Browsing by Author "Nawa, K"
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- ItemAntiferromagnetic order of ferromagnetically coupled dimers in the double pyrovanadate CaCoV2O7(Cornell University, 2021-08-02) Murasaki, R; Nawa, K; Okuyama, D; Avdeev, M; Sato, TJMagnetic properties of the pyrovanadate CaCoV2O7 have been studied by means of the bulk magnetization and neutron powder diffraction measurements. Magnetic susceptibility in the paramagnetic phase shows Curie-Weiss behavior with negative Weiss temperature ≃ −22.5 K, indicating dominant antiferromagnetic interactions. At TN = 3.44 K, CaCoV2O7 shows antiferromagnetic order, accompanied by a weak net ferromagnetic moment of ∼ 0.05 μB/Co2+. Neutron powder diffraction confirms the formation of antiferromagnetic order below TN. It was further confirmed from the magnetic structure determination that the two Co2+ ions in the adjacent edge-sharing octahedra have almost parallel (ferromagnetic) spin arrangement, indicative of a formation of a ferromagnetic spin dimer. The antiferromagnetic order is, in turn, stabilized by sizable inter-dimer antiferromagnetic interactions.
- ItemControlling the stoichiometry of the triangular lattice antiferromagnet Li1+xZn2-yMo3O8(The Physical Society of Japan, 2017-09-12) Sandvik, KE; Nawa, K; Okuyama, D; Reim, J; Avdeev, M; Matsuda, M; Sato, TJAn intriguing topic in condensed matter physics is exploring exotic ground states in frustrated quantum systems, such astriangular-or kagome-lattice antiferromagnets. In these systems competing interactions destabilize conventional magnetic order and lead to interesting nonmagnetic states, exemplified by the quantum spin liquid or resonating valence bond state.The compound LiZn2Mo3O8(LZMO) is an antiferromagnet with magnetic Mo3O13 clusters forming triangular layers well separated from each other. It is proposed to realize a condensed valence bond state [1]. For the stoichiometric composition, these clusters have an unpaired electron (spin 1/2) and hence the system may be an ideal playground for exploring exotic ground states in quantum triangular antiferromagnet. However, chemical disorder atLi/Zn cites in LZMO easily leads to off-stoichiometry that introduces unoccupied S= 1/2 sites in the triangular lattice.Partial control of the Zn composition has been achieved by earlier efforts involving electrochemical technique[2], however,full stoichiometry control has not been achieved as far as we know. Since the stoichiometry is the key to explore intrinsic physics in the quantum S= 1/2 triangular antiferromagnet, we undertook a thorough investigation on the relation between initial and final chemical composition for the LZMO system. Several LZMO compounds were prepared using solid-state reaction technique [1] with widely varied starting composition. After the reactions, the samples were washed by hydrochloric acid to remove excess ZnO. The composition of the obtained compounds was determined as Li1+xZn2-yMo3O8from ICP mass spectroscopy (ArcosEOP, Spectro), X-ray diffraction (Rigaku) and neutron powder diffraction (Echidna, ANSTO). Composition dependence on the magnetism was investigated using the magnetic property measurement system (MPMS-XL, Quantum Design). © 2017 The Physical Society of Japan
- ItemControlling the stoichiometry of the triangular lattice antiferromagnet Li1+xZn2−yMo3O8(Elsevier, 2019-01-03) Sandvik, KE; Okuyama, D; Nawa, K; Avdeev, M; Sato, TJThe control of the stoichiometry of Li1+xZn2−yMo3O8 was achieved by the solid-state-reaction. We found that the best sample that has the chemical composition Li0.95(4)Zn1.92(8)Mo3O8 was obtained from the starting nominal composition with Li : Zn : Mo : O=(1+w) : (2.8−w) : 3 : 8.6 with w=−0.1, indicating that the stoichiometry is greatly improved compared to those in the earlier reports. For larger w detailed structural analysis indicates that the mixed sites of Li and Zn are preferentially occupied by Li atoms, as well as the fraction of the non-magnetic secondary phase Zn2Mo3O8 decreases. Magnetic susceptibility of the improved stoichiometry powder samples shows a broad hump in the temperature range of 100
