arXiv subjects

Masahiro Kawamata

Publications and source records attributed to Masahiro Kawamata.

10 recordsLinked to original sources

arXiv2026

Dipolar-driven mean-field criticality in the ferrimagnet Eu$_2$MnSi$_2$O$_7$

We report mean-field critical behavior in Eu$_2$MnSi$_2$O$_7$, a melilite-type ferrimagnet with spin-only Eu$^{2+}$ and Mn$^{2+}$ moments and negligible orbital contributions. Magnetization measurements combined with neutron powder diffraction reveal critical exponents close to the mean-field values, indicating that long-range dipolar interactions govern the asymptotic critical behavior in this insulating ferrimagnet. The refined magnetic structure, described by the magnetic space group $P2_12_1^\prime2^\prime$, exhibits a tilted ferrimagnetic configuration driven by the Dzyaloshinskii-Moriya interaction, reflecting the noncentrosymmetric nature of the lattice. These results extend the applicability of mean-field theory to complex insulating magnets and establish Eu$_2$MnSi$_2$O$_7$ as a platform for exploring ferrimagnetism and long-range interactions. To our knowledge, this is the first insulating ferrimagnet in which dipolar interactions drive mean-field criticality.

cond-mat.str-el↗

arXiv2026

Refining hydrogen positions in α-FeOOH through combined neutron diffraction and computational techniques

The hydrogen positions and magnetic structure of goethite $α$-FeOOH, a key component of iron rust, were examined through neutron diffraction. All symmetry-allowed magnetic structures under the space group $Pnma$ with the magnetic wavevector $\vec{q}_{\rm m} = (0, 0, 0)$ r.l.u. were analysed using irreducible representation and magnetic space group approaches. The magnetic moments aligned along the $b$-axis form antiferromagnetic spin arrangements, as reproduced by first-principles calculations. Accurately determining the hydrogen positions is crucial for understanding the mechanism of catalytic reduction of CO$_2$ in $α$-FeOOH. These positions were precisely identified through diffraction and calculations, highlighting the effectiveness of using both methods for undeuterated compounds.

cond-mat.mtrl-sci↗

arXiv2026

Collinear Magnetic Structure in the Diamond Network Magnet EuTi$_2$Al$_{20}$

The magnetic structure of EuTi$_2$Al$_{20}$, in which magnetic Eu$^{2+}$ ions form a diamond network, was investigated using neutron and resonant X-ray diffraction on powder and single-crystal samples. The propagation vector was determined to be $\textbf{\textit{q}}_{\rm m}=(1,0,0)$~r.l.u. from these diffraction measurements. All possible magnetic structures in the space group $Fd\bar{3}m$ with this propagation vector were examined using the irreducible representation method and magnetic space group analysis. This magnetic structure was identified as a collinear antiferromagnetic structure with the magnetic space group $P_Inna$ (\#52.320) or $P_Inn2$ (\#34.164) under zero magnetic field. In these magnetic structure, frustration arises from competing magnetic interactions on the diamond network. These findings provide a concrete experimental reference for assessing the role of competing interactions in diamond-network magnets and motivate further studies of interaction-driven quantum states.

cond-mat.str-el↗

arXiv2026

Emergent Anomalous Hall Effect in the Eu-Based Compound with a Diamond Network: The Centrosymmetric Cubic Antiferromagnet EuTi$_2$Al$_{20}$

The centrosymmetric cubic compound EuTi$_2$Al$_{20}$, in which magnetic Eu ions form a diamond network, undergoes an antiferromagnetic transition at T$_N$ = 3.3 K and exhibits metamagnetic transitions at H$_{m1}$ = 1.7 T and H$_{m2}$ = 2.8 T for H || [100] at 1.9 K. Between these fields, the magnetization shows a step-like behavior, defining an intermediate field-induced phase (Phase~II). We investigated the electronic transport in Phase~II and found that both the resistivity and Hall resistivity are markedly enhanced, while remaining nearly field independent within the phase. Phase~II appears for all field directions, although its transport response shows moderate directional dependence. These features differ from the strongly orientation-selective behavior often observed in skyrmion-lattice phases of several 4f-electron compounds, suggesting that Phase II may host a field-induced spin texture with a topological character distinct from that of a conventional skyrmion lattice.

