Magnetic ordering and structural distortion in a PrFeAsO single crystal studied by neutron and x-ray scattering

We report the magnetic ordering and structural distortion in PrFeAsO crystals, the basis compound for one of the oxypnictide superconductors, using high-resolution x-ray diffraction, neutron diffraction, and x-ray resonant magnetic scattering (XRMS). We find the structural tetragonal-to-orthorhombic phase transition at ${T}_{\mathrm{S}}=147\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the AFM phase transition of the Fe moments at ${T}_{\mathrm{Fe}}=72\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and the Pr AFM phase transition at ${T}_{\mathrm{Pr}}=21\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Combined high-resolution neutron diffraction and XRMS show unambiguously that the Pr moments point parallel to the longer orthorhombic $\mathbit{a}$ axis and order antiferromagnetically along the $\mathbit{a}$ axis but ferromagnetically along the $\mathbit{b}$ and $\mathbit{c}$ directions in the stripelike AFM order. The temperature-dependent magnetic order parameter of the Pr moments shows no evidence for a reorientation of moments.