Experimental Study of New Universality Classes in Magnetic Phase Transition

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The purpose of the present research is to elucidate the phase transitions in low dimensional magnetic system, such as frustrated triangular lattice antiferromagnet and quantum spin one-dimensional antiferromagnet.We performed the experimental study on various model substances using nuclear magnetic resonance (NMR), electron spin resonance (ESR), neutron scattering, susceptibity and high field magnetization maeasuments.1.Triangular lattice antiferromagnetOur precise measurements between 1.4K and 300K under static magnetic fields up to 40 T,occasionally up to 110 T,revealed a rich variety of magnetic phase transitions in the stacked triangular lattice antiferromagnets ABX3 and AMO2, including a novel phase transition which belongs to a new universality class linked to chiral degeneracy.The magnetic field versus temoerature phase diagrams are classified into two types depending on their magnetic antisotropies.2.Quantum spin systemNonmagnetic groud states due to quantum spin effect were studied for one-dimensional Heisenberg systems.We observed remarkable differneces in the nonmagnetic impurity efect on the Haldane system, spin-Peierls system and alternating linear chain system and disscussed the physical sihnificance of the results.