Magnetic Structures of Pr0.8Lu0.2Mn2Ge2 and Pr0.6Lu0.4Mn2Ge2

The magnetic structures of Pr0.8Lu0.2Mn2Ge2 and Pr0.6Lu0.4Mn2Ge2 have been determined by neutron powder diffraction over the temperature range 10-450 K. On cooling from the paramagnetic region the Mn sublattice of Pr0.8Lu0.2Mn2Ge2 orders in a similar manner to PrMn2Ge2 with first ab-plane intralayer antiferromagnetism (AFl) below TNintra ~ 397 K followed by canted ferromagnetism (Fmc) at TCinter ~ 330 K and then a conical (Fmi) spin structure below Tc/c ~ 192 K. The transition at TCPr=35 K with related enhancement in magnetization, is assigned to the additional ferromagnetic contribution of the Pr sublattice leading to the combined (Fmc+F(Pr)). For Pr0.6Lu0.4Mn2Ge2 the transition from PM to AFl occurs at TNintra ~ 375 K while the canted ferromagnetic (Fmc) state forms at TCinter ~ 321 K . The increased Lu concentration of Pr0.6Lu0.4Mn2Ge2 destroys the incommensurate Fmi conical spin structure of Pr0.8Lu0.2Mn2Ge2. Rather, in common with similar mixed RT2X2 systems (e.g., La1-xYxMn2Si2, La1-xPrxMn2Si2), the Pr0.6Lu0.4Mn2Ge2 compound exhibits co-existence of the AFmc and Fmc phases on cooling from the pure Fmc state. Transformation to the combined ferromagnetic state (Fmc+F(Pr)) takes place on c-axis ordering of the Pr sublattice at TCPr ~ 31 K . In the region of phase co-existence, the Fmc unit cell is larger than the AFmc unit cell indicating strong magneto-structural coupling with a change of the lattice inducing a change of the magnetic state.