arXiv subjects

Akihiko Ikeda

Publications and source records attributed to Akihiko Ikeda.

At least 19 recordsLinked to original sources

arXiv2026

Pulse magnet of 10 T for power laser experiments with x-ray free-electron laser diagnostics

The importance of investigating magnetized plasmas/solids in extreme conditions has grown over the last decades, particularly in the field of high energy density physics (HEDP), such as laboratory astrophysics and inertial confinement fusion. However, up to now, the unique capabilities of an X-ray free-electron laser (XFEL), such as high brilliance and low divergence have never been exploited for this type of research. In this paper, we present the first platform developed at SACLA, Japan, that combines a high-power optical laser for generating matter under extreme conditions of pressure and temperature, an XFEL probe, and an external magnetic field. The high current is produced using a 2 kV, 4.8 kJ pulsed power system giving a maximum current of 10 kA which is synchronized with the optical laser and XFEL in a vacuum environment. It flows through a split-pair coil to generate a high magnetic field (10 T at 6 kA) which has 1 cm access every 45$^{\circ}$ in the equatorial plane and 90$^{\circ}$ in the poloidal one. This platform offers new opportunities to study high-energy-density matter in strong magnetic fields, including shock propagation, instability growth, and turbulent plasma dynamics.

physics.plasm-ph↗

arXiv2026

Non-linear diffusion and inhomogeneity of the magnetic field in single-turn coils: Insights from 3D multiphysics modeling

The single-turn coil method is a destructive pulsed magnet for generating over 100 T with a few $μ$-second pulse duration, and it inevitably causes the coil to explode. The temporal and spatial distributions of the electric current and magnetic field are highly inhomogeneous, arising from the skin effect, rapid temperature rise, and coil deformation. To grasp the dynamic phenomena in the single-turn coil, we conducted a finite element analysis using multiphysics simulation. We employed finite element method calculations using a fully 3D model of the single-turn coil with broken cylindrical symmetry. The calculated result revealed highly nonlinear diffusion of electric current, temperature, and magnetic fields, which are the sources of the inhomogeneous magnetic fields inside the single-turn coil in time and space.

cond-mat.mtrl-sci↗

arXiv2026

Efficiency in a repetitive pulse magnet

A repetitive-pulse magnet is a promising tool when combined with repetitive excitations, such as pulsed lasers. Technically, the repetition and the magnetic field values in a repetitive-pulse magnet are limited by the Joule heating in the coil. Here, we analytically examine the relationship between the coil's dimensions and its efficiency, assuming negligible heating of the coil, to design an optimized high-repetition, high-magnetic-field coil. We calculated the dependence of the maximum magnetic field, energy loss, pulse duration, form factor, impedance, and maximum current on the coil's geometry. We found that the smaller the coil, the more pulses and the more intense the magnetic fields we can obtain under a given condition. We argue that the obtained trend arises from a complex interplay among various parameters.

physics.app-ph↗

arXiv2026

Unified Description of Spin-Lattice Coupling and Thermodynamics in the Pyrochlore Heisenberg Antiferromagnet

We study an extended model to describe the spin-lattice coupling, incorporating individual vibrations of bonds and atomic sites alongside distance-dependent exchange interactions. The proposed spin Hamiltonian can be effectively considered as an interpolation between two well-established minimum models, the bond-phonon model and the site-phonon model. The extended model, which treats bond phonons and site phonons on comparable footing, well reproduces successive field-induced phase transitions as well as the thermodynamic properties of a three-up-one-down state in the pyrochlore-lattice Heisenberg antiferromagnet, including negative thermal expansion, an enhanced magnetocaloric effect, and a sharp specific-heat peak. The present approach is broadly applicable to various spin models, providing a framework for identifying the primary phonon modes responsible for spin-lattice coupling and for understanding complex magnetic phase diagrams.

