加藤 徹也

Magnetic susceptibility and torque measurements were carried out for the mixed system CsMn(BrxI1-x)3. CsMnBr3 and CsMnI3 are known to be triangular antiferromagnets with planar and axial anisotropies, respectively. The intermediate phase observed in CsMnI3 becomes narrower with increasing bromine concentration x and vanishes at xc = 0.19. For the sample with x = xc, in order to study the critical behavior of which is expected to belong to the chiral Heisenberg universality class, neutron scattering experiments were performed. The critical exponents were obtained as ・ﾀ = 0.29 ・} 0.01, ・ﾁ = 0.75 ・} 0.04 and ・ﾋ = 0.42 ・} 0.03. ? 1998 Elsevier Science B.V. All rights reserved.

  The crystal structure of the low-temperature phase V of the distorted-triangular-lattice antiferromagnet RbMnBr₃ is thought to be a zigzag-row model with the space group symmetry Pbca (Z = 8) which is deduced from the extinctions of the possible reflections in the diffraction experiments. The spin-ordered states of RbMnBr₃ under magnetic fields in the ab plane are suggested to be qualitatively explained by the row model studied by Zhitomirsky.

Raman spectra of some phonon modes of the hexagonal ferroelectriccrystal KNiCl3are obtained in the temperature range between 290 K and 590 K, which includes the structural phase transition point T2(=561 K) at which previous measurements of dielectric constant and spontaneouspolarization as a function of temperature had shown that KNiCl3 undergoes a transition between polar phases II and III. An optical birefringence measurement carried outas a complement to the present Raman scattering revealed that this transition is of second order. Towards this transition point, the totally symmetric phonon mode with the lowest frequency observed in the room-temperature phasewas found to soften with increasing temperature.The present results provide new information on the phase-transitionmechanism and the space groups of thehigher (II)- and lower (III)-symmetric phases around T2.

Neutron diffraction measurement of a distorted-triangular lattice antiferromagnet RbMnBr3 has been performed. The critical exponent β representing the magnetic ordering under zero field is evaluated as 0.28 ± 0.02. The Kawamura theory, which predicts that the chiral 'Z2 × S1' tetracriticality at TN(H = 0) for XY spins with the chiral degeneracy is not destroyed by introducing an orthorhombic lattice distortion, is found to be applicable to the criticality of RbMnBr3 at TN. © 1995 Elsevier Science B.V. All rights reserved.

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.