arXiv subjects

Yusuke Nambu

Publications and source records attributed to Yusuke Nambu.

At least 19 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

Tetrahedrally ferromagnetic correlations and a glassy-freezing anomaly in the breathing pyrochlore magnet $\mathrm{AgInCr_4S_8}$ with partial $A$-site disorder

We investigate the chromium breathing pyrochlore sulfide $\mathrm{AgInCr_4S_8}$, a chromium-based thiospinel, by synchrotron x-ray and neutron powder diffraction, dc magnetization, and heat capacity. Diffraction confirms the $F\bar{4}3m$ breathing structure with alternating large and small $\mathrm{Cr_4}$ tetrahedra, a large breathing ratio ($d^\prime/d = 1.106$ at 300 K), and substantial Ag/In intermixing on the $A$ sublattice ($\sim 16\%$). No structural transition or magnetic Bragg peaks are detected down to 1.5 K. An enlarged low-angle difference plot between the 1.5 and 20 K neutron diffraction patterns shows a weak broad diffuse-like enhancement, consistent with short-range or frozen correlated moments within the sensitivity of the present data. Susceptibility yields a positive Weiss temperature $\theta_{\mathrm{W}} = +92$ K and a moment enhancement in 30--60 K, while the magnetic entropy released by $\sim 30$ K approaches a scale of order $R\ln 13$, together consistent with the development of short-range tetrahedral ferromagnetic correlations and an effective $S = 6$ cluster-moment picture. A broad susceptibility cusp with ZFC--FC bifurcation and a low-temperature specific heat anomaly near 9 K indicate a phenomenological glassy-freezing anomaly without long-range order. $\mathrm{AgInCr_4S_8}$ provides a benchmark for the interplay of strong breathing distortion and quenched $A$-site disorder in chromium breathing pyrochlores.

cond-mat.str-el↗

arXiv2026

Single-crystal growth and magnetic, magnetoelectric, and optical properties of ferroaxial-type SrMn$_2$Ni$_6$Te$_3$O$_{18}$

Single crystals of SrMn$_2$Ni$_6$Te$_3$O$_{18}$, a member of the ferroaxial-type magnetic oxide family $AB_{2}C_{6}$Te$_3$O$_{18}$ ($A$ = Pb, Sr; $B$ = Mn, Cd; $C$ = Ni, Co), have been successfully grown, and their structural, magnetic, magnetoelectric, and optical properties have been systematically studied. Imaging of the spatial distribution of electric-field-induced optical rotation reveals that the single crystals preferentially form single ferroaxial (FA) domains. Magnetization and neutron diffraction measurements show that Mn$^{2+}$ and Ni$^{2+}$ magnetic moments order antiferromagnetically at $T_{\rm N}$ = 83 K, forming a $c$-axis collinear bidirector-type antiferromagnetic structure. All independent magnetoelectric tensor components allowed by the magnetic point group 6/$m^{\prime}$ have been detected, and the $\chi_{33}$ component exhibits a pronounced temperature-dependent anomaly, including a peak and a sign reversal. Preferential formation of single FA domains and a similar $\chi_{33}$ anomaly are also observed in the isostructural compound PbMn$_2$Ni$_6$Te$_3$O$_{18}$. These findings suggest that the ferroaxial and magnetic characteristics within this structural framework are robust against Sr-Pb replacement.

cond-mat.mtrl-sci↗

arXiv2026

Refining hydrogen positions in {\alpha}-FeOOH through combined neutron diffraction and computational techniques

The hydrogen positions and magnetic structure of goethite $\alpha$-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 $\alpha$-FeOOH. These positions were precisely identified through diffraction and calculations, highlighting the effectiveness of using both methods for undeuterated compounds.

cond-mat.mtrl-sci↗

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↗

arXiv2025

Direct demonstration of electric chirality control in a helimagnetic YMn$_6$Sn$_6$ by spin-polarized neutron scattering

The spiral handedness of magnetic moments, referred to as chirality, gives rise to emergent electromagnetic phenomena in helimagnets. In insulating helimagnets, known as multiferroics, the cycloidal spin structure induces electric polarization by utilizing the inverse Dzyaloshinskii-Moriya mechanism. Spin-polarized neutron diffraction experiments, which directly probe circular spin arrangements, clearly demonstrated that an electric field controlled the chirality in multiferroic helimagnets. On the other hand, it was unclear until recently how the chirality could be controlled in metallic helimagnets where a large electric field cannot be applied, while the chirality control technique in metallic helimagnets should enable the exploration of chirality-dependent spintronic functionalities. Recently, Jiang et al. succeeded in controlling the chirality of a spiral structure by the simultaneous application of a magnetic field and electric current in a metallic helimagnet, utilizing the nonreciprocal electronic transport as an indirect probe of chirality, highlighting the need for a neutron diffraction experiment that directly probes the chirality. Here, we directly demonstrate the chirality control in a metallic helimagnet YMn$_6$Sn$_6$ by means of spin-polarized neutron diffraction, which should give rise to a firm basis for the development of future helimagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2025

