Neutron diffraction study of the magnetic structures of CeMn(sub 2)Ge(sub 2) and CeMn(sub 2)Si(sub 2)

摘要

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.