Tetracritical behavior of CsMnBr3

We have used elastic neutron scattering to determine the magnetic phase diagram of CsMnBr3, an XY antiferromagnet on a stacked triangular lattice. The application of a field along (100) splits the antiferromagnetic transition and results in an intermediate phase of spin-flop character. This phase is extremely narrow near the zero-field transition at TN=8.32 K which is shown to be a tetracritical point. This suggests that the unusual critical properties of the zero-field phase transition may be due to its tetracriticality as opposed to belonging to a new universality class.