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Magnetic structure of the magnetoelectric material Ca2CoSi2O7

Detailed investigation of Ca2CoSi2O7 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 T-N approximate to 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 P2(1)2(1)2 with a 3 x 3 x 1 supercell compared with the high-temperature unmodulated state (tetragonal space group P (4) over bar2(1)m). We found that in zero magnetic field the magnetic space group is P2(1)2'(1)2' with antiferromagnetic order along the [100] or [010] axes for two types of 90(omicron) twin domains, while neighboring spins along the [001] axis are ordered ferromagnetically. A noncollinear spin arrangement due to small canting within the ab plane is allowed by symmetry and leads to the existence of the tiny spontaneous magnetization below T-N. The ordered moment with a magnitude of about 2.8 mu B/Co2+ at 2.5 K lies in the ab plane. Distinct differences between the magnetic structure of Ca2CoSi2O7 as compared to those of Ba2CoGe2O7 and Sr2CoSi2O7 are discussed.

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