Neutron study of magnetic phase transitions in TbCo2Si2 and DyCo2Si2

Neutron diffraction and magnetic measurements reveal a phase transition in TbCo2Si2 and DyCo2Si2 from a collinear antiferromagnetic AFI type order wit h the k-vector [001] to an incommensurate sine-modulated magnetic structure described by the wavevector k = (0, 0, 1-k(z)). The transition above occur s directly below the Neel temperature and k, is equal to 0.042(1) for TbCo2 Si2 and 0.049(1) for DyCo2Si2 In both phases magnetic moments localized on rare earth ions are parallel to the c-axis.