A double-spiral spin ordering in the helimagnet YBaCuFeO$_{5}$

Materials with a spiral spin ordering always show a rich phase diagram and can be a playground for studying the exotic physical properties associated with spiral magnetism. Using neutron elastic and resonant x-ray scattering on a high-quality single crystal YBaCuFeO$_{5}$, we demonstrate YBaCuFeO$_{5}$ to be a helimagnet consisting of a double-spiral spin ordering. YBaCuFeO$_{5}$ undergoes a commensurate to incommensurate magnetic phase transition at $T_{N2}$$\sim$ 175 K, and the incommensurate phase consists of two spin-ordered components. Both components have different periodicities but with the same propagating direction along the {\it c}-axis below $T_{N2}$. Using resonant x-ray scattering at the Fe and Cu \textit{K}-edges, we further demonstrate that both spiral spin orderings result from the Fe$^{3+}$ and Cu$^{2+}$, respectively, forming a double-spiral spin ordering structure. This can be understood to be caused by the coupling between both sublattices of Fe$^{3+}$ and Cu$^{2+}$ with the atomic lattice.

cond-mat.str-el↗

arXiv2025

Universal whirling magnetic orders in non-Heisenberg Tsai-type quasicrystal approximants

Magnetic orders of non-Heisenberg Tsai-type 1/1 approximant crystals (ACs) in the Au-Ga-Dy system were studied through bulk magnetization, neutron diffraction, and inelastic neutron scattering techniques. The results uncovered noncoplanar, ferromagnetic (FM) and antiferromagnetic (AFM) spin configurations whirling along [111] crystallographic axis, which is analogous to those observed in the Tb- and Ho-contained counterparts. The crystal electric field excitations similar to those in the Tb-based counterpart are also observed indicating the strong Ising-like magnetic anisotropy. These comprehensive experiments and analyses have revealed the existence of a universal mechanism that stabilizes noncoplanar FM and AFM structures in non-Heisenberg Tsai-type ACs, independent of the rare-earth species (Tb, Dy, Ho); FM intra-cluster interactions and strong Ising-like anisotropy.

cond-mat.str-el↗

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 $J_1=45.54(5)$ $\mu$eV, $J_2=0.52(1)$ $\mu$eV, $\Lambda=4.98(11)$ $\mu$eV, $D_{xy}=0.02(9)$ $\mu$eV, and $D_z=4.10(1)$ $\mu$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↗

arXiv2024

Current-induced sliding motion in a helimagnet MnAu$_2$

We found signatures of current-induced sliding motion in helimagnetic $\mathrm{Mn}\mathrm{Au}_2$ thin films. An abrupt change in differential resistivity occurred at a threshold bias current in the helimagnetic state, whereas it was absent in the induced ferromagnetic state. Broadband voltage noise also emerged above the threshold current in the helimagnetic state. Based on the similarity to canonical charge/spin density wave systems, we ascribed the origin of these phenomena to the sliding motion of the helimagnetic structure.

cond-mat.mtrl-sci↗

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↗

arXiv2023

Thermal Magnetoelectrics in all Inorganic Quasi-Two-Dimensional Halide Perovskites

From lithium-ion batteries to high-temperature superconductors, oxide materials have been widely used in electronic devices. However, demands of future technologies require materials beyond oxides, as anion chemistries distinct from oxygen can expand the palette of mechanisms and phenomena, to achieve superior functionalities. Examples include nitride-based wide bandgap semiconductors and halide perovskite solar cells, with MAPbBr3 being a representation revolutionizing photovoltaics research. Here, we demonstrate magnetoelectric behaviour in quasi-two-dimensional halides (K,Rb)3Mn2Cl7 through simultaneous thermal control of electric and magnetic polarizations by exploiting a polar-to-antipolar displacive transition. Additionally, our calculations indicate a possible polarization switching path including a strong magnetoelectric coupling, indicating halides can be excellent platforms to design future multiferroic and ferroelectric devices. We expect our findings to broaden the exploration of multiferroics to non-oxide materials and open access to novel mechanisms, beyond conventional electric/magnetic control, for coupling ferroic orders.

cond-mat.mtrl-sci↗

arXiv2022

Higher-order modulations in the skyrmion-lattice phase of Cu$_2$OSeO$_3$

Using small angle neutron scattering, we have investigated higher-order peaks in the skyrmion-lattice phase of Cu$_2$OSeO$_3$, in which two different skyrmion lattices, SkX1 and SkX2, are known to form. For each skyrmion-lattice phase, we observed two sets of symmetrically inequivalent peaks at the higher-order-reflection positions with the indices $(110)$ and $(200)$. Under the condition where the SkX1 and SkX2 coexist, we confirmed the absence of the scattering at $\mathbf{Q}$ positions combining reflections from the two phases, indicating a significantly weak double-scattering component. Detailed analysis of the peak profile, as well as the temperature and magnetic-field dependence of the peak intensity, also supports the intrinsic higher-order modulation rather than the parasitic double scattering. The two higher-order modulations show contrasting magnetic-field dependence; the former $(110)$ increases as the field is increased, whereas the latter $(200)$ decreases. This indicates that, in Cu$_2$OSeO$_3$, skyrmions are weakly distorted, and the distortion is field-dependent in a way that the dominant higher-order modulation switches from $(110)$ to $(200)$ under field. Monte Carlo simulations under sweeping external magnetic field qualitatively reproduce the observed magnetic-field dependence, and suggests that the higher-order modulations correspond to the superlattices of weak swirlings appearing in the middle of the original triangular-latticed skyrmions.

