MAGNETIC-STRUCTURE OF MGCU2O3 AND DOPING-INDUCED SPIN REORIENTATION IN MG1-X 2LIXCU2-X/2O3/

The magnetic properties of undoped and Li-doped MgCu2O3 single crystal s have been studied by magnetic-susceptibility and neutron-diffraction measurements. The pure compound is a semiconductor with an antiferrom agnetic ground state (T(N) = 95 K). Above T(N), short-range magnetic c orrelations within the Cu-O chains of the MgCu2O3 structure give rise to a predominantly one-dimensional (1D) magnetic behavior. This is rev ealed by the quantitative interpretation of the susceptibility measure ments. Below T(N), the 3D magnetic structure, derived from neutron-dif fraction experiments, can be described by an essentially collinear mod el. A small spin canting, however, exists, because adjacent CuO6 octah edra are strongly tilted with respect to each other. The magnetic stru cture can be decomposed into two independent magnetic sublattices, whi ch, in the mean-field approximation, are not coupled. The topology is similar to the one producing an infinitely degenerate state in antifer romagnetic fcc lattices. Doping experiments with Li clearly demonstrat e the importance of spin fluctuations and fluctuations of the local ex change fields for lifting the degeneracy in such a system. A remarkabl y small amount of Li (about 2 mole % Li) is sufficient to disturb the magnetic lattice in such a way that the almost collinear spin arrangem ent changes into an arrangement where the spins of one sublattice are strongly canted with respect to the spins of the other sublattice.