Experimental Study of New Ordered Phases in Magnetic Field for Stacked Triangular Lattice and Haldane System

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Ordering phenomena are classified to universality classes by symmetry of order parameter and spacial dimensionality. As for the magnetic transition, the symmetry of the order parameter is usually governed just by spin symmetry. Recent theoretical studies, however, have suggested that the concept of universality should be deepened more profoundly. Under such circumstances, we perfomed various experimental studies including nuclear magnetic resonance (NMR), electron spin resonance (ESR), neutron scattering and high field magnetization.The purpose of the present research is to elusidate the new ordered phases in the frustrated triangular lattice antiferromagnets and the one-dimensional Haldane systems.1. For the triangular systems, universality classes of CsMnBr3 and CsMnI3 were studied. Two compounds have the same hexagonal structure and almost the same magnetic parameters except the anisotropy which produces an essential difference in ordering process. We provided an experimental eviden … More ce that the phase transition in CsMnBr3 with XY anisotropy belongs to a new universality class characterized by the order parameter linked to a twofold discrete chiral degeneracy inherent to the frustrated triangular antiferromagnet as well as a continuous rotational degeneracy. On the contrary, in CsMnI3 with Ising anisotropy there exist two successive transitions which correspond to independent ordering of magnetic moments parallel and perpendicular to the Ising axis, respectively. The critical exponents at these transitions remarkably differ from those for CsMnBr3 reflecting an essential role of anisotropy in agreement with a theoretical prediction. The magnetic phase transitions in closely related substances, RbMnBr3 and CsNiBr3 were also studied.2. For the Haldane systems we studied magnetic properties of NENP and the related substances which are well approximated to one-dimensional Heisenberg antiferromagnet with integer spins and proved the existence of the Haldane phase with a novel ground state. The effect of magnetic field on the ground state properties were revealed from microscopic study. Less