This thesis describes the experimental investigation of the two magnetic insulators Ca10Cr7O28 and (NO)[Cu(NO3)3]. The magnetic properties are based on the spin-1/2 3d transition metals Cr5+ and Cu2+ which feature quenched orbitals leading to spin-only magnetic moments. The first part of the thesis deals with the two-dimensional, frustrated magnet Ca10Cr7O28. Detailed analysis of the crystal structure and magnetic properties was performed revealing the absence of long-range order down to the lowest accessible temperatures and the presence of slow dynamics. The magnetic excitations are diffuse and gapless at base temperature and zero-field consistent with a spin-liquid ground state. In the long-range ordered high-field phase the excitations however form sharp spin wave modes and the exchange parameters were obtained by fitting the spectra using linear spin wave theory. The resulting exchange constants yield a new type of frustrated structure based on ferromagnetic and antiferromagnetic bi-triangles coupled in a two-dimensional kagome lattice. (NO)[Cu(NO3)3] is a one-dimensional spin-1/2 Heisenberg antiferromagnetic chain compound with frustrated interchain coupling consistent with the Nersesyan-Tsvelik model. Competing nearest and next-nearest neighbor interchain interactions effectively decouple the chains and enhance the low-dimensionality as verified by the ideal one-dimensional behavior observed down to low energies and temperatures. It develops long-range magnetic order with an incommensurate spin density wave structure at a highly suppressed Néel temperature consistent with the presence of frustration. The phase transition is analyzed and the magnetic excitation spectrum is compared to recent theoretical models of the spin-1/2 Heisenberg antiferromagnetic chain. Together these two compounds show how frustration and low-dimensionality lead to complex magnetic behavior and unconventional magnetic ground states.
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
|Subjects:||Condensed matter physics, Electromagnetics|
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