UNIVERSALITY CLASS OF MAGNETIC PHASE-TRANSITION IN THE TRIANGULAR LATTICE ANTIFERROMAGNET CSMNI3

University classes of successive magnetic phase transitions in the triangular lattice antiferromagnet CsMnI3 are studied by measuring the critical exponents gamma and nu using neutron scattering. The critical exponents of the successive transitions at T(N1) = 11.2 K and T(N2) = 8.2 K, which correspond to orderings of magnetic moments parallel and perpendicular to the c-axis, respectively, are determined to be gamma parallel-to = 1.12 +/- 0.07, nu parallel-to = 0.59 +/- 0.03 (T(N1)) and gamma perpendicular-to = 1.04 +/- 0.03, nu perpendicular-to = 0.56 +/- 0.02 (T(N2)). These values and the beta-exponents beta parallel-to = 0.32 +/- 0.01, beta perpendicular-to = 0.35 +/- 0.01 measured in our previous work do not agree with a theory which predicted the three dimensional XY universality for both the transitions. This is in contrast with the satisfactory agreement of critical exponents of CsMnBr3 with a theory of the novel three-dimensional Z2 x S1 universality. A uniform magnetization parallel to the c-axis in terms of the three sublattices in the c-plane is found to increase slowly below T(N1).