Showing 1–50 of 56 results for author: Nambu, Y

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

Authors: Masahiro Kawamata, Maxim Avdeev, Yusuke Nambu

Abstract: 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 in… ▽ More 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. △ Less

Submitted 12 August, 2026; originally announced August 2026.

Comments: 8 pages, 4 figures

Journal ref: Physical Review Letters 137, 076703 (2026)

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

Authors: Yuya Haraguchi, Masahiro Kitamura, Masaki Gen, Yusuke Nambu, Akira Matsuo, Koichi Kindo, Masaki Kondo, Masashi Tokunaga, Hiroko Aruga Katori

Abstract: 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 in… ▽ More 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 $θ_{\mathrm{W}} = +92$ 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. △ Less

Submitted 28 May, 2026; originally announced May 2026.

Comments: 14 pages, 6 figures, accepted in Physical Review B

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

Authors: Ryoya Nakamura, Shinichiro Asai, Yusuke Nambu, Takatsugu Masuda, Kenta Kimura

Abstract: 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 th… ▽ More 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 $χ_{33}$ component exhibits a pronounced temperature-dependent anomaly, including a peak and a sign reversal. Preferential formation of single FA domains and a similar $χ_{33}$ 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. △ Less

Submitted 23 June, 2026; v1 submitted 23 April, 2026; originally announced April 2026.

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

Authors: Yusuke Nambu, Akihide Kuwabara, Masahiro Kawamata, Seira Mori, Megumi Okazaki, Kazuhiko Maeda

Abstract: 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… ▽ More 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. △ Less

Submitted 27 March, 2026; originally announced March 2026.

Comments: 8 pages, 5 figures

Journal ref: Dalton Transactions (Spotlight Collection: Mixed-Anion Compounds) (2006)

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

Authors: Harry Lane, Guratinder Kaur, Masahiro Kawamata, Yusuke Nambu, Lukas Keller, Russell A. Ewings, David J. Voneshen, Travis J. Williams, Helen C. Walker, Dwight Viehland, Peter M. Gehring, Chris Stock

Abstract: 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 sit… ▽ More 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$_4$. △ Less

Submitted 17 December, 2025; originally announced December 2025.

Comments: (19 pages; 9 figures)

Journal ref: Adv.Funct.Mater. 2025, e16830

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

Authors: Hidetoshi Masuda, Yutaro Yanagisawa, Kazuki Ohishi, Yusuke Nambu, Yoichi Nii, Yoshinori Onose

Abstract: 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, c… ▽ More 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. △ Less

Submitted 5 December, 2025; originally announced December 2025.

Comments: 18 pages, 9 figures

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

Authors: Yu-Hui Liang, Chun-Hao Lai, Chin-Wei Wang, Shinichiro Yano, Daisuke Okuyama, Taku J. Sato, Yusuke Nambu, Shih-Chang Weng, Yen-Chung Lai, Wei-Tin Chen, Kirrily C. Rule, Chao-Hung Du

Abstract: 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 comme… ▽ More 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. △ Less

Submitted 27 March, 2025; originally announced March 2025.

Comments: 29 pages, 6 figures

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

Authors: Farid Labib, Kazuhiro Nawa, Yusuke Nambu, Hiroyuki Takakura, Yoichi Ikeda, Kazuhiko Deguchi, Masato Matsuura, Asuka Ishikawa, Ryoichi Kajimoto, Kazuhiko Ikeuchi, Taku J. Sato, Ryuji Tamura

Abstract: 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- a… ▽ More 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. △ Less

Submitted 13 January, 2026; v1 submitted 3 March, 2025; originally announced March 2025.

Field-Induced Criticality in YbCu4Au

Authors: T. Taniguchi, K. Osato, H. Okabe, T. Kitazawa, M. Kawamata, S. Hashimoto, Y. Ikeda, Y. Nambu, D. P. Sari, I. Watanabe, J. G. Nakamura, A. Koda, J. Gouchi, Y. Uwatoko, S. Kittaka, T. Sakakibara, M. Mizumaki, N. Kawamura, T. Yamanaka, K. Hiraki, T. Sasaki, M. Fujita

Abstract: YbCu4Au is a unique material exhibiting multiple quantum fluctuations simultaneously. In this study, we investigated the field-induced criticality in YbCu4Au, based on comprehensive micro and macro measurements, including powder X-ray diffraction (XRD), neutron powder diffraction (NPD), nuclear magnetic resonance, magnetization, resistivity, specific heat, muon spin rotation relaxation (muSR), and… ▽ More YbCu4Au is a unique material exhibiting multiple quantum fluctuations simultaneously. In this study, we investigated the field-induced criticality in YbCu4Au, based on comprehensive micro and macro measurements, including powder X-ray diffraction (XRD), neutron powder diffraction (NPD), nuclear magnetic resonance, magnetization, resistivity, specific heat, muon spin rotation relaxation (muSR), and X-ray absorption spectroscopy (XAS). Single crystals of YbCu4Au were grown, and their crystal structure was determined using XRD, and NPD measurements. Magnetic successive transitions were observed below 1 T by specific heat, resistivity, NPD, and muSR measurements. XAS measurements further indicate that the valence of Yb ions (+2.93) remained unchanged above 2 T. Moreover, the change in quadrupole frequency observed in the previous study is attributable to the electric quadrupole, as the expected value of the electric quadrupole was finite under magnetic fields [S. Wada et al., Journal of Physics: Condensed Matter, 20, 175201 (2008).]. These experimental results suggest that YbCu4Au exhibited bicritical behavior near 1 T, arising from the competition between RKKY interaction, accounting for the magnetic phases, and the Zeeman effect. △ Less

Submitted 7 November, 2024; originally announced November 2024.

Comments: 9 pages, 8 figures

Magnetic excitations in the noncentrosymmetric magnet Sr2MnSi2O7

Authors: Masahiro Kawamata, Xiaoqi Pang, Hiroshi Murakawa, Seiko Ohira-Kawamura, Kenji Nakajima, Hidetoshi Masuda, Masaki Fujita, Noriaki Hanasaki, Yoshinori Onose, Yusuke Nambu

Abstract: 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 nea… ▽ More 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. △ Less

Submitted 30 October, 2024; originally announced October 2024.

Comments: 6 pages, 5 figures

Journal ref: Phys. Rev. Mat. 9, 014407 (2025)

Unusual spin dynamics in the van der Waals antiferromagnet FeGa2S4

Authors: Yifei Tang, Yoshihiko Umemoto, Yo Kawamoto, Masahiro Kawamata, Shinichiro Asai, Yoichi Ikeda, Masaki Fujita, Yusuke Nambu

Abstract: 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, non… ▽ More 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. △ Less

Submitted 28 October, 2024; originally announced October 2024.

Comments: 10 pages, 5 figures

Journal ref: J. Phys.: Condens. Matter 37 035801 (2024). [Focus Issue on Frustrated Quantum Magnets]

Magnetic structure of the noncentrosymmetric magnet Sr2MnSi2O7 through irreducible representation and magnetic space group analyses

Authors: Y. Nambu, M. Kawamata, X. Pang, H. Murakawa, M. Avdeev, H. Kimura, H. Masuda, N. Hanasaki, Y. Onose

Abstract: Magnetic structures of the noncentrosymmetric magnet Sr$_2$MnSi$_2$O$_7$ were examined through neutron diffraction for powder and single-crystalline samples, as well as magnetometry measurements. All allowed magnetic structures under the space group $P\bar{4}2_1 m$ with the magnetic wavevector $\vec{q}_{\rm m}=(0,0,1/2)$~r.l.u., were analysed via irreducible representation and magnetic space group… ▽ More Magnetic structures of the noncentrosymmetric magnet Sr$_2$MnSi$_2$O$_7$ were examined through neutron diffraction for powder and single-crystalline samples, as well as magnetometry measurements. All allowed magnetic structures under the space group $P\bar{4}2_1 m$ with the magnetic wavevector $\vec{q}_{\rm m}=(0,0,1/2)$~r.l.u., were analysed via irreducible representation and magnetic space group approaches. The compound is refined to have in-plane magnetic moments within the magnetic space group $Cmc2_1.1^{\prime}_c$ (\#36.177) under zero field, which can be altered to $P2_1 2_1 2_1.1^{\prime}_c$ (\#19.28) above $μ_0 H=0.067(5)$~T to align induced weak-ferromagnetic components within one layer on the $ab$-plane. All refined parameters are provided following the recent framework based upon the magnetic space group, which better conveys when exchanging crystallographic information for commensurate magnetic structures. △ Less

Submitted 5 October, 2024; originally announced October 2024.

