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Tricritical behavior in a stacked triangular lattice Ising antiferromagnet CsCoBr3

Mao2002.htm

Ming Mao, B.D. Gaulin, R.B. Rogge, and Z. Tun
Physical Review B 66, 184432 (2002)

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Journal
Abstract
Introduction
Notes by Dr. Zin Tun

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The Journal

Abstract

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I. Introduction

   The combination of antiferromagnetic (AF) interactions and certain lattice geometries based on triangles and tetrahedra are known to result in phenomena known broadly as geometric frustration.1 AF interactions between near neighbor spins on such local geometries cannot be simultaneously satisfied, leading to novel ground states in which the frustrated spins try to compromise the conflicting demands of their pairwise interactions. If the interactions are frustrated in all three dimensions, the magnetic system may have difficulties finding any ordered ground state. Such a situation is known to arise in a subset of pyrochlore antiferromagnets, for example, for which AF-coupled magnetic moments reside on a network of corner-sharing tetrahedra.2 (Mao2002intro01.gif)

   The ABX3 AF insulators occupy interesting territory within this subject area.3 While these materials are quasi-one-dimensional antiferromagnets, their three-dimensional crystalline lattice is simple hexagonal. The crystal structure of CsCoBr3 is shown in Fig. 1. As can be seen, a triad of Br- ions mediates a strong intiferromagnetic exchange interaction between neighboring Co2+ ions along c. However, their magnetically ordered phases at low temperatures are determined by relatively weak interactions with the a-b, basal plane. As such they are thought of, and referred to, as stacked triangular lattice (STL) antiferromagnets. (Mao2002intro02.gif)

   In the ABX3 STL antiferromagnets, the interactions along c are not frustrated, and these systems do undergo phase transitions at low temperatures to long-range ordered states. However, the geometrical frustration can manifest itself in novel critical phenomena at these phase transitions, as occurs in the case for the XY STL antiferromagnet  CsMnBr3. This novel critical phenomena has been experimentally observed,4 and explained in terms of a chiral-XY universality class5 which reflects both the XY symmetry of the Mn2+ spins and the Ising-like chiral symmetry of the local ordering of the spins into the 120 structure. (Mao2002intro03.gif)

   Antiferromagnetically coupled Ising spins on the STL have stronger manifestations of geometrical frustration than do continuous spins, as there is analog of the 120 spin structure with which a triad of neighbor XY spins may compromise. Theoretically, such classical, Ising STL systems have been studied,6-10 and are known to display at least two phase transitions as the temperature is lowered. At relatively high temperatures, the system undergoes a phase transition from a paramagnetic state to a novel three-sublattice AF structure, wherein one of the three long-range ordered sub-lattices remains paramagnetic. The three sublattice structure consists of up, down and paramagnetic sublattices. At lower temperatures the paramagnetic sublattice orders, either up or down, so as to form a ferrimagnetic sheet with the a-b plane. The ferrimagnetic sheets stack antiferromagnetically, so that there is no net moment for the overall structure. (Mao2002intro04.gif)

There are more in the introduction.

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Notes by Dr. Zin Tun

 

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