  Compounds based on the Fe2P structure have continued to attract interest
because of the interplay between itinerant and localized magnetism in a
non-centrosymmetric crystal structure, and because of the recent developments
of these materials for magnetocaloric applications. Here we report the growth
and characterization of mm size single crystals of FeMnP0.8Si0.2. Single
crystal x-ray diffraction, magnetization, resistivity, Hall and heat capacity
data are reported. Surprisingly, the crystals exhibit itinerant
antiferromagnetic order below 158 K with no hint of ferromagnetic behavior in
the magnetization curves and with the spins ordered primarily in the ab plane.
The room temperature resistivity is close to the Ioffe-Regel limit for a metal.
Single crystal x-ray diffraction indicates a strong preference for Mn to occupy
the larger pyramidal 3g site. The cation site preference in the as-grown
crystals and the antiferromagnetism are not changed after high temperature
anneals and a rapid quench to room temperature.
