Magnetic structure of the magnetoelectric material Ca^sub 2^ CoSi^sub 2^ O^sub 7

Detailed investigation of Ca2 CoSi2 O7 was performed in its low-temperature magnetoelectric state combining neutron diffraction with magnetization measurements on single crystals. The crystal and magnetic structures well below the antiferromagnetic transition temperature of TN ≈ 5.7 K were determined using neutron diffraction. Neutron diffraction data imply no structural phase transition from 10 K down to 2.5 K and are well described within the orthorhombic space group P21212 with a 3 × 3 × 1 supercell compared with the high-temperature unmodulated state (tetragonal space group P421m). We found that in zero magnetic field the magnetic space group is P212′12′ with antiferromagnetic order along the [100] or [010] axes for two types of 90∘ twin domains, while neighboring spins along the [001] axis are ordered ferromagnetically. A noncollinear spin arrangement due to small canting within the a b plane is allowed by symmetry and leads to the existence of the tiny spontaneous magnetization below TN. The ordered moment with a magnitude of about 2.8 μB/Co2+ at 2.5 K lies in the a b plane. Distinct differences between the magnetic structure of Ca2 CoSi2 O7 as compared to those of Ba2 CoGe2 O7 and Sr2 CoSi2 O7 are discussed.