MAGNETIC PHASE-TRANSITIONS IN A DISTORTED-TRIANGULAR LATTICE ANTIFERROMAGNET RBMNBR3

Neutron diffraction measurement of a distorted-triangular lattice anti ferromagnet RbMnBr3 has been performed. The critical exponent beta rep resenting the magnetic ordering under zero held is evaluated as 0.28 /- 0.02. The Kawamura theory, which predicts that the chiral 'Z(2) x S -1' tetracriticality at T-N(H = O)for XY spins with the chiral degener acy is not destroyed by introducing an orthorhombic lattice distortion , is found to be applicable to the criticality of RbMnBr3 at T-N.