Abstract
We determine the low-field ordered magnetic phases of the $S=1$ dimerized antiferromagnet ${\mathrm{Ba}}_{3}{\mathrm{Mn}}_{2}{\mathrm{O}}_{8}$ using single-crystal neutron diffraction. We find that for magnetic fields between ${\ensuremath{\mu}}_{0}H=8.80\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ and $10.56\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ applied along the $[1\overline{1}0]$ direction the system exhibits spin density wave order with incommensurate wave vectors of type $(\ensuremath{\eta},\ensuremath{\eta},\ensuremath{\epsilon})$. For ${\ensuremath{\mu}}_{0}Hg10.56\phantom{\rule{0.28em}{0ex}}\mathrm{T}$, 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.
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