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 macrosc opic magnetic and thermal measurements (such as magnetization, ac init ial magnetic susceptibility and linear thermal expansion) and microsco pic neutron-diffraction experiments. As a result, the magnetic phase d iagram has been determined over the whole temperature range. Large cha nges in the local Mn magnetic moments (e.g., Delta mu(Mn)/mu(Mn)approx imate to 16% in TbMn2Ge2) have been detected at the magnetic phase tra nsitions observed at low temperatures, approximate to 100-140 K, in th e x=0-0.4 alloys. This variation, together with the appearance of magn etic ordering in the rare-earth sublattice, has been related to the vo lume anomalies found (e.g., Delta V/V approximate to 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 (140 K<T<35 0 K) where two antifer romagnetic commensurate components within the ( 001) plane coexist with a ferromagnetic component along the c axis. Th e peculiar layered structure of the RMn2Ge2 compounds favors a cancell ation of the molecular field at the rare-earth sites in the case of an tiferromagnetic arrangements of the Mn sublattice, effectively isolati ng the R atoms and making a paramagnetic behavior of these possible de spite the presence of long-range order. The existence of a ferromagnet ic component in the Mn sublattice has been concluded to be indispensab le to allow the ordering of the rare-earth magnetic moments.