Incommensurate magnetism in the coupled spin tetrahedra system Cu2Te2O5Cl2

Neutron scattering studies on powder and single crystals have provided new evidence 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 T-N=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 similar to[0.15,0.42,1/2]. The Cu2+ ions carry a magnetic moment of 0.67(1)mu(B)/Cu2+ at 1.5 K and form a four-helix spin arrangement with two canted pairs within the tetrahedra. A domain redistribution is observed when a magnetic field is applied in the tetragonal plane (H(c)approximate to 0.5 T), but not for H parallel to c up to 4 T. The excitation spectrum is characterized by two well-defined modes, one completely dispersionless at 6 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.