arXiv subjects

Hideki Kuwahara

Publications and source records attributed to Hideki Kuwahara.

8 recordsLinked to original sources

arXiv2026

CE-antiferromagnetic electronic structure in LaSr$_2$Mn$_2$O$_7$ revealed by micro-focused angle-resolved photoemission spectroscopy and tight-binding models

We have investigated the electronic structure of LaSr$_2$Mn$_2$O$_7$ in the CE-type antiferromagnetic (CE-AFM) state below the Néel temperature using micro-focused angle-resolved photoemission spectroscopy ($μ$-ARPES) and tight-binding models. In agreement with the tight-binding calculations, we found a dispersive intensity around the X point in the $μ$-ARPES spectra, where the A-type antiferromagnetic (A-AFM) phase has no bands experimentally and theoretically, demonstrating that we have successfully captured the signatures of the CE-AFM band structure for the first time. Many observed features can be explained by the CE-AFM tight-binding bands, although some of them and the overall near-Fermi level intensity mapping can be explained by the A-AFM band structure. This indicates that the CE-AFM domain size would be no larger than the beam footprint size of a $\sim\!20$~{\textmu}m scale.

cond-mat.str-el↗

arXiv2026

Effect of Pb Substitution at the Mo site on the Magnetic Properties of the Polar Magnet Fe$_2$Mo$_3$O$_8$

The ternary transition-metal oxide Fe$_2$Mo$_3$O$_8$ is a polar magnet characterized by a layered structure of magnetic Fe honeycomb lattices and non-magnetic Mo kagome lattices. Whereas previous studies have primarily focused on the chemical substitution at the Fe sites to modulate the magnetic properties, the Mo sites have remained largely unexplored due to the strong spin-singlet trimerization of Mo$^{4+}$ ions. In this study, we investigated the effect of substituting non-magnetic Pb$^{4+}$ and Zr$^{4+}$ ions into the Mo sites to intentionally disrupt the Mo trimers. Our results reveal that the disruption of the Mo spin-singlet state induces active spins within the Mo layer, resulting in the emergence of a ferromagnetic-like behavior that persists even at room temperature. Quantitative analysis that takes into account the weight fraction of the main phase suggests an effective spin $S = 1/2$ state per active Mo ion upon trimer disruption. These findings demonstrate that controlling non-magnetic cluster states within a polar host via chemical substitution is a promising approach for designing room-temperature magnetoelectric materials.

cond-mat.str-el↗

arXiv2025

Magnetoelectric Effect Dependent on Electric Field Direction in a Pyroelectric Ferrimagnet CaBaCo$_4$O$_7$

This study investigates the dependence of the static magnetoelectric (ME) effect on the external field direction in the pyroelectric-ferrimagnet CaBaCo$_4$O$_7$, a topic that remains largely unexplored compared to dynamical nonreciprocal ME effects. We measured the magnetization with respect to the inherent polarization and found that an external electric field stabilizes the ferrimagnetic phase when applied parallel to the polarization, and destabilizes it when antiparallel. These results clearly demonstrate the electric-field-direction dependent control of the static ME effect, suggesting a new route to enhancing ME effects in pyroelectric-magnetic materials.

cond-mat.mtrl-sci↗

arXiv2014

Successive magnetic transitions of Ca$_2$CoSi$_2$O$_7$ in high magnetic fields

Magnetic and dielectric properties of åkermanite Ca$_2$CoSi$_2$O$_7$ single crystals were investigated in pulsed high magnetic fields. In magnetic fields along the $c$ axis, this material shows a magnetization plateau in a wide range of field below the saturation. Magnetization processes for fields along the $a$ and $b$ axes show multiple anomalies but different traces to each other, indicating the breaking of four-fold symmetry. Measurements of the magnetoelectric effects exhibit the changes in electric polarization according to the changes in the spin system. The experimentally determined quadratic magnetoelectric tensor is consistent with that expected in the crystal symmetry of orthorhombic $P2_12_12$.

cond-mat.str-el↗

arXiv2011

Anomalous Coexistence of Ferroelectric Phases ($P\parallel a$ and $P\parallel c$) in Orthorhombic Eu$_{1-y}$Y$_y$MnO$_3$ ($y>0.5$) Crystals