cond-mat.str-el↗

arXiv2026

Magneto-optical Kerr effect measurements under bipolar pulsed magnetic fields

The magneto-optical Kerr effect (MOKE) is a powerful probe of magnetism. Its contact-free optical nature makes it potentially well suitable for measurements under pulsed magnetic fields if various difficulties are overcome. In this paper, we report the establishment of MOKE measurements under bipolar pulsed magnetic fields up to 13.1 T. The accuracy of the setup was demonstrated by the excellent agreement with static-field results on the (001) surface of a Fe3O4 single crystal. Furthermore, clear hysteresis loops of various commercial permanent magnets were successfully observed. The capability for rapid characterization of hysteretic properties highlights the versatility of our pulsed-field MOKE setup for both fundamental materials science and engineering applications.

cond-mat.str-el↗

arXiv2026

Large magnetostriction and first-order phase transition induced by ultrahigh magnetic fields in V$_6$O$_{13}$

V$_6$O$_{13}$ exhibits the metal-insulator transition (MIT) at around 150 K with the formation of vanadium-vanadium (V-V) dimers. We measured the magnetostriction of V$_6$O$_{13}$ along the crystallographic $b$-axis in ultrahigh magnetic fields up to 186 T at various temperatures. An abrupt and large negative magnetostriction of $ΔL / L \sim 10^{-3}$ was observed above 120 T below 150 K which is attributed to the collapse of the V-V dimer. A pronounced hysteresis exceeding 50 T was observed, indicating a first-order phase transition. The field-induced collapse of the V-V dimer is expected to be accompanied by the insulator-to-metal transition. The robustness of the insulating phase against magnetic fields was discussed based on thermodynamics and the comparison with the case of VO$_2$.

cond-mat.str-el↗

arXiv2025

Magneto-optical Kerr-effect measurements under pulsed magnetic fields over 40 T using a compact sample fixture

The magnetic field is one of the most fundamental control parameters in materials science. A pulsed magnetic-field apparatus can generate high magnetic fields that are inaccessible by conventional DC-field magnets. One important issue is that measurement techniques compatible with pulsed fields are rather limited due to short pulse duration and large electromagnetic or mechanical noise originating from field pulses. The magneto-optical Kerr effect (MOKE), the change in the state of light polarization upon reflection from magnetic materials, has the potential to become a powerful tool for investigation of magnetic properties of a wide range of materials including non-transparent materials or thin films in pulsed fields. Nevertheless, since the MOKE response is typically very small, MOKE measurements under pulsed fields are quite challenging. Here, we present a new method to measure polar MOKE under high pulsed magnetic fields of 2-ms pulse width. The keys of this new technique are a ferrule-based compact sample-fiber fixture and a phase-resolved numerical lock-in analysis, combined with the high-resolution optical apparatus based on an all-fiber loop-less Sagnac interferometer. We succeeded in measuring MOKE signals from various ferromagnetic or ferrimagnetic samples above 40 T and down to 77 K, significantly extending the limits of previously reported pulse-field MOKE measurements. Our apparatus is simple enough to be compatible with larger-scale experiments in pulse-field facilities, thus becoming a new promising tool to optically investigate material properties in pulsed fields.

cond-mat.str-el↗

arXiv2025

Vector pulse magnet

The underlying symmetry of the crystal, electronic structure, and magnetic structure manifests itself in the anisotropy of materials' properties, which is a central topic of the present condensed matter research. However, it demands such a considerable effort to fill the explorable space that only a small part has been conquered. We report a vector pulse magnet (VPM) as an alternative experimental technique to control the direction of applied magnetic fields, which may complement the conventional methods with its characteristic features. The VPM combines a conventional pulse magnet and a vector magnet. The VPM can create vector pulsed magnetic fields and swiftly rotating pulsed magnetic fields. As a demonstration, the three-dimensional magnetoresistance measurement of a highly oriented pyrolytic graphite is carried out using the AC four-probe method at 4.5 K and 6 T. The two-dimensional electronic structure of graphite is visualized in the three-dimensional magnetoresistance data. One can uncover the rotational and time-reversal symmetry of materials using a VPM and a variety of measurement techniques.

