Magnetic ordering phenomena in some low-dimensional xy-like magnetic model systems

The physics of the one-dimensional (I-D) Ising, XY and Heisenberg systems as well as the two-dimensional (2-D) Heisenberg and XY systems differ from the 2-D Ising and three-dimensional (3-D) Heisenberg magnets in that they do not show magnetic order in the presence of magnetic short-range correlations for T --70 K. Small deviations from ideal behaviour induced by single-ion anisotropy, magnetic dipole interaction or intrachainar, intraplanar superexchange, can result in the introduction of long-range (3-D) magnetic ordering. The lattice geometry in conjunction with the anisotropy of the spin system may introduce magnetic frustration effects, chirality ordering or Kosterlitz-Thouless behaviour. In this thesis a collection of papers is presented on different aspects of the magnetic ordering behaviour in quasi 1-0 and 2-D magnetic model systems with XY -like anisotropy where in the case of the quasi I -0 hexagonal ABX3 ternary halides additional low-dimensional aspects are introduced in the magnetic ordering behaviour due to the 2-D network - a triangular lattice - of the magnetic ions perpendicular to the chain-axis. Consequently, frustration effects as well as chirality ordering can be observed for particular compounds. The magnetic ordering phenomena in the quasi 1-0 AFeX3 halides will be presented. These systems show singlet groundstate or induced moment behaviour. Their magnetic ordering behaviour can be influenced by an external order parameter such as an applied magnetic field (HI/c) or an applied uniaxial pressure (P//a). The ordering behaviour is strongly dependent on relatively small changes in the structural parameters. The magnetic ordering behaviour for a diluted induced-moment system is investigated for the solid solution RbFel-xMgxCl3 and mixed ferro-antiferromagnetic induced-moment system for the solid solution RbFeCI3-xBrx. The magnetic phase diagrams for these sytems have been determined. The effect of the magnetic ordering on the magnetic excitations was studied for the singlet groundstate system CsFeBr3 in the presence of an applied field along the chain direction and perpendicular to the chain direction as well as for the induced moment system RbFeBr3. The influence of a slight structural distortion of the trigonal basal plane and its influence on the magnetic ordering behaviour and excitations at the magnetic soft mode point have been investigated. In the case of triangular antiferromagnets with XY anisotropy, geometrical frustration leads to magnetic structures of the 120' type. The magnetic moments in such a system are condensed into the basal plane where the magnetic moments form plaquettes of spins which rotate 120' either clockwise or anti-clockwise. This chirality order introduces an extra degree of freedom which results in new universality classes for the critical exponents of such a type of system. On symmetry grounds it has been shown that the chiral ordering can be removed in the presence of an electric field along the direction of the magnetic ordering vector. Experimental evidence has been presented to underwrite this theoretical prediction. The 2-D XY model does not sustain long-range order for T - OK. However, Berezinskii, Kosterlitz and Thouless have shown that for this model a phase transition to low temperature phase does exist. This phase has an infinite correlation length but no spontaneous order. Evidence for the existence of such a phase in the quasi 2-D ferromagnet Rb2CrCl4 has been presented. A similar behaviour may be present in the in the quasi 2-D weak ferromagnets AMnP03.H20: A += NH4, ND4, K, CnH2n+ I. Magnetisation data are presented in support of this hypothesis.