Browsing by Author "Okuyama, D"
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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
- ItemHelical magnetic structure in cubic chiral crystal Pr5Ru3Al2(International Union of Crystallography, 2017-01-01) Okuyama, D; Makino, K; Avdeev, M; Ohishi, K; Yamauchi, K; Oguchi, T; Sato, TJHelical magnetic structure has recently attracted intrests because of the discovery of novel topological spin textures, forexample magnetic skyrmions and chiral magnetic soliton lattices. For such spin textures, a finite antisymmetricDzyaloshinsky–Moriya-type (cross product) interaction is crucial, activated in noncentrosymmetric crystals. Suchantisymmetric interactions have been studied mainly in 3d magnets. For example, in B20 compound (Mn,Fe,Co)Ge, theantisymmetric interaction is well investigated by the theoretical first principle calculation and found that the observed signinversion of the helicity of the helical magnetic structure by the magnetic ion substitution is quantitatively explained [1]. Incontrast, there are few studies investigating the antisymmetric interaction in 4f rare-earth based noncentrosymmetricmaterials. Murashova et al. reported the rare-earth based chiral compounds Re5Ru3Al2 with the space group I213 (Re = La,Pr) [2]. Nonetheless, their low temperature magnetism was largely unexplored. Powder Pr5Ru3Al2 was synthesized by the arc melting and high-frequency induction heating methods. The powder sampleswere annealed using muffle furnace and the high quality powder sample and single crystal were grown. In the magnetizationmeasurement using obtained Pr5Ru3Al2, the temperature dependence of the magnetic susceptibility is fitted by Curie-Weisslaw and the obtained Curie constant is close to the value for a free Pr3+ ion. At 4K, the antiferromagnetic transition isobserved [3]. To clarify the magnetic structure of Pr5Ru3Al2, we performed powder neutron diffraction using ECHIDNA inANSTO and single crystal small angle neutron scattering (SANS) using TAIKAN in J-PARC. The powder diffraction pattern at10 K is explained by the nuclear scattering of Pr5Ru3Al2. At 3 K, additional incommensurate magnetic peaks with thepropagation vector (q q q): q ~ 0.066 [r. l. u.] are observed. More noteworthy are the integrated intensities of theequivalent magnetic reflections around the nuclear 1 1 0 are not the same value. To explain the difference of the intensitiesbetween equivalent reflections, it is reasonable to conclude that the helical magnetic ordering takes place and the sign of itshelicity is determined by the sign of the crystal chirality. The magnetic structure determined by the magnetic representationand Rietveld analyses is shown in Fig. 1 (a). The composite magnetic structure obtained by adding the magnetic moments ofPr1, Pr2, Pr3, and Pr4 layers is the typical helical, as shown in Fig. 1 (b). In the SANS experiment using single crystalPr5Ru3Al2, the (q q q)-type magnetic reflection is also observed below 3.3 K. The band structure near Fermi energy iscalculated by the first principle calculation to determine the conduction band mediating RKKY interaction. From theseexperimental and theoretical results, the origin of the helical magnetic structure in Pr5Ru3Al2 will be discussed. © International Union of Crystallography
- ItemIncommensurate magnetic structure in the cubic noncentrosymmetric ternary compound Pr5Ru3Al2(The Physical Society of Japan, 2016-06-08) Makino, K; Okuyama, D; Avdeev, M; Sato, TJMagnetic susceptibility and neutron powder diffraction experiments have been performed on the noncentrosymmetric ternary compound Pr5Ru3Al2. The previously reported ferromagnetic transition at 24 K was not detected in our improved-quality samples. Instead, magnetic ordering was observed in the DC magnetic susceptibility at Tc≃3.8 K. The neutron powder diffraction experiment further indicates that an incommensurate magnetic structure is established below Tc with the magnetic modulation vector q≃(0.066,0.066,0.066) ©2016 The Physical Society of Japan
- 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
- 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
- ItemSmall angle neutron scatterings study on the cubic chiral crystal Pr5Ru3Al2(International Conference on Neutron Scattering, 2017-07-12) Makino, K; Okuyama, D; Avdeev, M; Ohishi, K; Yamauchi, K; Oguchi, T; Sato, TJThe helical magnetic structure has attracted renewed interests because of the discovery of novel topological spin textures, for example magnetic skyrmions and chiral magnetic soliton lattices. For such spin textures, a finite antisymmetric Dzyaloshinskii-Moriya-type interaction is crucial, activated in noncentrosymmetric crystals. To date, such antisymmetric interactions have been studied mainly in 3d magnets, and few studies have been performed on 4f rare-earth (RE) magnets. Recently, Murashova et al. reported the RE-based chiral compounds RE5Ru3Al2 (RE = La, Ce, and Pr) with the space group I213. Nonetheless, their low-temperature magnetism was largely unexplored. Combining detailed magnetization and small-angle-neutron-scattering (SANS) measurements, we have scrutinized magnetic orderings in the Pr5Ru3Al2 compound at low temperatures under finite magnetic fields. In the magnetization study, we found at least four ordered phases in the ranges 1.9 < T< 3.8 K and 0 < H < 2500 Oe. The SANS study, performed using TAIKAN at J-PARC, revealed that under zero external field, two phases exists, characterized by the two distinct directions of their magnetic modulation vectors, q1 = (q q q): |q1| ~ 0.12 r. l. u. below 3.3 K, and q2 = (q q 0) : |q2| ~ |q1| observed for 3.3 K < T < 3.8 K. The magnetic modulation vectors under finite fields were similarly assigned. A first-principle electronic structure calculation has been made to understand those low-temperature ordered phases.