Abstract
The universality class of successive magnetic phase transitions in the triangular lattice antiferromagnet CsMnI3 is studied by measuring the critical exponent β using neutron scattering. A weak Ising anisotropy brings about two successive transitions at TN1=11.2 K and TN2=8.2 K which correspond to ordering of magnetic moments parallel and perpendicular to the c-axis, respectively. The critical exponents for the parallel and perpendicular components of the sublattice magnetization, β⁄⁄=0.32±0.01 and β⊥=0.35±0.01, are close to or in excellent agreement with the exponent for the three dimensional XY symmetry of the order parameter in spite of the fact that the system has the Ising spin symmetry, in accord with a prediction. They remarkably differ from β=0.25±0.01 for CsMnBr3 which has an XY anisotropy and belongs to the three dimensional Z2×S1 universality.
