Instability of the Co-magnetic moment in Tm(Co,M)2 (M=Al,Si)

Tm(Co1-xAlx)2 compounds with x=0; 0.05; 0.10 and Tm(Co0.95Si0.05)2 have been studied by means of ac-susceptibility, magnetization, magnetostriction, electrical resistivity and neutron diffraction measurements. It is shown that the magnetic state of TmCo2 is characterized by the absence of the Co-magnetic moment and by the presence of the localized spin density fluctuations in the d-subsystem at low temperatures and is strongly affected by an applied magnetic field. The partial substitution of Al or Si for Co in TmCo2 leads to the appearance of the long range magnetic order and gives rise to the Co-magnetic moment up to MCo=1.25 μB. In Tm(Co0.9Al0.1)2, the field-induced phase transition in d-subsystem was observed at the critical field Bc∼17 T using high-field magnetization and magnetostriction measurements. This phase transition is associated with a collapse of the Co-magnetic moment. Instability of the Co-magnetic moment in TmCo2 at the substitution and in an applied magnetic field is explained by the closeness of the molecular field acting on the Co-subsystem to the critical value of the Co metamagnetic transition field. (orig.)