Comments: 7 pages, 4 figures

Journal ref: Acta Cryst. B 80, 393-400 (2024). [Focused Issue on Magnetic Structures]

Detailed dynamics of a moving magnetic skyrmion lattice in MnSi observed using a small-angle neutron scattering under an alternating electric current flow

Authors: D. Okuyama, M. Bleuel, Q. Ye, J. Krzywon, N. Nagaosa, A. Kikkawa, Y. Taguchi, Y. Tokura, J. D. Reim, Y. Nambu, T. J. Sato

Abstract: Lattice formation of swirling textures is ubiquitous in solid-state materials, such as a magnetic skyrmion lattice in chiral magnets. In the magnetic skyrmion lattices, their moving states and dynamics under external perturbations are still unrevealed, although a detailed understanding of the dynamics is crucial to realizing spintronic applications, such as magnetic domain-wall racetrack memory. H… ▽ More Lattice formation of swirling textures is ubiquitous in solid-state materials, such as a magnetic skyrmion lattice in chiral magnets. In the magnetic skyrmion lattices, their moving states and dynamics under external perturbations are still unrevealed, although a detailed understanding of the dynamics is crucial to realizing spintronic applications, such as magnetic domain-wall racetrack memory. Here, we report in detail on the transient state of a moving magnetic skyrmion lattice in bulk single-crystalline MnSi under alternating current (AC) using small-angle neutron scattering. A rotation and concomitant broadening of the spot width in the azimuthal direction of the magnetic skyrmion reflections originating from the plastic deformation of the magnetic skyrmion lattice were found only at low AC frequencies, whereas above the threshold AC frequency (ft ~ 0.12 Hz) the rotation was not observed, and the spot width becomes sharper. The observed complex response of the magnetic skyrmion reflections can be explained by the change in dislocation density in the magnetic skyrmion lattice. At frequencies higher than ft, the magnetic skyrmions oscillate removing the dislocations, indicating that the dislocation density is controlled by the AC frequency. △ Less

Submitted 26 April, 2024; originally announced April 2024.

Comments: 13 pages, 10 figures

Understanding spin currents from magnon dispersion and polarization: Spin-Seebeck effect and neutron scattering study on Tb3Fe5O12

Authors: Y. Kawamoto, T. Kikkawa, M. Kawamata, Y. Umemoto, A. G. Manning, K. C. Rule, K. Ikeuchi, K. Kamazawa, M. Fujita, E. Saitoh, K. Kakurai, Y. Nambu

Abstract: Magnon spin currents in the ferrimagnetic garnet Tb3Fe5O12 with 4f electrons were examined through the spin-Seebeck effect and neutron scattering measurements. The compound shows a magnetic compensation, where the spin-Seebeck signal reverses above and below Tcomp = 249.5(4) K. Unpolarized neutron scattering unveils two major magnon branches with finite energy gaps, which are well-explained in the… ▽ More Magnon spin currents in the ferrimagnetic garnet Tb3Fe5O12 with 4f electrons were examined through the spin-Seebeck effect and neutron scattering measurements. The compound shows a magnetic compensation, where the spin-Seebeck signal reverses above and below Tcomp = 249.5(4) K. Unpolarized neutron scattering unveils two major magnon branches with finite energy gaps, which are well-explained in the framework of spin-wave theory. Their temperature dependencies and the direction of the precession motion of magnetic moments, i.e. magnon polarization, defined using polarized neutrons, explain the reversal at Tcomp and decay of the spin-Seebeck signals at low temperatures. We illustrate an example that momentum- and energy-resolved microscopic information is a prerequisite to understanding the magnon spin current. △ Less

Submitted 1 April, 2024; originally announced April 2024.

Comments: 7 pages, 5 figures

Journal ref: Appl. Phys. Lett. 124, 132406 (2024). Featured Article & AIP Publishing Showcase

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

Authors: Yuta Kimoto, Hidetoshi Masuda, Takeshi Seki, Yoichi Nii, Jun-ichiro Ohe, Yusuke Nambu, Yoshinori Onose

Abstract: 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 canon… ▽ More 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. △ Less

Submitted 13 November, 2024; v1 submitted 29 February, 2024; originally announced March 2024.

Comments: 18 pages, 16 figures

Journal ref: Phys. Rev. Lett. 134, 056702 (2025)

Quantum Criticality in YbCu4Ni

Authors: Kotaro Osato, Takanori Taniguchi, Hirotaka Okabe, Takafumi Kitazawa, Masahiro Kawamata, Zhao Hongfei, Yoichi Ikeda, Yusuke Nambu, Dita Puspita Sari, Isao Watanabe, Jumpei G Nakamura, Akihiro Koda, Jun Gouchi, Yoshiya Uwatoko, Masaki Fujita

Abstract: 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 press… ▽ More 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. △ Less

Submitted 20 December, 2023; originally announced December 2023.

Comments: 8 pages, 5 figures

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

Authors: Tong Zhu, Xuezeng Lu, Takuya Aoyama, Koji Fujita, Yusuke Nambu, Takashi Saito, Hiroshi Takatsu, Tatsushi Kawasaki, Takumi Terauchi, Shunsuke Kurosawa, Akihiro Yamaji, Hao-Bo Li, Cedric Tassel, Kenya Ohgushi, James M. Rondinelli, Hiroshi Kageyama

Abstract: 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 p… ▽ More 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. △ Less

Submitted 15 November, 2023; originally announced November 2023.

Comments: 20 pages, 4 figures

Observation of field-induced single-ion magnetic anisotropy in a multiorbital Kondo alloy $\mathrm{(Lu,Yb)}\mathrm{Rh}_{2}\mathrm{Zn}_{20}$

Authors: T. Kitazawa, Y. Ikeda, T. Sakakibara, A. Matsuo, Y. Shimizu, Y. Tokunaga, Y. Haga, K. Kindo, Y. Nambu, K. Ikeuchi, K. Kamazawa, M. Ohkawara, M. Fujita

Abstract: We demonstrate field-induced single-ion magnetic anisotropy resulting from the multiorbital Kondo effect on the diluted ytterbium alloy $(\mathrm{Lu}_{1-x}\mathrm{Yb}_x)\mathrm{Rh}_2\mathrm{Zn}_{20}$. Single-ion anisotropic metamagnetic behavior is revealed in low-temperature regions where the local Fermi-liquid state is formed. Specific hea, low-field magnetic susceptibility, and resistivity indi… ▽ More We demonstrate field-induced single-ion magnetic anisotropy resulting from the multiorbital Kondo effect on the diluted ytterbium alloy $(\mathrm{Lu}_{1-x}\mathrm{Yb}_x)\mathrm{Rh}_2\mathrm{Zn}_{20}$. Single-ion anisotropic metamagnetic behavior is revealed in low-temperature regions where the local Fermi-liquid state is formed. Specific hea, low-field magnetic susceptibility, and resistivity indicate reproduction of the ground-state properties by the $\mathrm{SU}(N = 8)$ Kondo model with a relatively large $c$-$f$ hybridization of $T_{\mathrm{K}} = 60.9 \ \mathrm{K}$. Dynamical susceptibility measurements on $\mathrm{Yb}\mathrm{Rh}_2\mathrm{Zn}_{20}$ support realizing the multiorbital Kondo ground state in $(\mathrm{Lu}_{1-x}\mathrm{Yb}_x)\mathrm{Rh}_2\mathrm{Zn}_{20}$. The single-ion magnetic anisotropy becomes evident above $\sim5 \ \mathrm{T}$, which is lower than the isotropic Kondo crossover field of 22.7 T, verifying blurred low-lying crystal field states through the multiorbital Kondo effect. △ Less

Submitted 4 August, 2023; v1 submitted 17 January, 2023; originally announced January 2023.

