Magnetic neutron-scattering studies ofRbMnBr3

${\mathrm{RbMnBr}}_3$ is an XY-like antiferromagnet on a distorted stacked triangular lattice. At low temperatures and fields we show by neutron scattering that there is an incommensurate ordered structure with magnetic Bragg peaks at (h/8±δ,h/8±δ,l) where h and l are odd integers and δ=0.0183±0.0004. At low temperatures, a field of 3 T applied along the a axis produces a first-order phase transition to a commensurate structure corresponding to δ=0, so that the lattice parameter of the magnetic cell is eight times that of the nuclear cell. In addition to these two ordered structures, there is another incommensurate ordered phase found at high temperatures. The magnetic phase diagram shows a tetracritical point at zero field and T=8.5±0.1 K and a bicritical point at H=2.55±0.05 T and T=7.8±0.1 K. Inelastic neutron scattering was used to measure spin waves along the c axis and in the easy plane. The dispersion relations confirm that ${\mathrm{RbMnBr}}_3$ is quasi-one-dimensional, with much stronger exchange along c than in the ab plane.