Magnetic structures and magnetic phase diagram ofNdxTb1−xMn2Ge2

Abstract

The various magnetic structures and magnetic phase transitions in the series ${\mathrm{Nd}}_{\mathrm{x}}$${\mathrm{Tb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{2}$${\mathrm{Ge}}_{2}$ have been thoroughly studied by means of macroscopic magnetic and thermal measurements (such as magnetization, ac initial magnetic susceptibility and linear thermal expansion) and microscopic neutron-diffraction experiments. As a result, the magnetic phase diagram has been determined over the whole temperature range. Large changes in the local Mn magnetic moments (e.g., \ensuremath{\Delta}${\mathrm{\ensuremath{\mu}}}_{\mathrm{Mn}}$/${\mathrm{\ensuremath{\mu}}}_{\mathrm{Mn}}$\ensuremath{\approx}16% in ${\mathrm{TbMn}}_{2}$${\mathrm{Ge}}_{2}$) have been detected at the magnetic phase transitions observed at low temperatures, \ensuremath{\approx}100--140 K, in the x=0--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., \ensuremath{\Delta}V/V\ensuremath{\approx}0.3% in ${\mathrm{TbMn}}_{2}$${\mathrm{Ge}}_{2}$). A new magnetic structure of the Mn sublattice in the ${\mathrm{RMn}}_{2}$${\mathrm{X}}_{2}$ (R=rare earth, X=Si, Ge) family has been found in ${\mathrm{Nd}}_{0.4}$${\mathrm{Tb}}_{0.6}$${\mathrm{Mn}}_{2}$${\mathrm{Ge}}_{2}$ (140 K350 K) where two antiferromagnetic commensurate components within the (001) plane coexist with a ferromagnetic component along the c axis. The peculiar layered structure of the ${\mathrm{RMn}}_{2}$${\mathrm{Ge}}_{2}$ 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.