We have investigated the magnetic and dielectric properties of orthorhombic Eu$_{1-y}$Y$_y$MnO$_3$ ($0\leq y\leq 0.6$) single crystals without the presence of the 4$f$ magnetic moments of the rare-earth ions. In $y\geq 0.2$, the magnetic-structure driven ferroelectricity is observed. The ferroelectric transition temperature is steeply reducing with increasing $y$. In $y\geq 0.52$, two ferroelectric phases ($P\parallel a$ and $P\parallel c$) are coexistent at low temperatures. In these phases, ferroelectricity has different origin, which is evidenced by the distinctive poling-electric-field dependence of electric polarization. Namely, the electric polarization along the c axis ($P_c$) is easily saturated by a poling electric field, therefore $P_c$ is caused by the $bc$ spiral antiferromagnetic order. On the other hand, the electric polarization along the a axis ($P_a$) is probably attributed to the collinear $E$-type antiferromagnetic order, because $P_a$ is unsaturated even in a poling field of $10^6$ V/m.

cond-mat.str-el↗

arXiv2011

Electron- and Hole-Doping Effects on $A$-site Ordered NdBaMn$_2$O$_6$

We have investigated electron- and hole-doping effects on $A$-site ordered perovskite manganite NdBaMn$_2$O$_6$, which has the $A$-type (layered) antiferromagnetic (AFM) ground state. Electrons (holes) are introduced by partial substitution of Ba$^{2+}$ (Nd$^{3+}$) with Nd$^{3+}$ (Ba$^{2+}$). Electron-doping generates ferromagnetic (FM) clusters in the $A$-type AFM matrix. With increasing the electron-doping level, the volume fraction of the FM phase or the number of the FM clusters is abruptly increasing. In contrast, the $A$-type AFM phase is robust against the hole-doping, and no FM correlation is observed in the hole-doped NdBaMn$_2$O$_6$.

cond-mat.str-el↗

arXiv2010

The Effect of $f$-$d$ Magnetic Coupling in Multiferroic $R$MnO$_3$ Crystals

We have established detailed magnetoelectric phase diagrams of (Eu$_{0.595}$Y$_{0.405}$)$_{1-x}$Tb$_x$MnO$_3$ ($0 \le x \le 1$) and (Eu,Y)$_{1-x}$Gd$_x$MnO$_3$ ($0 \le x \le 0.69$), whose average ionic radii of $R$-site ($R$: rare earth) cations are equal to that of Tb$^{3+}$, in order to reveal the effect of rare earth 4$f$ magnetic moments on the magnetoelectric properties. In spite of the same $R$-site ionic radii, the magnetoelectric properties of the two systems are remarkably different from each other. A small amount of Tb substitution on $R$ sites ($x \sim 0.2$) totally destroys ferroelectric polarization along the a axis ($P_a$), and an increase in Tb concentration stabilizes the $P_c$ phase. On the other hand, Gd substitution ($x \sim 0.2$) extinguishes the $P_c$ phase, and slightly suppresses the $P_a$ phase. These results demonstrate that the magnetoelectric properties of $R$MnO$_3$ strongly depend on the characteristics of the rare earth 4$f$ moments.

cond-mat.str-el↗

arXiv2009

Order-disorder effect of A-site and oxygen-vacancy on magnetic and transport properties of Y1/4Sr3/4CoO3-delta

We have synthesized A-site ordered (AO)- and A-site disordered (AD)-Y1/4Sr3/4CoO3-delta (YSCO) with various oxygen deficiency delta, and have made a comparative study of the structural and physical properties. We have found that A-site (Y/Sr) ordering produces the unconventional oxygen-vacancy ordered (OO) structure, and that the magnetic and transport properties of both AO- and AD-YSCO strongly depend on the oxygen-vacancy (or excess oxygen) ordering pattern. AO-YSCO with a stoichiometric delta of 0.5 has the unconventional OO structure reflecting Y/Sr ordering pattern. With decreasing delta from 0.5, the overall averaged OO structure is essentially unchanged except for an increase of occupancy ratio for the oxygen-vacant sites. At delta = 0.34, excess oxygen atoms are ordered to form a novel superstructure, which is significant for the room-temperature ferromagnetism of AO-YSCO. In AD-YSCO, on the other hand, the quite different OO structure, which is of a brownmillerite-type, is found only in the vicinity of delta = 0.5.

cond-mat.str-el↗