Magnetic structure of the two-dimensional XY antiferromagnet Sr2CoSi2O7 studied using single-crystal neutron diffraction

We report a combined polarized and unpolarized neutron diffraction study on a multiferroic ${\mathrm{Sr}}_{2}{\mathrm{CoSi}}_{2}{\mathrm{O}}_{7}$ (SCSO) single crystal below and above the antiferromagnetic ordering temperature ${\mathrm{T}}_{N}$ = 6.5 K. Unpolarized neutron diffraction measurements at 15 K confirm the melilite-type tetragonal $P\overline{4}{2}_{1}m$ space group as the parent structure of SCSO. The low temperature study at 2.3 K, in contrast, reveals symmetry lowering with the orthorhombic $Cm{m}^{\ensuremath{'}}{2}^{\ensuremath{'}}$ and $P{2}_{1}{2}_{1}^{\ensuremath{'}}{2}^{\ensuremath{'}}$ magnetic space groups being equally possible. In these $Cm{m}^{\ensuremath{'}}{2}^{\ensuremath{'}}$ and $P{2}_{1}{2}_{1}^{\ensuremath{'}}{2}^{\ensuremath{'}}$ magnetic space groups we obtain a very similar ordered magnetic moment about 2.86 and $2.94\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}/{\mathrm{Co}}^{2+}$, respectively, which lies in the $\mathit{ab}$ plane. Our spin polarized flipping ratio measurements under an applied magnetic field of 6 T in the paramagnetic state support the results of our bulk magnetization data, indicating strong easy-plane spin anisotropy, responsible for the in-plane order below ${\mathrm{T}}_{N}$.