Phase formation and ferrimagnetism of GdCo9Si4

The phase relations of the GdCo13-xSix system have been studied by means of scanning microscopy, microprobe analysis and x-ray diffraction. Single-phase samples GdCo9 +/-delta Si4 -/+delta (structure type LaFe9Si4 with space group I4/mcm) are formed in a narrow composition range vertical bar delta vertical bar <= 0.2, where GdCo9Si4 forms as a fully ordered ternary compound. The magnetic properties of GdCo9Si4 have been investigated by ac susceptibility, magnetization, specific heat and resistivity measurements. These experiments reveal ferrimagnetism below about 47 K, which is analysed in terms of a two-sublattice molecular field model with a local moment Gd subsystem coupling antiparallel to the itinerant ferromagnetic Co 3d sublattice. The 3d-3d exchange of the latter is the driving force for the onset of long range magnetic order. The validity of the two-sublattice model is demonstrated by high field measurements, showing that the ferrimagnetic coupling starts to break up at a lower critical field mu H-0(1) similar or equal to 27 T. The magnetic ground state of GdCo9Si4 has been analysed on a microscopic basis via LSDA electronic structure calculations.