Natalia Perkins
MPIPKS, DRESDEN, GERMANY

In recent decades,there has been a lot of interest in geometrically frustrated systems. The pyrochlore lattice, which is composed by corner sharing tetrahedra, is a typical example of a highly frustrated three-dimensional structure. It is believed that Heisenberg AFM on a pyrochlore structure does not support a magnetically ordered ground state. However, in real systems, for example, in spinels, with general formula AB2O4, a magnetic ion can also possess an orbital degeneracy an addition to the spin one. The physical behaviour of such systems may be drastically different from that of pure spin models, as the occurence of an an orbital ordering camn modulate the spin exchange and thus lift the geometrical degeneracy of the underlying lattice. I will discuss two compounds with such properties: the B-spinels MgTi2O4 and ZnV2O4, where novel magnetic phases can occur as a ground state.