cond-mat.str-el↗

arXiv2025

Anisotropic Band-Split Magnetism in Magnetostrictive CoFe$_2$O$_4$

Single crystal spinel CoFe$_2$O$_4$ exhibits the largest room-temperature saturation magnetostriction among non-rare-earth compounds and a high Curie temperature ($T_c \sim 780$ K), properties that are critical to a wide range of industrial and medical applications. Neutron spectroscopy reveals a large band splitting ($\sim$ 60 meV) between two ferrimagnetic magnon branches, which is driven by site mixing between Co$^{2+}$ and Fe$^{3+}$ cations, and a significantly weaker magnetocrystalline anisotropy ($\sim$ 3 meV). Central to this behavior is the competition between extremely large mismatched molecular fields on the tetrahedral $A$-site and octahedral $B$-site sublattices and the single-ion anisotropy on the $B$-site. This creates a strong energetic anisotropy that locks the magnetic moment within each structural domain in place. As a result of these differing energy scales, switching structural domains is energetically favored over a global spin reorientation under applied magnetic fields, and this is what amplifies the magnetostrictive nature of CoFe$_2$O$_4$.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic excitations in the noncentrosymmetric magnet Sr2MnSi2O7

Magnetic excitations in the noncentrosymmetric magnet Sr$_2$MnSi$_2$O$_7$ were investigated through inelastic neutron scattering measurements. Major magnetic excitations are limited up to the energy transfer of 0.5 meV, and two magnon branches under zero magnetic field were well explained in the framework of linear spin-wave theory. The magnitudes of the square-lattice in-plane and inter-plane nearest-neighbor interactions, spin anisotropy term, and the Dzyaloshinskii-Moriya interaction are respectively estimated to be $J_1=45.54(5)$ $μ$eV, $J_2=0.52(1)$ $μ$eV, $Λ=4.98(11)$ $μ$eV, $D_{xy}=0.02(9)$ $μ$eV, and $D_z=4.10(1)$ $μ$eV, and calculations using these parameters reproduce experimental data quite well. Sr$_2$MnSi$_2$O$_7$ appears to have the smallest energy scale among the melilite-type compounds, and the small $J_2/J_1=0.0114(2)$ indicates the sufficient two-dimensionality.

cond-mat.str-el↗

arXiv2024

Unusual spin dynamics in the van der Waals antiferromagnet FeGa2S4

Spin dynamics in the van der Waals antiferromagnet FeGa$_2$S$_4$ with triangular lattices are investigated using magnetometry, neutron scattering, and muon spin relaxation measurements. The characteristic spin relaxation time is thoroughly clarified over thirteen orders of magnitude. Although the temperature dependence of DC and AC susceptibilities recalls a conventional spin-glass transition, nonlinear susceptibilities showing no divergences at the anomalous temperature, $T^{\ast}=16.87(7)$~K, deny that and instead hint at other mechanisms. Elastic neutron scattering together with previously measured muon results depict a slowly fluctuated ($\sim 10^{-5}$~sec) spin state above $T^{\ast}$. In juxtaposing the underlying simplest structure among frustrated magnets with an intricate hierarchy of time scales, FeGa$_2$S$_4$ can be a playground for studying temporal spin correlations in the two-dimensional limit.

cond-mat.str-el↗

arXiv2023

Quantum Criticality in YbCu4Ni

We report on the quantum criticality of YbCu$_4$Ni as revealed by our combined micro- and macro-measurements. We determine the crystal structure of YbCu$_4$Ni with site mixing by neutron diffraction measurements, which suggests the possible presence of Kondo disorder. However, decreasing the local spin susceptibility distribution and the development of spin fluctuations below 10 K at ambient pressure by muon spin rotation and relaxation measurements suggests that YbCu4Ni exhibits quantum criticality. Therefore, our experimental results indicate that YbCu4Ni is a new material that exhibits quantum criticality under a zero magnetic field and ambient pressure.

cond-mat.str-el↗

arXiv2021

Second-order generalized Monge--Ampère equations on a plane and its geometric singular solutions

In the present paper, we study some generalized Monge--Ampère equations in terms of special exterior differential systems on a jet space. Moreover, we construct geometric singular solutions of the generalized Monge--Ampère equations by using the method of Cauchy characteristics. Furthermore, we give criteria for geometric singular solutions to be right-left equivalent to the cuspidal edge, swallowtail and butterfly.

math.DG↗

arXiv2021

On a generalization of Monge-Ampère equations and Monge-Ampère systems

We discuss Monge-Ampère equations from the view point of differential geometry. It is known that a Monge-Ampère equation corresponds to a special exterior differential system on a 1-jet space. In this paper, we generalize Monge-Ampère equations and prove that a $(k+1)$st order generalized Monge-Ampère equation corresponds to a special exterior differential system on a $k$-jet space. Then its solution naturally corresponds to an integral manifold of the corresponding exterior differential system. Moreover, we verify that the Korteweg-de Vries (KdV) equation and the Cauchy-Riemann equations are examples of our equation.

math.DG↗