Browsing by Author "Büchner, B"

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    FeMn3Ge2Sn7O16 : a spin-liquid candidate with a perfectly isotropic 2-D kagomé lattice
    (Australian Institute of Physics, 2020-02-05) Allison, MC; Wurmehl, S; Büchner, B; Valla, J; Söhnel, T; Avdeev, M; Schmid, S; Ling, CD
    The compound Fe4Si2Sn7O16 has a hitherto unique crystal structure, consisting of ionic oxide layers based on edge-sharing FeO6 and Sn4+O6 octahedra alternating with layers of intermetallic character based on FeSn2+6 octahedra, separated by covalent SiO4 tetrahedra. [1,2] The ionic layers contain kagomé lattices of magnetic Fe2+ cations (octahedral crystal field, high-spin [HS] d6, S = 2) with perfect trigonal symmetry; while the intermetallic layers are non-magnetic because the Fe2+ is in the low-spin (S = 0) state. The formula is more correctly written as Fe4Si2Sn7O16 to differentiate the one LS-Fe2+ per formula unit in the intermetallic layer from the three HS-Fe2+ per formula unit in the kagomé oxide layer. Fe4Si2Sn7O16 also has a unique magnetic ground state below a Néel ordering temperature TN = 3.5 K, in which the spins on 2/3 of the Fe2+ sites in the kagomé oxide layers order antiferromagnetically, while 1/3 remain disordered and fluctuating down to at least 0.1 K. [3] The nature and origin of this unique “striped” partial spin-liquid state is unclear. The fact that it breaks trigonal symmetry, which the more conventional q = 0 or √3×√3 kagomé states would not, raises the possibility that the anisotropic distribution of the 6 unpaired spins on HS-Fe2+ (t2g4eg2) plays a role. To test this possibility, we have now synthesised an isotropic analogue with a kagomé lattice of HS Mn2+ (t2g3eg2), by co-substituting Ge4+ for Si4+ in the bridging/stannite layers to match the lattice dimensions between layers. We found that FeMn3Ge2Sn7O16 has the same “striped” magnetic ground state as Fe4Si2Sn7O16, in the same temperature range, ruling out this explanation. However, the zero-field striped structure is collinear for FeMn3Ge2Sn7O16 vs. non-collinear for Fe4Si2Sn7O16, which may indeed be a consequence of the change in anisotropy on the magnetic kagomé site, and suggests that FeMn3Ge2Sn7O16 is an even more ideal spin-liquid candidate than Fe4Si2Sn7O16. We also found that an external applied magnetic field lifts the degeneracy on the disordered site, giving rise to another ordered magnetic structure never before observed nor predicted on a kagomé lattice.
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    Growth of LiCoO2 single crystals by the TSFZ method
    (American Chemical Society, 2018-11-08) Nakamura, S; Maljuk, A; Maruyama, Y; Nagao, M; Watauchi, S; Hayashi, T; Anzai, Y; Furukawa, Y; Ling, CD; Deng, G; Avdeev, M; Büchner, B; Tanaka, I
    We have grown LiCoO2 single crystals by the traveling solvent floating zone (TSFZ) growth with Li-rich solvent, having observed the incongruent melting behavior of LiCoO2 between 1100 and 1300 °C. The optimum growth conditions in terms of atmosphere and solvent composition were determined to be Ar flow and an atomic ratio Li/Co 85:15, respectively. The crystals grown using a conventional-mirror-type furnace contained periodic inclusions of a Co–O phase due to the influence of Co–O phase segregation on the stability of the molten zone during growth. By using a tilted-mirror FZ furnace, inclusion-free LiCoO2 crystals of about 5 mm in diameter and 70 mm long were obtained at a tilting angle θ = 10°. The grown crystals were confirmed to be single-domain by neutron Laue diffraction. © 2018 American Chemical Society
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    Magnetic frustration in a quantum spin chain: the case of Linarite PbCuSO4(OH)(2)
    (American Physical Society, 2012-03-16) Willenberg, B; Schäpers, M; Rule, KC; Süllow, S; Reehuis, M; Ryll, H; Klemke, B; Kiefer, K; Schottenhamel, W; Büchner, B; Ouladdiaf, B; Uhlarz, M; Beyer, R; Wosnitza, J; Wolter, A U B
    We present a combined neutron diffraction and bulk thermodynamic study of the natural mineral linarite PbCuSO4(OH)(2), this way establishing the nature of the ground-state magnetic order. An incommensurate magnetic ordering with a propagation vector k = (0, 0.186, 1/2) was found below T-N = 2.8 K in a zero magnetic field. The analysis of the neutron diffraction data yields an elliptical helical structure, where one component (0.638 mu(B)) is in the monoclinic ac plane forming an angle with the a axis of 27(2)degrees, while the other component (0.833 mu(B)) points along the b axis. From a detailed thermodynamic study of bulk linarite in magnetic fields up to 12 T, applied along the chain direction, a very rich magnetic phase diagram is established, with multiple field-induced phases, and possibly short-range-order effects occurring in high fields. Our data establish linarite as a model compound of the frustrated one-dimensional spin chain, with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions. Long-range magnetic order is brought about by interchain coupling 1 order of magnitude smaller than the intrachain coupling. © 2012, American Physical Society.
