Browsing by Author "Nishimoto, S"

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    The frustrated quantum spin chain, linarite, in high magnetic fields
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) Willenberg, B; Nishimoto, S; Schaepers, M; Reehuis, M; Wolter, AUB; Drechsler, SL; Buechner, B; Studer, AJ; Rule, KC; Ouladdiaf, B; Suellow, S
    Linarite, PbCuSO4(OH)2 is a natural mineral ideally suited to the study of frustration in J1-J2 systems due to an accessible saturation field and the availability of large single crystals well suited to neutron investigations. In this one dimensional J1-J2 model, competing ferromagnetic nearest-neighbour interactions (J1>0) and antiferromagnetic next-nearest-neighbours (J2<0) can give rise to novel phenomena such as multiferroicity for spiral spin states. It is also predicted that materials which exhibit such frustrated magnetic interactions are likely to display evidence of spin-nematic states. The magnetic spin-nematic phase can be likened to the arrangement of molecules in nematic liquid crystal displays (LCD). The magnetic form of the spin-nematic state, involves the ordering of spin-quadrupole moments in the absence of conventional spin-dipole order such that the magnetic spins align spontaneously along a chosen axis while still fluctuating dynamically. In Linarite, the Cu2+ ions form spin S = 1/2 chains along the b direction with dominant nearest neighbour FM interactions and a weaker next-nearest-neighbour AFM coupling, resulting in a magnetically frustrated topology [1, 2]. We present a neutron scattering and magnetic property study of linarite revealing a helical magnetic ground state structure with an incommensurate propagation vector of (0 0.186 ½) below TN = 2.8K in zero magnetic field [3]. From detailed measurements in magnetic fields up to 12 T (B || b), a very rich magnetic phase diagram will be presented (Fig. 1) [4]. A two-step spin-flop transition is observed, transforming the helical magnetic ground state into a collinear structure. As well, a magnetic phase with sine-wave modulated moments parallel to the field direction was detected, enclosing the other long-range ordered phases, and which exhibits phase separation in high magnetic fields. Theoretical calculations imply that linarite possesses an xyz exchange anisotropy. Our data establish linarite as a model compound of the frustrated one-dimensional spin chain, with ferromagnetic nearest-neighbour and antiferromagnetic next-nearest-neighbour interactions. We shall also discuss the high field phase (marked “?” in the phase diagram of Fig. 1) in terms of the spin-nematic physics as well as the hard to access regions of the phase diagram, namely Region II.
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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.