Universality classes of magnetic phase transitions in the stacked triangular lattice antiferromagnets, CsMnBr3 and CsMnI3

Our recent results of the neutron scattering study on new universality classes of magnetic phase transitions in the stacked triangular lattice antiferromagnets, CsMnBr3 and CsMnI3 are summarized. For CsMnBr3 with XY spin symmetry a new phase transition characterized by the novel critical exponents β = 0.25(1), γ = 1.10(5) and ν = 0.57(3) was found in good agreement with the prediction for Z2xS1 or O(2) symmetry order parameter, giving an experimental evidence for the existence of a new universality class linked to chiral symmetry. A comparative study on an isomorphic compound CsMnI3 with Ising spin symmetry has revealed that the exponents, β// = 0.32(1) and βperpendicular = 0.35(1) for the orderings of the parallel and perpendicular spin components to the c-axis definitely differ from that for CsMnBr3 and much closer to theoretical β = 0.345 for the XY symmetry of the order parameter despite the fact the spin has an Ising symmetry. However, the exponents, γ// = 1.27(7), ν// = 0.59(3) and γperpendicular 1.04(3), ν/- = 0.56(2) do not show a meaningful difference from those of CsMnBr3 within the experimental errors and are not consistent with the expected XY universality. (author)