Magnetic Resonance Study of Spin Dynamics in Frustrated Lattices

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The purpose of the present research is to elucidate the spin fluctuation effects among the highly degenerated states of the frustrated systems, and to discover possible new phase transitions in the triangular lattice antiferromagnets (TAL-AF). In view of our expectation for that new order parameter may appear reflecting the frustration effects in the ordering process, we performed various experiments including nuclear magnetic resonance (NMR), electron spin resonance (ESR), neutron' scattering, optical measurement, high field magnetization. 1. For the quasi-one-dimensional (1D) hexagonal ABX compounds with various types of spin anisotropy, we found unusual magnetic orderings caused by the spin frustration among the magnetic chains. For the Ising AF, CsCoCl_3, we studied the longitudinal spin fluctuations due to domain-wall solitons from the microscopic point of view, and concluded that the peculiar partially-disordered phase is caused by the sublattice flipping mechanism due to propaga … More tion of the magnetic solitons. For the Heisenberg + weak Ising AF's, CsNiCl_3 and CsMnI_3, we elucidated the independent orderings of the longitudinal and transverse spin components in the successive phase transitions. For the XY AF, CsMnBr_3, from accurate measurements of various kinds of critical exponents, we proved, for the first time, that the phase transition is characterized by a new universality class associated with chiral symmetry originated from the transverse spin ordering.2. To provide a break-through for the study of ideal 2D TAL-AF, we also studied the magnetic properties of the new model substances, ABO_2 and AM(XO_4)_2 compounds. We succeeded in observing many characteristic properties such as magnetic excitations for the TAL-AF with classical Heisenberg spin, highly-developed 2D short-range ordering, and a characteristic 1/3 magnetization plateau associated with a ferrimagnetic intermediate state, through the ESR, neutron scattering and high field magnetization measurements. Less