Neutron diffraction study of the magnetic structures of CeMn₂Ge₂ and CeMn₂Si₂

We have performed high-resolution neutron powder diffraction measurements on CeMn₂Si₂ and CeMn₂Ge₂ at temperatures between 12 and 550 K. Our measurements indicate that at high temperatures both compounds are paramagnetic. Below T_N=380 K CeMn₂Si₂ becomes a collinear AF, with a structure similar to that reported by Siek et al. in which the magnetic propagation vector is σ=(0 0 1). CeMn₂Ge₂, on the other hand, exhibits two different magnetic transitions. At T_N≈415 K there is a transition to a collinear AF phase characterized by the commensurate propagation wave vector τ=(1 0 1). At T_C≈318 there is a transition to a conical structure with a ferromagnetic component along the c axis and a helical component in the ab plane. The helical component is characterized by the incommensurate propagation vector τ=(1 0 1- q_z), where q_z is temperature dependent.