Comments: 14 pages, 12 figures, 6 pages of supplementary material

Journal ref: Phys. Rev. B 108, 085105 (2023)

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

Authors: Johannes D. Reim, Shinnosuke Matsuzaka, Koya Makino, Seno Aji, Ryo Murasaki, Daiki Higashi, Daisuke Okuyama, Yusuke Nambu, Elliot P. Gilbert, Norman Booth, Shinichiro Seki, Yoshinori Tokura, Taku J Sato

Abstract: 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 th… ▽ More 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. △ Less

Submitted 18 April, 2022; originally announced April 2022.

Comments: 13 pages, 14 figures

Expanding Edges of Quantum Hall Systems in a Cosmology Language -- Hawking Radiation from de Sitter Horizon in Edge Modes

Authors: Masahiro Hotta, Yasusada Nambu, Yuuki Sugiyama, Kazuhiro Yamamoto, Go Yusa

Abstract: Expanding edge experiments are promising to open new physics windows of quantum Hall systems. In a static edge, the edge excitation, which is described by free fields decoupled with the bulk dynamics, is gapless, and the dynamics preserve conformal symmetry. When the edge expands, such properties need not be preserved. We formulate a quantum field theory in 1+1 dimensional curved spacetimes to ana… ▽ More Expanding edge experiments are promising to open new physics windows of quantum Hall systems. In a static edge, the edge excitation, which is described by free fields decoupled with the bulk dynamics, is gapless, and the dynamics preserve conformal symmetry. When the edge expands, such properties need not be preserved. We formulate a quantum field theory in 1+1 dimensional curved spacetimes to analyze the edge dynamics. We propose methods to address the following questions using edge waveforms from the expanding region: Does the conformal symmetry survive? Is the nonlinear interaction of the edge excitations induced by edge expansion? Do the edge excitations interact with the bulk excitations? We additionally show that the expanding edges can be regarded as expanding universe simulators of two-dimensional dilaton-gravity models, including the Jackiw-Teitelboim gravity model. As an application, we point out that our theoretical setup might simulate emission of analog Hawking radiation with the Gibbons-Hawking temperature from the future de Sitter horizon formed in the expanding edge region. △ Less

Submitted 7 April, 2022; v1 submitted 8 February, 2022; originally announced February 2022.

Comments: A subtitle and arguments about Hawking radiation in de Sitter space are added

Breakdown of Linear Spin-wave Theory and Existence of Spinon Bound States in the Frustrated Kagome Lattice Antiferromagnet

Authors: K. Matan, T. Ono, S. Ohira-Kawamura, K. Nakajima, Y. Nambu, T. J. Sato

Abstract: Spin dynamics of the spin-1/2 kagome lattice antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$ was studied using high-resolution, time-of-flight inelastic neutron scattering. The flat mode, a characteristic of the frustrated kagome antiferromagnet, and the low-energy dispersive mode, which is dominated by magnons, can be well described by the linear spin-wave theory. However, the theory fails to describe thr… ▽ More Spin dynamics of the spin-1/2 kagome lattice antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$ was studied using high-resolution, time-of-flight inelastic neutron scattering. The flat mode, a characteristic of the frustrated kagome antiferromagnet, and the low-energy dispersive mode, which is dominated by magnons, can be well described by the linear spin-wave theory. However, the theory fails to describe three weakly dispersive modes between 9 and 14 meV. These modes could be attributed to two-spinon bound states, which decay into free spinons away from the zone center and at a high temperature, giving rise to continuum scattering. △ Less

Submitted 8 February, 2022; originally announced February 2022.

Comments: 4 figures and Supplementary Information

Journal ref: Phys. Rev. B 105, 134403 (2022)

Triplon current generation in solids

Authors: Yao Chen, Masahiro Sato, Yifei Tang, Yuki Shiomi, Koichi Oyanagi, Takatsugu Masuda, Yusuke Nambu, Masaki Fujita, Eiji Saitoh

Abstract: 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… ▽ More 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. △ Less

Submitted 1 September, 2021; originally announced September 2021.

Journal ref: Nat Commun 12, 5199 (2021)

Neutron Scattering Study on Yttrium Iron Garnet for Spintronics

Authors: Yusuke Nambu, Shin-ichi Shamoto

Abstract: 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 iro… ▽ More 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~$μ$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. △ Less

Submitted 29 June, 2021; originally announced June 2021.

Comments: 15 pages, 17 figures

Journal ref: J. Phys. Soc. Jpn. 90, 081002 (2021)

Glasslike phonon excitation caused by ferroelectric structural instability

Authors: Y. Ishii, A. Yamamoto, N. Sato, Y. Nambu, S. Ohira-Kawamura, N. Murai, T. Mori, S. Mori

Abstract: 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… ▽ More 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. △ Less

Submitted 5 April, 2021; originally announced April 2021.

Comments: 6 pages, 4 figures

Dichotomy Between Orbital and Magnetic Nematic Instabilities in BaFe2S3

Authors: Suguru Hosoi, Takuya Aoyama, Kousuke Ishida, Yuta Mizukami, Kazuki Hashizume, Satoshi Imaizumi, Yoshinori Imai, Kenya Ohgushi, Yusuke Nambu, Motoi Kimata, Shojiro Kimura, Takasada Shibauchi

Abstract: 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 re… ▽ More 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. △ Less

Submitted 17 November, 2020; originally announced November 2020.

Comments: 7 pages 5 figures, to be published in Phys. Rev. Res

Journal ref: Phys. Rev. Research 2, 043293 (2020)

Competing spin modulations in a magnetically frustrated semimetal EuCuSb

Authors: Hidefumi Takahashi, Kai Aono, Yusuke Nambu, Ryoji Kiyanagi, Takuya Nomoto, Masato Sakano, Kyoko Ishizaka, Ryotaro Arita, Shintaro Ishiwata

Abstract: 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 metal… ▽ More 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. △ Less

Submitted 14 October, 2020; originally announced October 2020.

Comments: 12 pages, 4 figures, Physical review B in press

Neutron polarisation correction to triple-axis data with analytical derivations

Authors: Yusuke Nambu, Mechthild Enderle, Tobias Weber, Kazuhisa Kakurai

Abstract: Polarised neutron scattering is the method of choice to study magnetism in condensed matter. Polarised neutrons are typically very low in flux, and complex experimental configurations further reduce the count rate. Neutron polarisation corrections would therefore be needed. Here we analytically derive formulae of the corrected partial differential scattering cross-sections. The analytical method i… ▽ More Polarised neutron scattering is the method of choice to study magnetism in condensed matter. Polarised neutrons are typically very low in flux, and complex experimental configurations further reduce the count rate. Neutron polarisation corrections would therefore be needed. Here we analytically derive formulae of the corrected partial differential scattering cross-sections. The analytical method is designed for the longitudinal polarisation analysis, and the correction generally holds for time-independent polarised neutrons with a triple-axis spectrometer. We then apply the correction to recent results of our $P_x$ experiment on Y$_3$Fe$_5$O$_{12}$. Although there is a difficulty with the experimental determination of inefficiency parameters of neutron spin polarisers and flippers, the correction appears to work properly. △ Less

Submitted 6 May, 2023; v1 submitted 28 June, 2020; originally announced June 2020.

