with various kinds of order. The discovery of a sequence of fractional magnetization plateaus in the Shastry-Sutherland compound SrCu2(BO3)2 has played a central role in the high-field research on quantum materials, but so far this system could only be probed up to half the saturation value of the magnetization. Here, we report the first experimental and theoretical investigation of this compound up to the saturation magnetic field of 140 T and beyond. Using ultrasound and magnetostriction techniques combined with extensive tensor-network calculations (iPEPS), several spin-supersolid phases are revealed between the 1/2 plateau and saturation (1/1 plateau). Quite remarkably, the sound velocity of the 1/2 plateau exhibits a drastic decrease of -50%, related to the tetragonal-to-orthorhombic instability of the checkerboard-type magnon crystal. The unveiled nature of this paradigmatic quantum system is a new milestone for exploring exotic quantum states of matter emerging in extreme conditions.

cond-mat.str-el↗

arXiv2021

Glasslike phonon excitation caused by ferroelectric structural instability

Quest for new states of matter near an ordered phase is a promising route for making modern physics forward. By probing thermal properties of a ferroelectric (FE) crystal Ba1-xSrxAl2O4, we have clarified that low-energy excitation of acoustic phonons is remarkably enhanced with critical behavior at the border of the FE phase. The phonon spectrum is significantly damped toward the FE phase boundary and transforms into glasslike phonon excitation which is reminiscent of a boson peak. This system thus links long-standing issues of amorphous solids and structural instability in crystals to pave the way to controlling lattice fluctuation as a new tuning parameter.

cond-mat.mtrl-sci↗

arXiv2020

Electronic charge transfer driven by spin cycloidal structure

Muon spin rotation and resonant soft X-ray scattering experiments on prototype multiferroics RMn2O5 (R = Y, Sm) are used to demonstrate that the local electric displacements are driven by the spin-current (SC) mechanism. Small local electric displacements were evaluated by observing spin polarization at ligand O ions, for which implanted muons served as an extremely sensitive probe. Our results for YMn2O5 provide evidence that the spin polarization of O ions forming a spin cycloid chain with Mn spins increases in proportion to the vector spin chirality (Si x Sj ) of the Mn ions. This relationship strongly indicates that the charge transfer between O and Mn ions is driven by the SC mechanism, which leads to the ferroelectricity accompanying O spin polarization.

cond-mat.str-el↗

arXiv2020

Lorentz microscopy and small-angle electron diffraction study of magnetic textures in La$_{1-x}$Sr$_x$MnO$_3$ (0.15 $< x <$ 0.30): the role of magnetic anisotropy

Magnetic textures in the ferromagnetic phases of La$_{1-x}$Sr$_x$MnO$_3$ for 0.15 $< x <$ 0.30 have been investigated by Lorentz microscopy combined with small-angle electron diffraction experiments. Various types of magnetic textures characterized by stripe, plate-shaped, and cylindrical (magnetic bubble) domains were found. Two distinct types of magnetic stripe domains appeared in the orthorhombic structure with an inversion symmetry of La$_{0.825}$Sr$_{0.175}$MnO$_3$, depending significantly on magnetocrystalline anisotropy. Based on in-situ observations as functions of temperature and the strength of the external magnetic field, a magnetic field-temperature phase diagram was constructed, showing the stabilization of magnetic bubbles in the ferromagnetic phase of La$_{0.825}$Sr$_{0.175}$MnO$_3$.

cond-mat.str-el↗

arXiv2020

Field-temperature phase diagram of magnetic bubbles spanning charge$/$orbital ordered and metallic phases in La$_{1-x}$Sr$_x$MnO$_3$ ($x = 0.125$)

We report formation of magnetic textures in the ferromagnetic (FM) phase of La$_{1-x}$Sr$_x$MnO$_3$ for $x =$ 0.125; these textures are magnetic bubbles, magnetic stripe domains, and forced FM states. In situ Lorentz microscopy (LM) observations show that magnetic bubbles exist in the FM insulating phase accompanying the formation of the charge$/$orbital ordering (CO$/$OO). Furthermore, stable magnetic bubbles still exist in an intermediate temperature region between the CO$/$OO ($T_{CO} =$ 155 K) and FM ($T_c =$ 190 K) transition temperatures. These magnetic bubbles are believed to originate from the magnetocrystalline anisotropy and the dipole-dipole interaction in the FM phase. Based on in situ LM observations as a function of both temperature and the strength of the external magnetic field applied, a magnetic field-temperature phase diagram is constructed, exhibiting the stabilizing regions of the magnetic bubbles in the FM phase of La$_{0.875}$Sr$_{0.125}$MnO$_{3}$.

