Browsing by Author "Asai, S"

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    Dimensional reduction by geometrical frustration in a cubic antiferromagnet composed of tetrahedral clusters
    (Springer Nature, 2021-07-19) Okuma, R; Kofu, M; Asai, S; Avdeev, M; Koda, A; Okabe, H; Hiraishi, M; Takeshita, S; Kojima, KM; Kadono, R; Masuda, T; Nakajima, K; Hiroi, Z
    Dimensionality is a critical factor in determining the properties of solids and is an apparent built-in character of the crystal structure. However, it can be an emergent and tunable property in geometrically frustrated spin systems. Here, we study the spin dynamics of the tetrahedral cluster antiferromagnet, pharmacosiderite, via muon spin resonance and neutron scattering. We find that the spin correlation exhibits a two-dimensional characteristic despite the isotropic connectivity of tetrahedral clusters made of spin 5/2 Fe3+ ions in the three-dimensional cubic crystal, which we ascribe to two-dimensionalisation by geometrical frustration based on spin wave calculations. Moreover, we suggest that even one-dimensionalisation occurs in the decoupled layers, generating low-energy and one-dimensional excitation modes, causing large spin fluctuation in the classical spin system. Pharmacosiderite facilitates studying the emergence of low-dimensionality and manipulating anisotropic responses arising from the dimensionality using an external magnetic field. © 2021, The Author(s)
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    A layered wide-gap oxyhalide semiconductor with an infinite ZnO2 square planar sheet: Sr2ZnO2Cl2
    (Royal Society of Chemistry, 2017-03-13) Su, Y; Tsujimoto, Y; Miura, A; Asai, S; Avdeev, M; Ogino, H; Ako, M; Belik, AA; Masuda, T; Uchikoshi, T; Yamaura, K
    A new square-planar zinc oxyhalide, Sr2ZnO2Cl2, was successfully synthesized using a high-pressure method. Absorption spectroscopy revealed an indirect band gap of 3.66 eV. Electronic structure calculations indicated a strong hybridization between Zn 3dx2−y2 and O 2p orbitals, which is distinct from tetrahedrally coordinated ZnO. © Royal Society of Chemistry 2021
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    Magnetic order in the rare-earth ferroborate CeFe3(BO3)4
    (American Physical Society, 2018-12-04) Hayashida, S; Asai, S; Kato, D; Hasegawa, S; Avdeev, M; Cao, H; Masuda, T
    We have studied the magnetic order of the rare-earth ferroborate CeFe3(BO3)4 through the thermodynamic and the neutron diffraction measurements. The heat capacity and the magnetic susceptibility revealed antiferromagnetic magnetic ordering at 29 K. In the neutron powder diffraction data, we observed the magnetic Bragg peaks indexed by the commensurate (CM) propagation vector kCM=(0,0,32) and the incommensurate (ICM) vector kICM=(0,0,32+δ). The incommensurability δ increases with decreasing the temperature, and is evaluated to be 0.04556(16) at 3.7 K. Magnetic structure analysis reveals that the magnetic moments aligning in the ab plane form the collinear antiferromagnetic structure having kCM and helical structure having kICM. Detailed measurements of the magnetic susceptibility exhibit an additional anomaly at 27 K. Furthermore, the temperature dependence of the neutron diffraction profile on the single-crystal sample shows that the ICM and CM ordering occurs at 29 and 26 K, respectively. These results suggest a phase separation state between the collinear and helical structures. The multiferroicity of CeFe3(BO3)4 is discussed on the basis of the determined magnetic structure. ©2018 American Physical Society
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    Magnetic ordering of the buckled honeycomb lattice antiferromagnet Ba2NiTeO6
    (American Physical Society, 2016-01-19) Asai, S; Soda, M; Kasatani, K; Ono, T; Avdeev, M; Masuda, T
    We investigate the magnetic order of the buckled honeycomb lattice antiferromagnet Ba2NiTeO6 and its related antiferromagnet Ba3NiTa2O9 by neutron diffraction measurements. We observe magnetic Bragg peaks below the transition temperatures, and identify propagation vectors for these oxides. A combination of representation analysis and Rietveld refinement leads to a collinear magnetic order for Ba2NiTeO6 and a 120∘ structure for Ba3NiTa2O9. We find that the spin model of the bilayer triangular lattice is equivalent to that of the two-dimensional buckled honeycomb lattice having magnetic frustration. We discuss the magnetic interactions and single-ion anisotropy of Ni2+ ions for Ba2NiTeO6 in order to clarify the origin of the collinear magnetic structures. Our calculation suggests that the collinear magnetic order of Ba2NiTeO6 is induced by the magnetic frustration and easy-axis anisotropy. ©2016 American Physical Society
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    Stripy order in buckled honeycomb lattice antiferromagnet Ba2NiTeO6
    (International Conference on Neutron Scattering, 2017-07-12) Asai, S; Soda, M; Kasatani, K; Ono, T; Avdeev, M; Garlea, VO; Winn, B; Masuda, T
    Ba NiTeO is a rare experimental realization of a buckled honeycomb lattice antiferromagnet. The nearest-neighbor and next-nearest-neighbor interactions in the honeycomb lattice are comparative due to the buckled geometry, leading to magnetic frustration. A magnetic transition is observed at 8.6 K in the susceptibility and heat capacity measurements [1]. The frustration parameter /T is 18.6, where is Weiss temperature and is the magnetic transition temperature. In order to investigate the low temperature state we performed neutron scattering experiments. In the diffraction profile magnetic Bragg peaks are observed at < , and the propagation vector is identified as (0, 1/2,1). Combination of the representation analysis and Rietveld refinement reveals that a collinear stripy structure [2] is realized [3]. Our calculation suggests that the stabilization of the stripy structure instead of spiral structure is ascribed to the competition between magnetic frustration and easy-axis type anisotropy. In the inelastic neutron spectrum at 2 K a magnetic excitation with an energy gap of 2 meV is observed. Spin-wave calculation based on two-dimensional frustrated honeycomb lattice antiferromagnet having easy-axis anisotropy reproduces the experimental data. The obtained parameters are consistent with Weiss temperature estimated from the bulk magnetic susceptibility measurement.