Comments: 8 pages, 2 figures

Journal ref: J. Phys.: Conf. Ser. 2481 012004 (2023)

Impact of the lattice on magnetic properties and possible spin nematicity in the S=1 triangular antiferromagnet NiGa$_2$S$_4$

Authors: Michael E. Valentine, Tomoya Higo, Yusuke Nambu, Dipanjan Chaudhuri, Jiajia Wen, Collin Broholm, Satoru Nakatsuji, Natalia Drichko

Abstract: NiGa$_2$S$_4$ is a triangular lattice S=1 system with strong two-dimensionality of the lattice, actively discussed as a candidate to host spin-nematic order brought about by strong quadrupole coupling. Using Raman scattering spectroscopy we identify a phonon of E$_g$ symmetry which can modulate magnetic exchange $J_1$ and produce quadrupole coupling. Additionally, our Raman scattering results demo… ▽ More NiGa$_2$S$_4$ is a triangular lattice S=1 system with strong two-dimensionality of the lattice, actively discussed as a candidate to host spin-nematic order brought about by strong quadrupole coupling. Using Raman scattering spectroscopy we identify a phonon of E$_g$ symmetry which can modulate magnetic exchange $J_1$ and produce quadrupole coupling. Additionally, our Raman scattering results demonstrate a loss of local inversion symmetry on cooling, which we associate with sulfur vacancies. This will lead to disordered Dzyaloshinskii-Moriya interactions, which can prevent long range magnetic order. Using magnetic Raman scattering response we identify 160~K as a temperature of an upturn of magnetic correlations. The temperature below 160~K, but above 50~K where antiferromagnetic magnetic start to increase, is a candidate for spin-nematic regime. △ Less

Submitted 12 May, 2020; originally announced May 2020.

Journal ref: Phys. Rev. Lett. 125, 197201 (2020)

Observation of the Magnon Polarization

Authors: Y. Nambu, J. Barker, Y. Okino, T. Kikkawa, Y. Shiomi, M. Enderle, T. Weber, B. Winn, M. Graves-Brook, J. M. Tranquada, T. Ziman, M. Fujita, G. E. W. Bauer, E. Saitoh, K. Kakurai

Abstract: We measure the mode-resolved direction of the precessional motion of the magnetic order, i.e., magnon polarization, via the chiral term of inelastic polarized neutron scattering spectra. The magnon polarisation is important in spintronics, affecting thermodynamic properties such as the magnitude and sign of the spin Seebeck effect. The observation of both signs of magnon polarization in Y3Fe5O12 a… ▽ More We measure the mode-resolved direction of the precessional motion of the magnetic order, i.e., magnon polarization, via the chiral term of inelastic polarized neutron scattering spectra. The magnon polarisation is important in spintronics, affecting thermodynamic properties such as the magnitude and sign of the spin Seebeck effect. The observation of both signs of magnon polarization in Y3Fe5O12 also gives direct proof of its ferrimagnetic nature. The experiments agree very well with atomistic simulations of the scattering cross section. △ Less

Submitted 27 November, 2019; originally announced November 2019.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 125, 027201 (2020)

Magnetic structure and high-field magnetization of the distorted kagome lattice antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$

Authors: K. Matan, T. Ono, G. Gitgeatpong, K. de Roos, P. Miao, S. Torii, T. Kamiyama, A. Miyata, A. Matsuo, K. Kindo, S. Takeyama, Y. Nambu, P. Piyawongwatthana, T. J. Sato, H. Tanaka

Abstract: High-resolution time-of-flight powder neutron diffraction and high-field magnetization were measured to investigate the magnetic structure and existence of a field-induced magnetic phase transition in the distorted kagome antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$. Upon cooling from room temperature, the compound undergoes a structural phase transition at $T_\textrm{t}=185$ K from the rhombohedral spa… ▽ More High-resolution time-of-flight powder neutron diffraction and high-field magnetization were measured to investigate the magnetic structure and existence of a field-induced magnetic phase transition in the distorted kagome antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$. Upon cooling from room temperature, the compound undergoes a structural phase transition at $T_\textrm{t}=185$ K from the rhombohedral space group $R\bar{3}m$ with the perfect kagome spin network to the monoclinic space group $P2_1/n$ with the distorted kagome planes. The distortion results in three inequivalent exchange interactions among the $S=1/2$ Cu$^{2+}$ spins that magnetically order below $T_\textrm{N}=20.2$ K. Magnetization measured with a magnetic field applied within the kagome plane reveals small in-plane ferromagnetism resulting from spin canting. On the other hand, the out-of-plane magnetization does not show a clear hysteresis loop of the ferromagnetic component nor a prominent anomaly up to 170 T, with the exception of the subtle knee-like bend around 90 T, which could indicate the 1/3 magnetization plateau. The combined analysis using the irreducible representations of the magnetic space groups and magnetic structure refinement on the neutron powder diffraction data suggests that the magnetic moments order in the magnetic space group $P2_1'/n'$ with the all-in-all-out spin structure, which by symmetry allows for the in-plane canting, consistent with the in-plane ferromagnetism observed in the magnetization. △ Less

Submitted 15 January, 2021; v1 submitted 4 May, 2019; originally announced May 2019.

Comments: 14 pages, 9 figures, published version with typos corrected

Journal ref: Phys. Rev. B 99, 224404 (2019)

Polar State induced by Block-type Lattice Distortions in BaFe2Se3 with Quasi-One-Dimensional Ladder Structure

Authors: Takuya Aoyama, Satoshi Imaizumi, Takuya Togashi, Yoshifumi Sato, Kazuki Hashizume, Yusuke Nambu, Yasuyuki Hirata, Masakazu Matsubara, Kenya Ohgushi

Abstract: Temperature dependent crystal structures of the quasi-one-dimensional ladder material BaFe2Se3 are examined. Combining the optical second harmonic generation (SHG) experiments and neutron diffraction measurements, we elucidate the crystal structure with Pmn2_1 space group in the low-temperature phase below Ts2 = 400 K, further above Neel temperature. This low-temperature phase loses the spatial in… ▽ More Temperature dependent crystal structures of the quasi-one-dimensional ladder material BaFe2Se3 are examined. Combining the optical second harmonic generation (SHG) experiments and neutron diffraction measurements, we elucidate the crystal structure with Pmn2_1 space group in the low-temperature phase below Ts2 = 400 K, further above Neel temperature. This low-temperature phase loses the spatial inversion symmetry, where a resultant macroscopic polarization emerges along the rung direction. The transition is characterized by block-type lattice distortions with the magneto-striction mechanism. Change in the electrical resistivity and the magnetic susceptibility across the polar-nonpolar transition also suggests a modification of the electronic states reflecting the structural instability. Consistency and discrepancy with the existing theory are discussed. △ Less

Submitted 28 February, 2019; originally announced February 2019.

Journal ref: Phys. Rev. B 99, 241109 (2019)

Deformation of the Magnetic Skyrmion Lattice in MnSi under Electric Current Flow

Authors: D. Okuyama, M. Bleuel, J. S. White, Q. Ye, J. Krzywon, G. Nagy, Z. Q. Im, I. Zivkovic, M. Bartkowiak, H. M. Ronnow, S. Hoshino, J. Iwasaki, N. Nagaosa, A. Kikkawa, Y. Taguchi, Y. Tokura, D. Higashi, J. D. Reim, Y. Nambu, T. J. Sato

Abstract: Using small-angle neutron scattering (SANS), we investigate the deformation of the magnetic skyrmion lattice in bulk single-crystalline MnSi under electric current flow. A significant broadening of the skyrmion-lattice-reflection peaks was observed in the SANS pattern for current densities greater than a threshold value j_t ~ 1 MA/m^2 (10^6 A/m^2). We show this peak broadening to originate from a… ▽ More Using small-angle neutron scattering (SANS), we investigate the deformation of the magnetic skyrmion lattice in bulk single-crystalline MnSi under electric current flow. A significant broadening of the skyrmion-lattice-reflection peaks was observed in the SANS pattern for current densities greater than a threshold value j_t ~ 1 MA/m^2 (10^6 A/m^2). We show this peak broadening to originate from a spatially inhomogeneous rotation of the skyrmion lattice, with an inverse rotation sense observed for opposite sample edges aligned with the direction of current flow. The peak broadening (and the corresponding skyrmion lattice rotations) remain finite even after switching off the electric current. These results indicate that skyrmion lattices under current flow experience significant friction near the sample edges, and plastic deformation due to pinning effects, these being important factors that must be considered for the anticipated skyrmion-based applications in chiral magnets at the nanoscale. △ Less

Submitted 19 July, 2018; originally announced July 2018.