cond-mat.str-el↗

arXiv2020

Formation mechanisms of magnetic bubbles in an M-type hexaferrite: the role of chirality reversals at domain walls

We examined the formation mechanisms of magnetic bubbles in an M-type hexaferrite via Lorentz microscopy. When magnetic fields were perpendicularly applied to a thin sample of BaFe$_{12-x-0.05}$Sc$_x$Mg$_{0.05}$O$_{19}$ ($x = 1.6$), Bloch lines, which were identified as reversals of domain-wall chirality, appeared, and magnetic bubbles were formed when the magnetic stripes were pinched off at these Bloch lines. The number of Bloch lines increased with the amount of Sc in BaFe$_{12-x-0.05}$Sc$_x$Mg$_{0.05}$O$_{19}$ probably because of the reduction in magnetic anisotropy. A Lorentz microscopic observation revealed that Bloch lines with high magnetostatic energy may play an important role in the formation of magnetic bubbles.

cond-mat.mtrl-sci↗

arXiv2019

Quantum magnetisms in uniform triangular lattices Li2AMo3O8 (A = In, Sc)

Molecular based spin-1/2 triangular lattice systems such as LiZn$_2$Mo$_3$O$_8$ have attracted research interest. Distortions, defects, and intersite disorder are suppressed in such molecular-based magnets, and intrinsic geometrical frustration gives rise to unconventional and unexpected ground states. Li$_2$$A$Mo$_3$O$_8$ ($A$ = In or Sc) is such a compound where spin-1/2 Mo$_3$O$_{13}$ clusters in place of Mo ions form the uniform triangular lattice. Their ground states are different according to the $A$ site. Li$_2$InMo$_3$O$_8$ undergoes conventional $120^\circ$ long-range magnetic order below $T_\text{N}=12$~K whereas isomorphic Li$_2$ScMo$_3$O$_8$ exhibits no long-range magnetic order down to 0.5~K. Here, we report exotic magnetisms in Li$_2$InMo$_3$O$_8$ and Li$_2$ScMo$_3$O$_8$ investigated by muon spin rotation ($μ$SR) and inelastic neutron scattering (INS) spectroscopies using polycrystalline samples. Li$_2$InMo$_3$O$_8$ and Li$_2$ScMo$_3$O$_8$ show completely different behaviors observed in both $μ$SR and INS measurements, representing their different ground states. Li$_2$InMo$_3$O$_8$ exhibits spin wave excitation which is quantitatively described by the nearest neighbor anisotropic Heisenberg model based on the $120^\circ$ spin structure. In contrast, Li$_2$ScMo$_3$O$_8$ undergoes short-range magnetic order below 4~K with quantum-spin-liquid-like magnetic fluctuations down to the base temperature. Origin of the different ground states is discussed in terms of anisotropies of crystal structures and magnetic interactions.

cond-mat.str-el↗

arXiv2019

Glass-like features of crystalline solids in the quantum critical regime

There has been growing interest in structural quantum phase transitions and quantum fluctuations of phonons in the research area of condensed matter physics. Here, we report the observation of glass-like features in the lattice heat capacity of a stuffed tridymite-type crystal, Ba1-xSrxAl2O4, a candidate compound of quantum paraelectrics. Substitutional chemical suppression of the ferroelectric phase transition temperature (TC) of Ba1-xSrxAl2O4 results in the disappearance of the TC at x=0.07. For the compositional window of x=0.2-0.5, the lattice heat capacity is enhanced below approximately 10 K and diverges from the T3-scaling law below 2.5 K. Synchrotron X-ray diffraction experiments on single crystals reveal the weakly correlated disorder in the crystal structure that survives down to low temperature; this disorder is responsible for the observed excess heat capacity. These features can be considered one of the manifestations of structural quantum fluctuations.

cond-mat.mtrl-sci↗