Magnetic structures and magnetic phase diagram of NdxTb1-xMn2Ge2

The various magnetic structures and magnetic phase transitions in the series NdxTb1-xMn2Ge2 have been thoroughly studied by means of macroscopic magnetic and thermal measurements (such as magnetization, ac initial magnetic susceptibility and linear thermal expansion) and microscopic neutron-diffraction experiments. As a result, the magnetic phase diagram has been determined over the whole temperature range. Large changes in the local Mn magnetic moments (e.g., ΔμMn/μMn∼16% in TbMn2Ge2) have been detected at the magnetic phase transitions observed at low temperatures, ∼100 endash 140 K, in the x=0 endash 0.4 alloys. This variation, together with the appearance of magnetic ordering in the rare-earth sublattice, has been related to the volume anomalies found (e.g., ΔV/V∼0.3% in TbMn2Ge2). A new magnetic structure of the Mn sublattice in the RMn2X2 (R=rare earth, X=Si, Ge) family has been found in Nd0.4Tb0.6Mn2Ge2 (140K<T<350 K) where two antiferromagnetic commensurate components within the (001) plane coexist with a ferromagnetic component along the c axis. The peculiar layered structure of the RMn2Ge2 compounds favors a cancellation of the molecular field at the rare-earth sites in the case of antiferromagnetic arrangements of the Mn sublattice, effectively isolating the R atoms and making a paramagnetic behavior of these possible despite the presence of long-range order. The existence of a ferromagnetic component in the Mn sublattice has been concluded to be indispensable to allow the ordering of the rare-earth magnetic moments. copyright 1997 The American Physical Society