New Universality Class of Antiferromagnetic Phase Transition in CsMnBr3

The critical exponents γ and ν of an XY antiferromagnet on a layered-triangular lattice, CsMnBr 3 , have been measured by neutron scattering. They are γ=1.10±0.05 and ν=0.57±0.03 in the reduced temperature range 0.015< ε < 0.1 above the Néel temperature. These and the previously measured β show that this phase transition belongs to the universality class characterized by the O(2) symmetry order-parameter in three dimensions, as calculated by Kawamura.