Incommensurate magnetism in the coupled spin tetrahedra system Cu2Te2O5Cl2.pdf

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Incommensurate magnetism in the coupled spin tetrahedra system Cu2Te2O5Cl2.pdf

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Incommensurate magnetism in the coupled spin tetrahedra system Cu2Te2O5Cl2

1Incommensurate magnetism in the coupled spin tetrahedra system Cu2Te2O5Cl2 O. Zaharko*1, H. M. Ronnow1, A. Daoud-Aladine1, S. Streule1, F. Juranyi1, J. Mesot1 H. Berger2 P. J. Brown3 1Laboratory for Neutron Scattering, ETHZ PSI, CH-5232 Villigen PSI, Switzerland 2Institute de Physique de la Matière Complexe, EPFL, CH-1015 Lausanne, Switzerland 3Institute Laue-Langevin, 156X, F-38042 Grenoble Cèdex, France PACS: 75.30-m, 61.12.Ld, 75.10.Jm Abstract Neutron scattering studies on powder and single crystals have provided new evidences for unconventional magnetism in Cu2Te2O5Cl2. The compound is built from tetrahedral clusters of S=1/2 Cu2+ spins located on a tetragonal lattice. Magnetic ordering, emerging at TN=18.2 K, leads to a very complex multi-domain, most likely degenerate, ground state, which is characterized by an incommensurate (ICM) wave vector k~[0.15, 0.42,1/2]. The Cu2+ ions carry a magnetic moment of 0.67(1) μB/ Cu 2+ at 1.5 K and form a four helices spin arrangement with two canted pairs within the tetrahedra. A domain redistribution is observed when a magnetic field is applied in the tetragonal plane (Hc≈0.5 T), but not for H||c up to 4 T. The excitation spectrum is characterized by two well- defined modes, one completely dispersionless at 6.0 meV, the other strongly dispersing to a gap of 2 meV. The reason for such complex ground state and spin excitations may be geometrical frustration of the Cu2+ spins within the tetrahedra, intra- and inter-tetrahedral couplings having similar strengths and strong Dzyaloshinski-Moriya anisotropy. Candidates for the dominant intra- and inter-tetrahedral interactions are proposed. Introduction Reduced dimensionality, geometrical frustration and low spin values often lead to quantum fluctuations resulting in interesting new ground states and spin dynamics [1]. The most famous examples are based on triangular units (triangular and kagomé lattices [2]) in two dimensions (2D) and tetrahedral clusters (FCC and pyrochlore