cond-mat.str-el↗

arXiv2021

Triplon current generation in solids

A triplon refers to a fictitious particle that carries angular momentum $S = 1$ corresponding to the elementary excitation in a broad class of quantum dimerized spin systems. Such systems without magnetic order have long been studied as a testing ground for quantum properties of spins. Although triplons have been found to play a central role in thermal and magnetic properties in dimerized magnets with singlet correlation, a spin angular momentum flow carried by triplons, a triplon current, has not been detected yet. Here we report spin Seebeck effects induced by a triplon current: triplon spin Seebeck effect, using a spin-Peierls system CuGeO$_3$. The result shows that the heating-driven triplon transport induces spin current whose sign is positive, opposite to the spin-wave cases in magnets. The triplon spin Seebeck effect persists far below the spin-Peierls transition temperature, being consistent with a theoretical calculation for triplon spin Seebeck effects.

cond-mat.mtrl-sci↗

arXiv2021

Neutron Scattering Study on Yttrium Iron Garnet for Spintronics

Spin current -- a flow of the spin degree of freedom in matter -- has vital importance in spintronics. Propagation of the spin current ranges over a whole momentum space; however, generated spin currents are mainly detected in the long-wavelength limit. To facilitate practical uses of spintronics and magnonics, microscopic understanding of the spin current is necessary. We here address yttrium iron garnet, which is a well-employed ferrimagnet for spintronics, and review {\it in re} the momentum- and energy-resolved characteristics of its magnetism. Using {\it unpolarized} neutrons, we refined its detailed crystal and magnetic structure, and examined magnetic excitations through four decades (10~$\mu$eV-100~meV) using chopper spectrometers in J-PARC, Japan. We also measured mode-resolved directions of the precessional motion of the magnetic moment, i.e., magnon polarization, which carries the spin current in insulators through {\it polarized} neutron scattering, using a triple-axis spectrometer in ILL, France. The magnon polarization is a hitherto untested fundamental property of magnets, affecting the thermodynamic properties of the spin current. Our momentum- and energy-resolved experimental findings provide an intuitive understanding of the spin current and demonstrate the importance of neutron scattering techniques for spintronics and magnonics.

cond-mat.mtrl-sci↗

arXiv2020

Dichotomy Between Orbital and Magnetic Nematic Instabilities in BaFe2S3

Nematic orders emerge nearly universally in iron-based superconductors, but elucidating their origins is challenging because of intimate couplings between orbital and magnetic fluctuations. The iron-based ladder material BaFe2S3, which superconducts under pressure, exhibits antiferromagnetic order below TN ~ 117K and a weak resistivity anomaly at T* ~ 180K, whose nature remains elusive. Here we report angle-resolved magnetoresistance (MR) and elastoresistance (ER) measurements in BaFe2S3, which reveal distinct changes at T*. We find that MR anisotropy and ER nematic response are both suppressed near T*, implying that an orbital order promoting isotropic electronic states is stabilized at T*. Such an isotropic state below T* competes with the antiferromagnetic order, which is evidenced by the nonmonotonic temperature dependence of nematic fluctuations. In contrast to the cooperative nematic orders in spin and orbital channels in iron pnictides, the present competing orders can provide a new platform to identify the separate roles of orbital and magnetic fluctuations.

cond-mat.supr-con↗

arXiv2020

Competing spin modulations in a magnetically frustrated semimetal EuCuSb

The competing magnetic ground states of the itinerant magnet EuCuSb, which has a hexagonal layered structure, were studied via magnetization, resistivity, and neutron diffraction measurements on single-crystal samples. EuCuSb has a three-dimensional semimetallic band structure as confirmed by band calculation and angle-resolved photoelectron spectroscopy, consistent with the nearly isotropic metallic conductivity in the paramagnetic state. However, below the antiferromagnetic transition temperature of TN1 (8.5 K), the resistivity, especially along the hexagonal axis, increases significantly. This implies the emergence of anisotropic magnetic ordering coupled to the conducting electrons. Neutron diffraction measurements show that the Eu spins, which order ferromagnetically within each layer, are collinearly modulated (up-up-down-down) along the hexagonal axis below TN1, followed by the partial emergence of helical spin modulation below TN2 (6 K). Based on the observation of anomalous magnetoresistance with hysteretic behavior, we discuss the competing nature of the ground state inherent in a frustrated Heisenberg-like spin system with a centrosymmetric structure.

cond-mat.str-el↗