Critical behaviour of the linear birefringence in 3d-Heisenberg antiferromagnets : RbMnF3, KNiF3 and KCoF3

The linear birefringence of stress induced single antiferromagnetic T-domains of RbMnF₃, KNiF₃ and KCoF₃ has been measured in the vicinity of T_N. We deduce the critical behaviour in such 3d-Heisenberg systems, showing a symmetry breaking at the transition involving a unique wave vector of the order parameter in the Brillouin zone. The critical exponent $\text{β}$ of the linear birefringence has been found to be larger than 2β, in agreement with the theoretical predictions of G. Gehring.