Magnetic structure of the two-dimensional XY antiferromagnet Sr 2 CoSi 2 O 7 studied using single-crystal neutron diffraction

We report a combined polarized and unpolarized neutron diffraction study on a multiferroic Sr2⁢CoSi2⁢O7 (SCSO) single crystal below and above the antiferromagnetic ordering temperature T𝑁 = 6.5 K. Unpolarized neutron diffraction measurements at 15 K confirm the melilite-type tetragonal 𝑃⁢ ‾ 4 ⁢21⁢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 𝐶⁢𝑚⁢𝑚′⁢2′ and 𝑃⁢21⁢2′ 1⁢2′ magnetic space groups being equally possible. In these 𝐶⁢𝑚⁢𝑚′⁢2′ and 𝑃⁢21⁢2′ 1⁢2′ magnetic space groups we obtain a very similar ordered magnetic moment about 2.86 and 2.94𝜇𝐵/Co2+, respectively, which lies in the 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 T𝑁.