Journal ref: Commun. Phys. 2, 79 (2019)

Pressure-Induced Metallization in Iron-Based Ladder Compounds Ba$_{1-x}$Cs$_x$Fe$_2$Se$_3$

Authors: Takafumi Hawai, Chizuru Kawashima, Kenya Ohgushi, Kazuyuki Matsubayashi, Yusuke Nambu, Yoshiya Uwatoko, Taku J. Sato, Hiroki Takahashi

Abstract: Electrical resistivity measurements have been performed on the iron-based ladder compounds Ba$_{1-x}$Cs$_x$Fe$_2$Se$_3$ ($x$ = 0, 0.25, 0.65, and 1) under high pressure. A cubic anvil press was used up to 8.0 GPa, whereas further higher pressure was applied using a diamond anvil cell up to 30.0 GPa. Metallic behavior of the electrical conductivity was confirmed in the $x$ = 0.25 and 0.65 samples f… ▽ More Electrical resistivity measurements have been performed on the iron-based ladder compounds Ba$_{1-x}$Cs$_x$Fe$_2$Se$_3$ ($x$ = 0, 0.25, 0.65, and 1) under high pressure. A cubic anvil press was used up to 8.0 GPa, whereas further higher pressure was applied using a diamond anvil cell up to 30.0 GPa. Metallic behavior of the electrical conductivity was confirmed in the $x$ = 0.25 and 0.65 samples for pressures greater than 11.3 and 14.4 GPa, respectively, with the low-temperature $\log T$ upturn being consistent with weak localization of 2D electrons due to random potential. At pressures higher than 23.8 GPa, three-dimensional Fermi-liquid-like behavior was observed in the latter sample. No metallic conductivity was observed in the parent compounds BaFe$_2$Se$_3$ ($x $ = 0) up to 30.0 GPa and CsFe$_2$Se$_3$ ($x$ = 1) up to 17.0 GPa. The present results indicate that the origins of the insulating ground states in the parent and intermediate compounds are intrinsically different; the former is a Mott insulator, whereas the latter is an Anderson insulator owing to the random substitution of Cs for Ba. △ Less

Submitted 16 December, 2016; originally announced December 2016.

Comments: 5 pages, 3 figures

Thermal stability and irreversibility of skyrmion-lattice phases in Cu$_2$OSeO$_3$

Authors: Koya Makino, Johannes D. Reim, Daiki Higashi, Daisuke Okuyama, Taku J. Sato, Yusuke Nambu, Elliot P. Gilbert, Norman Booth, Shinichiro Seki, Yoshinori Tokura

Abstract: Small angle neutron scattering measurements have been performed to study the thermodynamic stability of skyrmion-lattice phases in Cu$_2$OSeO$_3$. We found that the two distinct skyrmion-lattice phases [SkX(1) and SkX(2) phases] can be stabilized through different thermal histories; by cooling from the paramagnetic phase under finite magnetic field, the SkX(2) phase is selected. On the other hand,… ▽ More Small angle neutron scattering measurements have been performed to study the thermodynamic stability of skyrmion-lattice phases in Cu$_2$OSeO$_3$. We found that the two distinct skyrmion-lattice phases [SkX(1) and SkX(2) phases] can be stabilized through different thermal histories; by cooling from the paramagnetic phase under finite magnetic field, the SkX(2) phase is selected. On the other hand, the 30$^{\circ}$-rotated SkX(1) phase becomes dominant by heating the sample from the ordered conical phase under finite field. This difference in stabilization is surprisingly similar to the irreversibility observed in spin glasses. The zero-field cooling results in the co-existence of the two phases. It is further found that once one of the skyrmion-lattice phases is formed, it is hardly destabilized. This indicates unusual thermal stability of the two skyrmion-lattice phases originating from an unexpectedly large energy barrier between them. △ Less

Submitted 3 February, 2017; v1 submitted 22 August, 2016; originally announced August 2016.

Comments: 9 pages, 10 figures

Journal ref: Phys. Rev. B 95, 134412 (2017)

Spin Fluctuations from Hertz to Terahertz on a Triangular Lattice

Authors: Yusuke Nambu, Jason S. Gardner, Douglas E. MacLaughlin, Chris Stock, Hitoshi Endo, Seth Jonas, Taku J. Sato, Satoru Nakatsuji, Collin Broholm

Abstract: The temporal magnetic correlations of the triangular lattice antiferromagnet NiGa$_2$S$_4$ are examined through thirteen decades ($10^{-13}-1$~sec) using ultra-high-resolution inelastic neutron scattering, muon spin relaxation, AC and nonlinear susceptibility measurements. Unlike the short-ranged {\it spatial} correlations, the temperature dependence of the {\it temporal} correlations show distinc… ▽ More The temporal magnetic correlations of the triangular lattice antiferromagnet NiGa$_2$S$_4$ are examined through thirteen decades ($10^{-13}-1$~sec) using ultra-high-resolution inelastic neutron scattering, muon spin relaxation, AC and nonlinear susceptibility measurements. Unlike the short-ranged {\it spatial} correlations, the temperature dependence of the {\it temporal} correlations show distinct anomalies. The spin fluctuation rate decreases precipitously upon cooling towards $T^{\ast}=8.5$~K, but fluctuations on the microsecond time scale then persist in an anomalous dynamical regime for 4 K $<T\le T^{\ast}$. As this time scale exceeds that of single site dynamics by six orders of magnitude, these fluctuations bear evidence of emergent degrees of freedom within the short-range correlated incommensurate state of NiGa$_2$S$_4$. △ Less

Submitted 15 September, 2015; originally announced September 2015.

Journal ref: Phys. Rev. Lett. 115, 127202 (2015)

Pressure-induced Superconductivity in the Iron-based Ladder Material BaFe2S3

Authors: Hiroki Takahashi, Akira Sugimoto, Yusuke Nambu, Touru Yamauchi, Yasuyuki Hirata, Takateru Kawakami, Maxim Avdeev, Kazuyuki Matsubayashi, Fei Du, Chizuru Kawashima, Hideto Soeda, Satoshi Nakano, Yoshiya Uwatoko, Yutaka Ueda, Taku J. Sato, Kenya Ohgushi

Abstract: All the iron-based superconductors identified to date share a square lattice composed of Fe atoms as a common feature, despite having different crystal structures. In copper-based materials, the superconducting phase emerges not only in square lattice structures but also in ladder structures. Yet iron-based superconductors without a square lattice motif have not been found despite being actively s… ▽ More All the iron-based superconductors identified to date share a square lattice composed of Fe atoms as a common feature, despite having different crystal structures. In copper-based materials, the superconducting phase emerges not only in square lattice structures but also in ladder structures. Yet iron-based superconductors without a square lattice motif have not been found despite being actively sought out. Here, we report the discovery of pressure-induced superconductivity in the iron-based spin-ladder material BaFe2S3, a Mott insulator with striped-type magnetic ordering below ~120 K. On the application of pressure this compound exhibits a metal-insulator transition at about 11 GPa, followed by the appearance of superconductivity below Tc = 14 K, right after the onset of the metallic phase. Our findings indicate that iron-based ladder compounds represent promising material platforms, in particular for studying the fundamentals of iron-based superconductivity. △ Less

Submitted 21 July, 2015; originally announced July 2015.