- ItemCrystal structure and magnetic properties of the breathing kagome ising antiferromagnet Yb3Ni11Ge4.63(The Physical Society of Japan, 2020-07-31) Takahashi, M; Nawa, K; Okuyama, D; Nojiri, H; Frontzek, MD; Avdeev, M; Yoshida, M; Ueta, D; Yoshizawa, H; Sato, TJWe have investigated magnetic properties of the rare-earth based intermetallic compound Yb3Ni11Ge4.63 (YNG), where Yb3+ ions form a breathing-kagome lattice. Single-site 4f electron wavefunctions of the ground-state doublet are deduced from magnetization and heat capacity measurements. The Weiss temperatures are quite anisotropic as Θa = −0.01(2) K (H ∥ a) and Θc = −0.67(2) K (H ∥ c), indicating Ising-like spin–spin interactions dominating in this compound. Low-temperature neutron diffraction confirms absence of magnetic Bragg peaks down to 0.05 K, whereas enhancement of broad peaks below 0.8 K was observed, a signature of short-range spin correlations. Competing interactions on the breathing-kagome lattice would suppress a magnetic long-range order in YNG. ©2020 The Physical Society of Japan
- ItemDegenerate ground state in the classical pyrochlore antiferromagnet Na3Mn(CO3)2Cl(American Physical Society, 2018-10-18) Nawa, K; Okuyama, D; Avdeev, M; Nojiri, H; Yoshida, M; Ueta, D; Yoshizawa, H; Sato, TJIn an ideal classical pyrochlore antiferromagnet without perturbations, an infinite degeneracy in a ground state leads to the absence of magnetic order and a spin-glass transition. Here we present Na3Mn(CO3)2Cl as a new candidate compound where classical spins are coupled antiferromagnetically on the pyrochlore lattice and report its structural and magnetic properties. The temperature dependences of the magnetic susceptibility and heat capacity and the magnetization curve are consistent with those of an S=5/2 pyrochlore lattice antiferromagnet with nearest-neighbor interactions of 2 K. Neither an apparent signature of a spin-glass transition nor magnetic order is detected in magnetization and heat capacity measurements or powder neutron diffraction experiments. On the other hand, antiferromagnetic short-range order of the nearest neighbors is evidenced by the Q dependence of the diffuse scattering which develops around 0.85Å−1. A high degeneracy near the ground state in Na3Mn(CO3)2Cl is supported by the magnetic entropy, estimated as almost 4JK−2mol−1 at 0.5 K. ©2018 American Physical Society
- ItemExperimental observation of long-range magnetic order in icosahedral quasicrystals(American Chemical Society, 2021-11-17) Tamura, R; Ishikawa, A; Suzuki, S; Kotajima, T; Tanaka, Y; Seki, T; Shibata, N; Yamada, T; Fujii, T; Wang, CW; Avdeev, M; Nawa, K; Okuyama, D; Sato, TJQuasicrystals (QCs), first discovered in 1984, generally do not exhibit long-range magnetic order. Here, we report on long-range magnetic order in the real icosahedral quasicrystals (i QCs) Au–Ga–Gd and Au–Ga–Tb. The Au65Ga20Gd15i QC exhibits a ferromagnetic transition at TC = 23 K, manifested as a sharp anomaly in both magnetic susceptibility and specific heat measurements, along with an appearance of magnetic Bragg peak below TC. This is the first observation of long-range magnetic order in a real quasicrystal, in contrast to the spin-glass-like behaviors observed for the other magnetic quasicrystals found to date. Moreover, when Gd is replaced by Tb, i.e., for the Au65Ga20Tb15i QC, a ferromagnetic behavior is still retained with TC = 16 K. Although the sharp anomaly in the specific heat observed for the Au65Ga20Gd15i QC becomes broadened upon Tb substitution, neutron diffraction experiments clearly show marked development of magnetic Bragg peaks just below TC, indicating long-range magnetic order for the Au65Ga20Tb15i QC also. Our findings can contribute to the further investigation of exotic magnetic orders formed on real quasiperiodic lattices with unprecedented highest global symmetry, i.e., icosahedral symmetry. © 2021 The Authors - CC BY. Published by American Chemical Society