cond-mat.mtrl-sci↗

arXiv2025

X-ray free-electron laser observation of giant and anisotropic magnetostriction in $β$-O$_{2}$ at 110 Tesla

In strong magnetic fields beyond 100 T, the significant Zeeman energy competes with the lattice interactions, where a considerable magnetostriction is expected. However, the microscopic observation of the magnetostriction above 100 T has been hindered due to the short pulse duration of $μ$-seconds and the coil's destruction. Here, we report the observation of the giant and anisotropic magnetostriction of $\sim 1$ % at 110 T in the spin-controlled crystal, $β$-O$_{2}$, by combining the single-shot diffraction of x-ray free-electron laser (XFEL) and the newly developed portable 100 T generator (PINK-02). The very soft and anisotropic response of $β$-O$_{2}$ should originate in the competing van der Waals force and exchange interaction, and also the frustration of spin and lattice on the triangular network. The XFEL experiment above 100 T using PINK-02 enables microscopic investigations on materials' properties at high magnetic fields, providing insights into how spins contribute to the stability of crystal structures.

cond-mat.str-el↗

arXiv2025

Experimental and Numerical Studies of the Collapse of Dense Clouds Induced by Herbig-Haro Stellar Jets

This study investigates the influence of Herbig-Haro jets on initiating star formation in dense environments. When molecular clouds are nearing gravitational instability, the impact of a protostellar jet could provide the impetus needed to catalyze star formation. A high-energy-density experiment was carried out at the LULI2000 laser facility, where a supersonic jet generated by a nanosecond laser was used to compress a foam or plastic ball, mimicking the interaction of a Herbig-Haro jet with a molecular cloud. Simulations using the 3D radiation hydrodynamics code TROLL provided comprehensive data for analyzing ball compression and calculating jet characteristics. After applying scaling laws, similarities between stellar and experimental jets were explored. Diagnostic simulations-including density gradient, emission, and X-ray radiographies-showed strong agreement with experimental data. The results of the experiment, supported by simulations, demonstrate that the impact of a protostellar jet on a molecular cloud could reduce the Bonnor-Ebert mass by approximately 9%, thereby initiating collapse.

astro-ph.HE↗

arXiv2025

Crystal-field magnetostriction of the spin ice under ultrahigh magnetic fields

We present a comprehensive study of the magnetoelastic properties of the Ising pyrochlore oxide Ho$_{2}$Ti$_{2}$O$_{7}$, 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 $B_{\rm cf} \sim 65$ T. The transverse magnetostriction exhibits a contrasting behavior, highlighting the anisotropic nature of the CF striction. We identify distinct timescales of spin dynamics and CF-phonon dynamics by applying a magnetic field with different field-sweep rates. 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.

cond-mat.str-el↗

arXiv2024

A concise 40 T pulse magnet for condensed matter experiments

There is a growing interest in using pulsed high magnetic field as a controlling parameter of physical phenomena in various scientific disciplines, such as condensed matter physics, particle physics, plasma physics, chemistry and biological studies. We devised a concise and portable pulsed magnetic field generator that produces a 40 T field with a pulse duration of 2 ms. It is assembled using only off-the-shelf components and a homemade coil that leverages small computers, Raspberry Pi, and Python codes. It allows for straightforward modification for general purposes. As working examples, we show representative applications in condensed matter experiments of magnetoresistance, magnetization, and magnetostriction measurements for graphite, NdNi$_{2}$P$_{2}$, and NdCo$_{2}$P$_{2}$, respectively, with the maximum magnetic field of 41 T and the lowest temperature of 4.2 K.

cond-mat.mtrl-sci↗

arXiv2024

A review on magnetic field induced spin crossover in LaCoO$_{3}$ up to 600 T

\lco{} is known for its two-step spin crossover as a function of temperature. Despite efforts spanning over half a century, the origin of this phenomenon is still debated, particularly regarding how the microscopic spin states are involved in the observed macroscopic two-step spin crossover. High magnetic field studies on LaCoO$_{3}$ are performed because the magnetic field-induced spin crossover is induced, where the magnetic excited states become more stable in high magnetic fields than the non-magnetic ground states. This review focuses on the findings in LaCoO$_{3}$ at high magnetic fields over the last decade. A complex phase diagram has been revealed at high magnetic fields instead of solving the conventional problem of LaCoO$_{3}$. It suggests that appreciable spin state correlations are in play in LaCoO$_{3}$. The possibility of exciton condensation is also discussed.