Comments: in Nature Materials, 20 July 2015

Temperature and composition phase diagram in the iron-based ladder compounds Ba1-xCsxFe2Se3

Authors: Takafumi Hawai, Yusuke Nambu, Kenya Ohgushi, Fei Du, Yasuyuki Hirata, Maxim Avdeev, Yoshiya Uwatoko, Yurina Sekine, Hiroshi Fukazawa, Jie Ma, Songxue Chi, Yutaka Ueda, Hideki Yoshizawa, Taku J. Sato

Abstract: We investigated the iron-based ladder compounds (Ba,Cs)Fe$_2$Se$_3$. Their parent compounds, BaFe$_2$Se$_3$ and CsFe$_2$Se$_3$, have different space groups, formal valences of Fe and magnetic structures. Electrical resistivity, specific heat, magnetic susceptibility, X-ray diffraction and powder neutron diffraction measurements were conducted to obtain temperature and composition phase diagram of… ▽ More We investigated the iron-based ladder compounds (Ba,Cs)Fe$_2$Se$_3$. Their parent compounds, BaFe$_2$Se$_3$ and CsFe$_2$Se$_3$, have different space groups, formal valences of Fe and magnetic structures. Electrical resistivity, specific heat, magnetic susceptibility, X-ray diffraction and powder neutron diffraction measurements were conducted to obtain temperature and composition phase diagram of this system. Block magnetism observed in BaFe$_2$Se$_3$ is drastically suppressed with Cs doping. In contrast, stripe magnetism observed in CsFe$_2$Se$_3$ is not so fragile against Ba doping. New type of magnetic structure appears in intermediate compositions, which is similar to stripe magnetism of CsFe$_2$Se$_3$, but inter-ladder spin configuration is different. Intermediate compounds show insulating behavior, nevertheless finite $T$-linear contribution in specific heat was obtained at low temperatures. △ Less

Submitted 27 March, 2015; originally announced March 2015.

Comments: 12 pages, 13 figures

Anisotropic inplane spin correlation in the parent and Co-doped BaFe2As2: a neutron scattering study

Authors: S. Ibuka, Y. Nambu, T. Yamazaki, M. D. Lumsden, T. J. Sato

Abstract: Antiferromagnetic spin fluctuations were investigated in the normal states of the parent ($x = 0$), under-doped ($x = 0.04$) and optimally-doped ($x = 0.06$) Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals using inelastic neutron scattering technique. For all the doping levels, quasi-two-dimensional antiferromagnetic fluctuations were observed as a broad peak localized at ${\it Q} = (1/2, 1/2, l)$.… ▽ More Antiferromagnetic spin fluctuations were investigated in the normal states of the parent ($x = 0$), under-doped ($x = 0.04$) and optimally-doped ($x = 0.06$) Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals using inelastic neutron scattering technique. For all the doping levels, quasi-two-dimensional antiferromagnetic fluctuations were observed as a broad peak localized at ${\it Q} = (1/2, 1/2, l)$. At lower energies, the peak shows an apparent anisotropy in the $hk0$ plane; longitudinal peak widths are considerably smaller than transverse widths. The anisotropy is larger for the higher doping level. These results are consistent with the random phase approximation (RPA) calculations taking account of the orbital character of the electronic bands, confirming that the anisotropic nature of the spin fluctuations in the normal states is mostly dominated by the nesting of Fermi surfaces. On the other hand, the quasi-two-dimensional spin correlations grow much rapidly for decreasing temperature in the $x = 0$ parent compound, compared to that expected for nearly antiferromagnetic metals. This may be another sign of the unconventional nature of the antiferromagnetic transition in BaFe$_2$As$_2$. △ Less

Submitted 20 October, 2014; originally announced October 2014.

Comments: 9 pages, 6 figures

Journal ref: Physica C, 507 (2014) 25-30

Large Negative Quantum Renormalization of Excitation Energies in the Spin-1/2 Kagome Lattice Antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$

Authors: T. Ono, K. Matan, Y. Nambu, T. J. Sato, K. Katayama, S. Hirata, H. Tanaka

Abstract: Magnetic excitations in the spin-$\frac{1}{2}$ distorted kagome lattice antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$, which has an ordered ground state owing to the strong Dzyaloshinskii-Moriya interaction, were studied using inelastic neutron scattering. Although the spin-wave dispersion can be qualitatively understood in terms of linear spin-wave theory (LSWT), the excitation energies are renormalized… ▽ More Magnetic excitations in the spin-$\frac{1}{2}$ distorted kagome lattice antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$, which has an ordered ground state owing to the strong Dzyaloshinskii-Moriya interaction, were studied using inelastic neutron scattering. Although the spin-wave dispersion can be qualitatively understood in terms of linear spin-wave theory (LSWT), the excitation energies are renormalized by a factor of approximately 0.6 from those calculated by LSWT, almost irrespective of the momentum transfer. This inadequacy of LSWT, which is attributed to quantum fluctuations, provides evidence of negative quantum renormalization in the spin-$\frac{1}{2}$ kagome lattice antiferromagnet. △ Less

Submitted 18 February, 2014; v1 submitted 7 November, 2013; originally announced November 2013.

Comments: 8 pages, 6 figures, including Supplemental Materials

Journal ref: J. Phys. Soc. Jpn. 83, 043701 (2014)

Ghost modes and continuum scattering in the dimerized distorted kagome lattice antiferromagnet Rb$_2$Cu$_3$SnF$_{12}$

Authors: K. Matan, Y. Nambu, Y. Zhao, T. J. Sato, Y. Fukumoto, T. Ono, H. Tanaka, C. Broholm, A. Podlesnyak, G. Ehlers

Abstract: High intensity pulsed neutron scattering reveals a new set of magnetic excitations in the pinwheel valence bond solid state of the distorted kagome lattice antiferromagnet Rb$_2$Cu$_3$SnF$_{12}$. The polarization of the dominant dispersive modes (2 meV $<\hbarω< $7 meV) is determined and found consistent with a dimer series expansion with strong Dzyaloshinskii-Moriya interactions ($D/J=0.18$). A w… ▽ More High intensity pulsed neutron scattering reveals a new set of magnetic excitations in the pinwheel valence bond solid state of the distorted kagome lattice antiferromagnet Rb$_2$Cu$_3$SnF$_{12}$. The polarization of the dominant dispersive modes (2 meV $<\hbarω< $7 meV) is determined and found consistent with a dimer series expansion with strong Dzyaloshinskii-Moriya interactions ($D/J=0.18$). A weakly dispersive mode near 5 meV and shifted "ghosts" of the main modes are attributed to the enlarged unit cell below a $T=215$ K structural transition. Continuum scattering between 8 meV and 10 meV might be interpreted as a remnant of the kagome spinon continuum [T.-H. Han et al., Nature 492, 406 (2012)] △ Less

Submitted 28 December, 2013; v1 submitted 17 May, 2013; originally announced May 2013.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. B 89, 024414 (2014)

Structural and magnetic properties in the quantum S=1/2 dimer systems Ba3(Cr1-xVx)2O8 with site disorder

Authors: Tao Hong, L. Y. Zhu, X. Ke, V. O. Garlea, Y. Qiu, Y. Nambu, H. Yoshizawa, M. Zhu, G. E. Granroth, A. T. Savici, Zheng Gai, H. D. Zhou

Abstract: We report a comprehensive study of dc susceptibility, specific heat, neutron diffraction, and inelastic neutron scattering measurements on polycrystalline Ba3(Cr1-xVx)2O8 samples, where x=0, 0.06, 0.15, and 0.53. A Jahn-Teller structure transition occurs for x=0, 0.06, and 0.15 samples and the transition temperature is reduced upon vanadium substitution from 70(2) K at x=0 to 60(2) K at x=0.06 and… ▽ More We report a comprehensive study of dc susceptibility, specific heat, neutron diffraction, and inelastic neutron scattering measurements on polycrystalline Ba3(Cr1-xVx)2O8 samples, where x=0, 0.06, 0.15, and 0.53. A Jahn-Teller structure transition occurs for x=0, 0.06, and 0.15 samples and the transition temperature is reduced upon vanadium substitution from 70(2) K at x=0 to 60(2) K at x=0.06 and 0.15. The structure becomes less distorted as x increases and such transition disappears at x=0.53. The observed magnetic excitation spectrum indicates that the singlet ground state remains unaltered and spin gap energy Δ=1.3(1) meV is identical within the instrument resolution for all x. In addition, the dispersion bandwidth W decreases with increase of x. At x=0.53, W is reduced to 1.4(1) meV from 2.0(1) meV at x=0. △ Less

Submitted 29 April, 2013; v1 submitted 22 April, 2013; originally announced April 2013.

