Neutron diffraction study of the magnetic structures of CeMn{sub 2}Ge{sub 2} and CeMn{sub 2}Si{sub 2}

The magnetic properties of the layered compounds of the form RMn{sub 2}X{sub 2} (R = Rare Earth, X = Si, Ge) have been thought to be sensitive to the intralayer Mn-Mn distance. Thus it has been reported that the Mn moments in CeMn{sub 2}Si{sub 2} are aligned antiferromagnetically (AF) below T{sub N} = 380K, while the Mn moments in CeMn{sub 2}Ge{sub 2} are ferromagnetic (FM) below T{sub C} = 316K. Recently, however, there has been some debate about the actual magnetic structures of this family of compounds, and for this reason the authors have performed high-resolution neutron powder diffraction measurements on these compounds for temperatures between 12K and 550K. The measurements indicate that at high temperatures both compounds are paramagnetic. Below T{sub N} = 380K CeMn{sub 2}Si{sub 2} becomes a collinear AF, with a structure similar to that reported by Siek et al. in which the magnetic propagation vector is {tau} = (0 0 1). CeMn{sub 2}Ge{sub 2} on the other hand, exhibits two different magnetic transitions. At T{sub N} {approx} 415K there is a transition to a collinear AF phase characterized by the commensurate propagation wavevector {tau} = (1 0 1). At T{sub C} = 318K 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 {tau} = (1 0 1-q{sub z}), where q{sub z} is temperature dependent. These findings are consistent with the recent results of Welter et al