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    Magnetic properties and exchange integrals of the frustrated chain cuprate linarite PbCuSO4(OH)2
    (American Physical Society, 2012-01-05) Wolter, AUB; Lipps, F; Schäpers, M; Drechsler, SL; Nishimoto, S; Vogel, R; Kataev, V; Büchner, B; Rosner, H; Schmitt, M; Uhlarz, M; Skourski, Y; Wosnitza, J; Süllow, S; Rule, KC
    We present a detailed study in the paramagnetic regime of the frustrated s = 1/2 spin-compound linarite PbCuSO4(OH)(2) with competing ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchange interactions. Our data reveal highly anisotropic values for the saturation field along the crystallographic main directions, with similar to 7.6, similar to 10.5, and similar to 8.5 T for the a, b, and c axes, respectively. In the paramagnetic regime, this behavior is explained mainly by the anisotropy of the g factor, but leaving room for an easy-axis exchange anisotropy. Within the isotropic J(1)-J(2) spin model, our experimental data are described by various theoretical approaches, yielding values for the exchange interactions J(1) similar to -100 K and J(2) similar to 36 K. These main intrachain exchange integrals are significantly larger as compared to the values derived in two previous studies in the literature and shift the frustration ratio alpha = J(2)/vertical bar J(1)vertical bar approximate to 0.36 of linarite closer to the one-dimensional critical point at 0.25. Electron spin resonance (ESR) and nuclear magnetic resonance (NMR) measurements further prove that the static susceptibility is dominated by the intrinsic spin susceptibility. The Knight shift as well as the broadening of the linewidth in ESR and NMR at elevated temperatures indicate a highly frustrated system with the onset of magnetic correlations far above the magnetic ordering temperature T-N = 2.75(5) K, in agreement with the calculated exchange constants. © 2012, American Physical Society.
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    Thermodynamic properties of the anisotropic frustrated spin-chain compound linarite PbCuSO4(OH)2
    (American Physical Society, 2013-11-15) Schäpers, M; Wolter, AUB; Drechsler, SL; Nishimoto, S; Müller, KH; Abdel-Hafiez, M; Schottenhamel, W; Büchner, B; Richter, J; Ouladdiaf, B; Uhlarz, M; Beyer, R; Skourski, Y; Wosnitza, J; Rule, KC; Ryll, H; Klemke, B; Kiefer, K; Reehuis, M; Willenberg, B; Süllow, S
    We present a comprehensive macroscopic thermodynamic study of the quasi-one-dimensional (1D) s = 1/2 frustrated spin-chain system linarite. Susceptibility, magnetization, specific heat, magnetocaloric effect, magnetostriction, and thermal-expansion measurements were performed to characterize the magnetic phase diagram. In particular, for magnetic fields along the b axis five different magnetic regions have been detected, some of them exhibiting short-range-order effects. The experimental magnetic entropy and magnetization are compared to a theoretical modeling of these quantities using density matrix renormalization group (DMRG) and transfer matrix renormalization group (TMRG) approaches. Within the framework of a purely 1D isotropic model Hamiltonian, only a qualitative agreement between theory and the experimental data can be achieved. Instead, it is demonstrated that a significant symmetric anisotropic exchange of about 10% is necessary to account for the basic experimental observations, including the three-dimensional (3D) saturation field, and which in turn might stabilize a triatic (three-magnon) multipolar phase. © 2013, American Physical Society.