cond-mat.str-el↗

arXiv2023

Magnetic field induced valence change in Eu(Co$_{1-x}$Ni$_{x}$)$_{2}$P$_{2}$ up to 60 T

The solid solution 122 compounds, Eu(Co$_{1-x}$Ni$_{x}$)$_{2}$P$_{2}$, show valence transition between divalent state and intermediate valence states at Eu, which is firmly correlated to multiple degrees of freedom in the solid such as the isostructural transition between the collapsed tetragonal (cT) and uncollapsed tetragonal (ucT) structures, $3d$ magnetism, and the formation of P-P dimers. To gain insights into the correlated behavior, we investigate the effect of high magnetic fields on the samples of $x = 0.4$ and $0.5$ using magnetostriction and magnetization measurements up to 60 T. The samples are in the Eu valence fluctuating regime, where the possible structural transition from cT to ucT may be induced by the Eu valence change under the magnetic fields. For both samples, magnetostriction smoothly increases with increasing magnetic fields. The behavior is in good agreement with the calculated results using the interconfigurational fluctuation (ICF) model that describes the valence change. This indicates that $ΔL$ represents the change of the Eu valence state in these compounds. Magnetization curves for both compounds show good agreement with the ICF model at high magnetic fields. In contrast, in the low magnetic field region, magnetization curves do not agree with the ICF model. These results indicate that the Eu valence changes manifest themselves in the magnetization curves at high magnetic fields and that the magnetism of the $3d$ electrons manifests itself in the magnetization at low magnetic fields. Hence, we conclude that the valence change occurs within the Eu valence fluctuation regime coupled with the cT structure. Thereby, we believe that the transition to ucT structure which is firmly coupled with the divalent Eu state does not occur within the magnetic field range of the present study. (Continued)

cond-mat.str-el↗

arXiv2023

Signature of spin-triplet exciton condensations in LaCoO$_{3}$ at ultrahigh magnetic fields up to 600 T

Bose-Einstein condensation of electron-hole pairs, exciton condensation, has been effortfully investigated since predicted 60 years ago. Irrefutable evidence has still been lacking due to experimental difficulties in verifying the condensation of the charge neutral and non-magnetic spin-singlet excitons. Whilst, condensation of spin-triplet excitons is a promising frontier because spin supercurrent and spin-Seebeck effects will be observable. A canonical cobaltite LaCoO$_{3}$ under very high magnetic fields is a propitious candidate, yet to be verified. Here, we unveil the exotic phase diagram of LaCoO$_{3}$ up to 600 T generated using the electromagnetic flux compression method and the state-of-the-art magnetostriction gauge. We found the continuous magnetostriction curves and a bending structure, which suggest the emergence of two distinct spin-triplet exciton condensates. By constructing a phenomenological model, we showed that quantum fluctuations of excitons are crucial for the field-induced successive transitions. The spin-triplet exciton condensation in a cobaltite, which is three-dimensional and thermally equilibrated, opens up a novel venue for spintronics technologies with spin-supercurrent such as a spin Josephson junction.

cond-mat.str-el↗

arXiv2023

Rhombic skyrmion lattice coupled with orthorhombic structural distortion in EuAl$_{4}$

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.

cond-mat.str-el↗

arXiv2023

Single crystal growths and magnetic properties of hexagonal polar semimetals RAuGe (R = Y, Gd-Tm, and Lu)

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.

cond-mat.mtrl-sci↗

arXiv2022

Anisotropic magnetotransport properties coupled with spiral spin modulation in a triangular-lattice magnet EuZnGe

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.

cond-mat.str-el↗