加藤 徹也

The neutron-diffraction and magnetization measurements between 1.4 and 20 K under static magnetic fields up to 12 T have revealed a variety of magnetic phase transitions in the stacked triangular-lattice antiferromagnets ABX(3), including a novel phase transition which belongs to a new universality class linked to chiral degeneracy. The magnetic field versus temperature phase diagrams are classified into two types depending on their magnetic anisotropies.

A hexagonal antiferromagnet RbFeBr3 was found to undergo successive structural phase transitions at T I (=109.0 K) and T c (=34.4 K) and to be ferroelectric along the hexagonal c-axis below T c. RbFeBr3 is a new type of ferroelectric crystal, the crystal structure of which is characterized by linear chains of face-sharing octahedra (FeBr6)4- along the c-axis (h-h-h packing). The ferroelectricity in RbFeBr3 is attributed to possible shifts of two-thirds of the chains antiparallel to the remaining one-third along the c-axis, which are possible due to the crystal symmetry of P63cm below T c.

Neutron diffraction measurements of single-crystal RbMnBr3 have been performed under magnetic fields applied parallel to the #*-axis of the prototype lattice, where the structural phase transitions are not taken into account. The critical line of a field-induced phase transition, at which the modulation wave number of the spin structure changes, has been obtained in the magnetic phase diagram for 4.2Tell K and 0#5.0T, in addition to the in-c-plane spin-flop phase transition. distorted triangular lattice antiferromagnet, neutron diffraction, magnetic phase diagram, modulated spin structure, field-induced phase transition. © 1993, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

The magnetic properties of RbVBr3 with a deformed CsNiCl3 structure were investigated by means of the susceptibility and torque measurements. RbVBr3 undergoes the successive transitions, $T_{\text{N}1}{=}28.0$ K and $T_{\text{N}2}{=}21.3$ K and 19.0 K on heating and cooling, respectively. A large hysteresis was observed for the transition at $T_{\text{N}2}$. Metastable states appear between the intermediate phase and low-temperature phase. The intermediate phase has the weak ferromagnetic moment of the order of $10^{-5}$ $\mu_{\text{B } }$ perpendicular to the $c$-axis, while the low-temperature phase does not. It is compatible with the crystal structure that the weak moment due to the Dzyaloshinsky-Moriya interaction appears in linear chains. It is concluded that in the intermediate phase the spins form a collinear structure, probably ferrimagnetic one in the $c$-plane and a triangular one in the low-temperature phase.

The temperature dependence of birefringence DELTA-n(T) of a triangular lattice antiferromagnet RbMnBr3 has been measured together with comparative measurements of DELTA-n's of related compounds CsMgCl3, CsMgBr3, RbMgBr3, CsMnI3, CsMnBr3- A structural phase transition of RbMnBr3 at T(s2) = 220 K with symmetry-breaking in the hexagonal c-plane has been found. The low-temperature structure below T(s2) is suggested as orthorhombic, which can be the origin of an incommensurate spin structure of RbMnBr3. Critical exponents of magnetic specific heat are determined as alpha' = 0.22 +/- 0.06, alpha = 0.42 +/- 0.16, which are in fair agreement with the values of the Z2 x S1 universality class.