Field-induced spin density wave and spiral phases in a layered antiferromagnet

We determine the low-field ordered magnetic phases of the S=1 dimerized antiferromagnet Ba3Mn2O8 using single-crystal neutron diffraction. We find that for magnetic fields between μ0H=8.80T and 10.56T applied along the [11¯0] direction the system exhibits spin density wave order with incommensurate wave vectors of type (η,η,ε). For μ0H>10.56T, the magnetic order changes to a spiral phase with incommensurate wave vectors only along the [hh0] direction. For both field-induced ordered phases, the magnetic moments are lying in the plane perpendicular to the field direction. The nature of these two transitions is fundamentally different: the low-field transition is a second-order transition to a spin density wave ground state, while the one at higher field, toward the spiral phase, is of first order.