Comments: 11 pages, 7 figures, some typos are corrected

Journal ref: Physical Review B 87, 144427 (2013)

Spin dynamics and spin freezing in the triangular lattice antiferromagnets FeGa2S4 and NiGa2S4

Authors: Songrui Zhao, P. Dalmas de R/'eotier, A. Yaouanc, D. E. MacLaughlin, J. M. Mackie, O. O. Bernal, Y. Nambu, T. Higo, S. Nakatsuji

Abstract: Magnetic susceptibility and muon spin relaxation (muSR) experiments have been carried out on the quasi-2D triangular-lattice spin S = 2 antiferromagnet FeGa2S4. The muSR data indicate a sharp onset of a frozen or nearly-frozen spin state at T* = 31(2) K, twice the spin-glass-like freezing temperature T_f = 16(1) K. The susceptibility becomes field dependent below T*, but no sharp anomaly is observ… ▽ More Magnetic susceptibility and muon spin relaxation (muSR) experiments have been carried out on the quasi-2D triangular-lattice spin S = 2 antiferromagnet FeGa2S4. The muSR data indicate a sharp onset of a frozen or nearly-frozen spin state at T* = 31(2) K, twice the spin-glass-like freezing temperature T_f = 16(1) K. The susceptibility becomes field dependent below T*, but no sharp anomaly is observed in any bulk property. A similar transition is observed in muSR data from the spin-1 isomorph NiGa2S4. In both compounds the dynamic muon spin relaxation rate lambda_d(T) above T* agrees well with a calculation of spin-lattice relaxation by Chubukov, Sachdev, and Senthil in the renormalized classical regime of a 2D frustrated quantum antiferromagnet. There is no firm evidence for other mechanisms. At low temperatures lambda_d(T) becomes temperature independent in both compounds, indicating persistence of spin dynamics. Scaling of lambda_d(T) between the two compounds is observed from ~T_f to ~1.5T*. Although the muSR data by themselves cannot exclude a truly static spin component below T*, together with the susceptibility data they are consistent with a slowly-fluctuating "spin gel" regime between T_f and T*. Such a regime and the absence of a divergence in lambda_d(T) at T* are features of two unconventional mechanisms: (1) binding/unbinding of Z_2 vortex excitations, and (2) impurity spins in a nonmagnetic spin-nematic ground state. The absence of a sharp anomaly or history dependence at T* in the susceptibility of FeGa2S4, and the weakness of such phenomena in NiGa2S4, strongly suggest transitions to low-temperature phases with unconventional dynamics. △ Less

Submitted 14 August, 2012; originally announced August 2012.

Comments: 13 pages, 6 figures, accepted for publication in Physical Review B

Evidence for an exotic magnetic transition in the triangular spin system FeGa2S4

Authors: P. Dalmas de Reotier, A. Yaouanc, D. E. MacLaughlin, Songrui Zhao, T. Higo, S. Nakatsuji, Y. Nambu, C. Marin, G. Lapertot, A. Amato, C. Baines

Abstract: We report positive muon spin relaxation measurements on the triangular lattice magnetic system FeGa2S4. A magnetic transition not previously detected by specific heat and magnetic susceptibility measurements is found in zero field at T^* \simeq 30 K. It is observed through the temperature dependencies of the signal amplitude and the spin-lattice relaxation rate. This transition is therefore not a… ▽ More We report positive muon spin relaxation measurements on the triangular lattice magnetic system FeGa2S4. A magnetic transition not previously detected by specific heat and magnetic susceptibility measurements is found in zero field at T^* \simeq 30 K. It is observed through the temperature dependencies of the signal amplitude and the spin-lattice relaxation rate. This transition is therefore not a conventional magnetic phase transition. Since persistent spin dynamics is observed down to 0.1 K, the ground state cannot be of the canonical spin-glass type, which could be suggested from hysteresis effects in the bulk susceptibility below T_f \simeq 16 K. These results are compared to those found for the isomorph NiGa2S4. It is argued that the fate of the transition, which has been interpreted in terms of the Z_2 topological transition in this latter system, is probably different in FeGa2S4. △ Less

Submitted 12 April, 2012; originally announced April 2012.

Comments: 5 pages, 3 figures

Journal ref: Phys. Rev. B 85, 140407(R) (2012)

Block magnetism coupled with local distortion in the iron-based spin-ladder compound BaFe2Se3

Authors: Yusuke Nambu, Kenya Ohgushi, Shunpei Suzuki, Fei Du, Maxim Avdeev, Yoshiya Uwatoko, Koji Munakata, Hiroshi Fukazawa, Songxue Chi, Yutaka Ueda, Taku J Sato

Abstract: Magnetism in the insulating BaFe$_2$Se$_3$ was examined through susceptibility, specific heat, resistivity and neutron diffraction measurements. After formation of a short-range magnetic correlation, a long-range ordering was observed below $T_{\rm N}\sim 255$ K. The transition is obscured by bulk properties. Magnetic moments ($\parallel a$) are arranged to form a Fe$_4$ ferromagnetic unit, and ea… ▽ More Magnetism in the insulating BaFe$_2$Se$_3$ was examined through susceptibility, specific heat, resistivity and neutron diffraction measurements. After formation of a short-range magnetic correlation, a long-range ordering was observed below $T_{\rm N}\sim 255$ K. The transition is obscured by bulk properties. Magnetic moments ($\parallel a$) are arranged to form a Fe$_4$ ferromagnetic unit, and each Fe$_4$ stacks antiferromagnetically. This block magnetism is of the third type among magnetic structures of ferrous materials. The magnetic ordering drives unusually large distortion via magnetoelastic coupling. △ Less

Submitted 23 February, 2012; originally announced February 2012.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 85, 064413 (2012)

Ferro-quadrupolar ordering in PrTi$_2$Al$_{20}$

Authors: Taku J Sato, Soshi Ibuka, Yusuke Nambu, Teruo Yamazaki, Akito Sakai, Satoru Nakatsuji

Abstract: Origin of the non-magnetic phase transition in PrTi$_2$Al$_{20}$, reported earlier in the macroscopic study, has been asserted microscopically using elastic and inelastic neutron scattering techniques. It has been shown spectroscopically that the crystalline-electric-field ground state is a non-magnetic $Γ_3$ doublet, whereas the excited states are two triplets ($Γ_4$ and $Γ_5$) and a singlet (… ▽ More Origin of the non-magnetic phase transition in PrTi$_2$Al$_{20}$, reported earlier in the macroscopic study, has been asserted microscopically using elastic and inelastic neutron scattering techniques. It has been shown spectroscopically that the crystalline-electric-field ground state is a non-magnetic $Γ_3$ doublet, whereas the excited states are two triplets ($Γ_4$ and $Γ_5$) and a singlet ($Γ_1$). The diffraction experiment under external magnetic field shows that the non-magnetic transition is indeed ferro-quadrupolar ordering, which takes place as a consequence of cooperative removal of the ground-state-doublet degeneracy. It is therefore concluded that PrTi$_2$Al$_{20}$ is another rare example of Pr compounds exhibiting non-magnetic quadrupolar order. △ Less

Submitted 18 August, 2011; originally announced August 2011.