- ItemFrustrated magnetism in the J1−J2 honeycomb lattice compounds MgMnO3 and ZnMnO3 synthesized via a metathesis reaction(American Physical Society, 2019-12-17) Haraguchi, Y; Nawa, K; Michioka, C; Ueda, J; Matsuo, A; Kindo, K; Avdeev, M; Sato, TJ; Yoshimura, KWe investigated the magnetic properties of the ilmenite-type manganates MgMnO3 and ZnMnO3, both of which are composed of a honeycomb lattice of magnetic Mn4+ ions. Both compounds show antiferromagnetic order with weak ferromagnetic moments. In particular, MgMnO3 exhibits a magnetization “reversal” behavior which can be described by the N-type ferrimagnetism in the Néel's classification. The relationship between the magnetic properties and the crystal and magnetic structures probed by the neutron diffraction experiments indicates that the two honeycomb lattice magnets have different J1−J2 parameter sets, placing them in the distinct regions in the phase diagram; both nearest neighbor (NN) and next nearest neighbor (NNN) exchange interactions are antiferromagnetic in MgMnO3, while NN and NNN interactions become ferromagnetic and antiferromagnet, respectively, in ZnMnO3. ©2019 American Physical Society
- ItemMagnetic properties of the quasicrystal approximant Au65Ga21Tb14(American Physical Society, 2023-05-31) Nawa, K; Avdeev, M; Ishikawa, A; Takakura, H; Wang, CW; Tamura, R; Okuyama, D; Satao, TJ; Murasaki, RThe magnetic properties of the quasicrystal approximant Au65Ga21Tb14 were investigated using magnetization and neutron diffraction experiments. The temperature dependences of the magnetic susceptibility and magnetization curve indicate dominant ferromagnetic interactions, whereas a whirling antiferromagnetic order was observed in neutron diffraction experiments. In the antiferromagnetic phase, the magnetic moments are aligned almost perpendicular to a pseudofivefold symmetry axis, which corresponds to the easy-axis direction of a Tb atom. Magnetic properties similar to those of Au72Al14Tb14 in spite of the substantial difference in the Au concentration suggest the robustness of the easy-axis anisotropy against the chemical environment. ©2023 American Physical Society
- ItemMagnetic structure study of the sawtooth chain antiferromagnet Fe2Se2O7(Springer Nature, 2021-12-15) Nawa, K; Avdeev, M; Berdonosov, P; Sobolev, A; Presniakov, I; Aslandukova, A; Kozlyakova, E; Vasiliev, AN; Shchetinin, I; Sato, TJA magnetic structure of the sawtooth-chain antiferromagnet Fe2Se2O7 was investigated by magnetization measurements, single crystalline and powder neutron diffraction experiments, and a further analysis on the Mössbauer spectra. These experiments revealed a nearly collinear antiferromagnetic structure with magnetic moments aligned along the b-axis, indicating dominant antiferromagnetic exchanges between Fe(1)–Fe(2) and Fe(2)–Fe(3) sites. The magnon dispersion relation derived from the linear spin wave approximation suggests the possible flat band nature of magnons. © 2021, The Author(s) This article is licensed under a Creative Commons Attribution 4.0 International License.
- ItemMagnetism of classical pyrochlore antiferromagnet Na3Mn(CO3)2Cl(American Physical Society, 2020-03-03) Nawa, K; Okuyama, D; Avdeev, M; Nojiri, H; Yoshida, M; Ueta, D; Yoshizawa, H; Sato, TJPyrochlore antiferromagnets have attracted interests in terms of unconventional ground states and spin excitations owing to competing interactions. When its magnetism is dominated by classical spins coupled by Heisenberg interactions, a spin liquid state is expected as its ground state because of infinite degeneracy in the ground state (R. Moessner and J. T. Chalker, Phys. Rev. Lett. 80, 2929 (1998); Phys. Rev. B 58, 12049 (1998).). In this research, we report structural and magnetic properties together with low-temperature neutron diffraction patterns on a new pyrochlore antiferromagnet Na3Mn(CO3)2Cl. The structure of Na3Mn(CO3)2Cl is isotypic with that of the Co-analogue Na3Co(CO3)2Cl, which exhibits all-in-all-out magnetic order below 1.5 K (Z. Fu et al., Phys. Rev. B 87, 214406 (2013)). On the other hand, no magnetic Bragg peak indicating a magnetic order was detected down to 0.05 K in Na3Mn(CO3)2Cl. A high degeneracy near the ground state is suggested by a magnetic entropy estimated from heat capacity experiments and enhancement of diffuse scattering from neutron diffraction experiments. © 2021 American Physical Society