Search for novel characteristics of triangular lattice antiferromagnets exhibiting dielectric anomaly

作为该基金的一个研究项目，作者提出了一项研究，旨在阐明结构相变产生的自旋阻挫的减少对六角三角晶格ABX_3反铁磁体的磁相变和临界现象的影响。本文对KNiCl_3型晶格畸变体系的磁性、介电、光学和比热等性质进行了测量。这些畸变晶格的原型晶体是空间群为P6_3/mmc的CsNiCl_3型晶格。结果发现了一些与介电和磁性质有关的新现象，如相当复杂的连续结构相变、反铁磁有序和铁电有序共存以及与磁相变有关的介电异常。特别是，在某些情况下，这些现象与介电和磁性质有关。KNiCl_3体系的介电性能可分为 ...更多信息 分成三组。其中KNiCl_3、RbMnBr_3和TlFeCl_3属于一类，其特征是在一定温度范围内观察到铁电性，在磁转变点处介电常数随温度的变化没有反常现象。第二类是RbFeBr_3，在低温下铁电性和反铁磁性共存。RbVBr_3和RbCoBr_3属于第三类，介电常数的温度依赖性在磁连续相变点附近出现反常峰。RbCoBr_3在37 K时同时具有铁电和弱铁磁两种性质的发现似乎是最有启发性的。此外，垂直于Ising对称轴的弱磁矩的出现是非常令人惊讶的，因为六方晶系ACoX_3型系统中的Co^2+离子具有Ising类磁各向异性，并且横向分量与Ising类长程有序的建立无关。这一现象可以归因于最近邻Co^2+离子之间的Dzyaloshinskii-Moriya相互作用使xy分量沿着六方c轴倾斜，这是KNiCl_3型晶格固有的晶体对称性所允许的。测量外磁场下的介电常数或电场下的磁化率，理解这些新奇而复杂的现象。少

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