Electronic properties and the appearance of magnetism in the (Tb_1-xY_x)Co₂ compounds

We present experimental results obtained from low temperature specific heat and magnetic measurements on the (Tb_1-xY_x)Co₂ pseudo-binary system. Two main concentration ranges are distinguished: i) 0≤x≤0.8, ii) 0.8≤x≤1.0. The first of these corresponds to the ferrimagnetic alloys with decreasing Curie temperatures and mean cobalt moment when x increases. The density of states at the Fermi level N(E_F), determined from specific heat measurements, show values of the same order of magnitude for x ≤ 0.6. This little change of N(E_F) over such a large concentration range is due to the location of E_F in the 3d Co sub-band with small contributions from the 4d of Y and 5d or the Tb electrons. The concentration range 0.8≤x≤1.0 corresponds to the onset of ferrimagnetism. Here high γ values are observed, favouring the Stoner condition for magnetism of cobalt sub-band, plus enhancement effects due to spin glass behaviour which appears to be untypical: instead of T, a T ^{3 2} behaviour is observed for the magnetic specific heat at low temperatures. This may be due to the existence of spin waves in a spin glass regime as predicted by recent theories.