Search for novel characteristics of triangular lattice antiferromagnets exhibiting dielectric anomaly

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As a research program of this grant the present authors proposed a study of clarifying how the reduction of spin frustration produced by structural phase transitions influences on magnetic phase transitions and critical phenomena of hexagonal triangular lattice ABX_3 antiferromagnets. For the term of this investigation, couples of measurements have been carried out on magnetic, dielectric, optical and specific heat properties for systems with the KNiCl_3 type lattice deformation. The prototype crystal from which those distorted lattice are derived is the CsNiCl_3 type lattice with the space group P6_3/mmc.As a result, novel phenomena concerning both to dielectric and to magnetic properties have been found. For instance, they are rather complex successive structural phase transitions, coexistence of antiferromagnetic and ferrolectric orderings, and dielectric anomaly in association with magnetic phase transitions.In particular, dielectric properties of the KNiCl_3 systems can be classif … More ied into three groups. One to which KNiCl_3, RbMnBr_3 and TlFeCl_3 belong has a feature that ferroelectricity is observed in some temperature range and the temperature dependence of dielectric constant shows no anomaly at magnetic transition points. The second to which RbFeBr_3 belong has that ferroelectricity and antiferromagnetism coexists in low temperatures. The third to which RbVBr_3 and RbCoBr_3 belong has that the temperature dependence of dielectric constant displays anomalous peaks in the vicinity of magnetic successive phase transition points. A finding that RbCoBr_3 sets at 37K in ferroelectric and weak ferromagnetic orders simultaneously seems to be most provocative. Furthermore the appearance of weak magnetic moments perpendicular to Ising symmetry axis is quite surprising, because Co^<2+> ions in hexagonal ACoX_3 type systems have been so far known to have Ising-like magnetic anisotropy and the transverse components are irrelevant to establishing of Ising-like long range order. This phenomenon can be attributed to the canting of xy components through the Dzyaloshinskii-Moriya interaction between the nearest neighbor Co^<2+> ions along the hexagonal c axis allowed by the crystal symmetry intrinsic to the KNiCl_3 type crystal lattice.Based on the present results, a measurement of dielectric constant under an external magnetic field or that of magnetic susceptibility under an electric field will be performed for understanding those novel and complex phenomena consistently. Less