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College Park, Maryland      June 6 - 10 , 2004

W2-D4 (2:30 PM): Cooperative ordering of gapped and gapless spin networks in Cu2Fe2Ge4O13

T. Masuda (Condensed Matter Sciences Division, Oak Ridge National Laboratory), B. Grenier (DRFMC/SPSMS/MDN Cntre dEtudes Nucleares), A. Zheludev (Condensed Matter Sciences Division, Oak Ridge National Laboratory), S. Imai, K. Uchinokura (Department of Advanced Materials Science, the University of Tokyo), E Ressouche (DRFMC/SPSMS/MDN Cntre dEtudes Nucleares), S. Park (NIST Center for Neutron Research)

The unusual magnetic properties of a novel low-dimensional quantum ferrimagnet Cu2Fe2Ge4O13 are studied by neutron scattering and bulk methods. The crystal structure of this material contains chains of Fe3+ ions intercalated by paired Cu2+ sites. The magnetic susceptibility in the high temperature region is described by the simple sum of classical S=5/2 spin chains and S=1/2 quantum dimers. In the 3D ordered state neutron diffraction reveals strongly reduced moments on Cu2+ and large moments on Fe3+, respectively. The observed linear relation between mCu and mFe is consistent with the Cu2+-spins being bound in a gapped spin-singlet state. Low-energy spin excitations up to 10 meV energy transfer are studied by cold neutron inelastic scattering. The measured dispersion relation can be understood as that of Fe3+ spin chains with effective inter-chain coupling. A combination of experimental technique reveals that the material can be described in terms of two subsystems with distinct energy scales. Magnetic ordering in this case is a cooperative phenomenon caused by interactions between these two units.

This work was carried out under DOE Contract No. DE-AC05-00OR22725.

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