High field investigation on the ferrimagnetic systems GdCo9-xSi4+x (-0.2 ≤ x≤ 0.2) and TbCo9Si4

GdCo9-xSi4+x (x = -0.2 to x = 0.2) and TbCo9Si4 exhibit the tetragonal LaFe9Si4 structure (space group I4/mcm). Magnetically these samples are ferrimagnetic with ordering temperatures close to 50K. The ferrimagnetic magnetization is found to decrease with decreasing Si content, thus, revealing an increasing cobalt sublattice magnetisation. The antiparallel coupling of the Gd- and Co-sublattice magnetisations breaks up at lower critical fields Hl well above 20 T. From the lower critical field Hl as well as from the slope ΔM/ΔH for fields above Hlthe inter-sublattice molecular coefficient can be determined which is approximately constant within this solid solution