ONSET OF FERROMAGNETISM IN DISORDERED V-NI, RH-NI, PT-NI AND PD-NI ALLOYS

Detailed measurements of the magnetization of disordered V-Ni, Rh-Ni and Pt-Ni alloys as a function of the temperature and the magnetic field, around the critical concentration for the onset of ferromagnetism, are presented and analysed, together with some data from the literature (particularly Beille's results for Pd-Ni under hydrostatic pressure). We show that instead of being fundamentally responsible for the magnetic transitions, the unavoidable Ni rich clusters may be viewed as perturbing them. Thus we take the Landau theory of phase transitions as the basis of a very simple model where the alloy consists of an homogeneous matrix interacting with a few superparamagnetic clusters. The Landau parameters, A(T, c, p) and B, which characterize this matrix, may differ markedly from those obtained from the conventional analysis of the magnetic data by means of Arrott plots. This model applies to all the alloys mentioned above, especially to Pd-Ni; here the complex behaviour of the magnetization M(H, T, c, p) is well reproduced, with a critical concentration as high as ∼4at% Ni.