Comments: 7pages, 7 figure, and 1 table, submitted to PRB

Journal ref: Phys. Rev. B 86, 184419 (2012)

Incommensurate Magnetism in FeAs Strips: Neutron Scattering from CaFe$_4$As$_3$

Authors: Yusuke Nambu, Liang L. Zhao, Emilia Morosan, Kyoo Kim, Gabriel Kotliar, Pawel Zajdel, Mark A. Green, William Ratcliff, Jose A. Rodriguez-Rivera, Collin Broholm

Abstract: Magnetism in the orthorhombic metal CaFe$_4$As$_3$ was examined through neutron diffraction for powder and single crystalline samples. Incommensurate (${\bm q}_{\rm m}\approx (0.37-0.39)\times{\bm b}^{\ast}$) and predominantly longitudinally ($\parallel b$) modulated order develops through a 2nd order phase transition at $T_{\rm N}=89.63(6)$ K with a 3D Heisenberg-like critical exponent… ▽ More Magnetism in the orthorhombic metal CaFe$_4$As$_3$ was examined through neutron diffraction for powder and single crystalline samples. Incommensurate (${\bm q}_{\rm m}\approx (0.37-0.39)\times{\bm b}^{\ast}$) and predominantly longitudinally ($\parallel b$) modulated order develops through a 2nd order phase transition at $T_{\rm N}=89.63(6)$ K with a 3D Heisenberg-like critical exponent $β=0.365(6)$. A 1st order transition at $T_2=25.6(9)$ K is associated with the development of a transverse component, locking ${\bm q}_{\rm m}$ to $0.375(2){\bm b}^{\ast}$, and increasing the moments from 2.1(1) to 2.2(3) $μ_{\rm B}$ for Fe$^{2+}$ and from 1.3(3) to 2.4(4) $μ_{\rm B}$ for Fe$^+$. The {\it ab-initio} Fermi surface is consistent with a nesting instability in cross-linked FeAs strips. △ Less

Submitted 18 December, 2010; originally announced December 2010.

Comments: 5 pages, 5 figures, accepted for publication in Phys. Rev. Lett

Incommensurate short-range order in the S=1 triangular antiferromagnet NiGa2S4

Authors: C. Stock, S. Jonas, C. Broholm, S. Nakatsuji, Y. Nambu, K. Onuma, Y. Maeno, J. -H. Chung

Abstract: Neutron scattering is used to investigate spin correlations in ultra pure single crystals of the S=1 triangular lattice NiGa2S4. Despite a Curie-Weiss temperature of Theta_CW=-80(2) K, static (tau > 1 ns) short range (xi_ab=26(3) Å) incommensurate order prevails for T>1.5 K. The incommensurate modulation Q_0=(0.155(3),0.155(3),0), Theta_CW, and the spin wave velocity (c=4400 m/s) can be accounted… ▽ More Neutron scattering is used to investigate spin correlations in ultra pure single crystals of the S=1 triangular lattice NiGa2S4. Despite a Curie-Weiss temperature of Theta_CW=-80(2) K, static (tau > 1 ns) short range (xi_ab=26(3) Å) incommensurate order prevails for T>1.5 K. The incommensurate modulation Q_0=(0.155(3),0.155(3),0), Theta_CW, and the spin wave velocity (c=4400 m/s) can be accounted for by antiferromagnetic third-nearest neighbor interactions J_3=2.8(6) meV and ferromagnetic nearest neighbor coupling J_1=-0.35(9) J_3. Inter-plane correlations are limited to nearest neighbors and weakened by an in-plane field. These observations show that the short range ordered glassy phase that has been observed in a number of highly degenerate systems, can persist near the clean limit. △ Less

Submitted 16 June, 2010; originally announced June 2010.

Comments: 4 pages, 4 figures, to be published in Physical Review Letters

Journal ref: Phys. Rev. Lett. 105, 037402 (2010)

High Field ESR and Magnetization of the Triangular Lattice Antiferromagnet NiGa2S4

Authors: Hironori Yamaguchi, Shojiro Kimura, Masayuki Hagiwara, Yusuke Nambu, Satoru Nakatsuji, Yoshiteru Maeno, Akira Matsuo, Koichi Kindo

Abstract: We report the experimental and the analytical results of electron spin resonance (ESR) and magnetization in high magnetic fields up to about 68 T of the quasi two-dimensional triangular lattice antiferromagnet NiGa$_2$S$_4$. From the temperature evolution of the ESR absorption linewidth, we find a distinct disturbing of the development of the spin correlation by $Z_2$-vortices between 23 K and… ▽ More We report the experimental and the analytical results of electron spin resonance (ESR) and magnetization in high magnetic fields up to about 68 T of the quasi two-dimensional triangular lattice antiferromagnet NiGa$_2$S$_4$. From the temperature evolution of the ESR absorption linewidth, we find a distinct disturbing of the development of the spin correlation by $Z_2$-vortices between 23 K and 8.5 K. Below $T_{\rm{v}}=8.5$ K, spin-wave calculations based on a 57$^{\circ}$ spiral spin order well explains the frequency dependence of the ESR resonance fields and high field magnetization processes for $H$$\parallel$$c$ and $H$$\perp$$c$, although the magnetization for $H$$\perp$$c$ at high fields is different from the calculated one. Furthermore, we explain the field independent specific heat with $T^2$-dependence by the same spin-wave calculation, but the magnitude of the specific heat is much less than the observed one. Accordingly, these results suggest the occurrence of a $Z_2$ vortex-induced topological transition at $T_{\rm{v}}$ and may indicate quantum effects beyond the descriptions based on the above classical spin models. △ Less

Submitted 15 February, 2010; originally announced February 2010.

Successive phase transitions and phase diagrams of the quasi-two-dimensional triangular antiferromagnet Rb4Mn(MoO4)3

Authors: Rieko Ishii, Shu Tanaka, Keisuke Onuma, Yusuke Nambu, Masashi Tokunaga, Toshiro Sakakibara, Naoki Kawashima, Yoshiteru Maeno, Collin Broholm, Dixie P. Gautreaux, Julia Y. Chan, Satoru Nakatsuji

Abstract: Comprehensive experimental studies by magnetic, thermal and neutron measurements have clarified that Rb4Mn(MoO4)3 is a model system of a quasi-2D triangular Heisenberg antiferromagnet with an easy-axis anisotropy, exhibiting successive transitions across an intermediate collinear phase. As a rare case for geometrically frustrated magnetism, quantitative agreement between experiment and theory is… ▽ More Comprehensive experimental studies by magnetic, thermal and neutron measurements have clarified that Rb4Mn(MoO4)3 is a model system of a quasi-2D triangular Heisenberg antiferromagnet with an easy-axis anisotropy, exhibiting successive transitions across an intermediate collinear phase. As a rare case for geometrically frustrated magnetism, quantitative agreement between experiment and theory is found for complete, anisotropic phase diagrams as well as magnetic properties. △ Less

Submitted 24 December, 2009; originally announced December 2009.

Comments: 4 pages, 5 figures

Journal ref: Europhysics Letters 94, 17001 (2011)

Unconventional spin freezing and fluctuations in the frustrated antiferromagnet NiGa2S4

Authors: D. E. MacLaughlin, Y. Nambu, S. Nakatsuji, R. H. Heffner, Lei Shu, O. O. Bernal, K. Ishida

Abstract: Muon spin rotation (muSR) experiments reveal unconventional spin freezing and dynamics in the two-dimensional (2D) triangular lattice antiferromagnet NiGa2S4. Long-lived disordered Ni-spin freezing (correlation time > 10-6 s at 2 K) sets in below T_f = 8.5 +- 0.5 K with a mean-field-like temperature dependence. The observed exponential temperature dependence of the muon spin relaxation above T_f… ▽ More Muon spin rotation (muSR) experiments reveal unconventional spin freezing and dynamics in the two-dimensional (2D) triangular lattice antiferromagnet NiGa2S4. Long-lived disordered Ni-spin freezing (correlation time > 10-6 s at 2 K) sets in below T_f = 8.5 +- 0.5 K with a mean-field-like temperature dependence. The observed exponential temperature dependence of the muon spin relaxation above T_f is strong evidence for 2D critical spin fluctuations. Slow Ni spin fluctuations coexist with quasistatic magnetism at low temperatures but are rapidly suppressed for fields > 10 mT, in marked contrast with the field-independent specific heat. The muSR and bulk susceptibility data indicate a well-defined 2D phase transition at T_f, below which NiGa2S4 is neither a conventional magnet nor a singlet spin liquid. △ Less

Submitted 6 November, 2008; originally announced November 2008.

Comments: 5 pages, 4 figures, to be published in Phys. Rev. B