The absence of critical concentration for ferromagnetism in disordered PdNi-type alloys

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Abstract
The generalized Landau model is applied to disordered strongly 'paramagnetic' alloys. It is shown that there is a close analogy between the susceptibility of such magnetic alloys and the Green's function of ordinary (nonmagnetic) disordered binary alloys. The Green's function analogy is used to prove that within the generalized Landau theory, disordered alloys similar to PdNi do not exhibit a critical concentration for ferromagnetism. The magnetization of such alloys is estimated on the basis of a simple statistical model and it is proved quite rigorously that a spontaneous magnetization exists even in the so called 'paramagnetic' region (below 2.3% Ni for PdNi). These general conclusions are illustrated by a CPA calculation of the susceptibility of PdNi alloys. In contrast to previous CPA calculations, it is found that the CPA susceptibility never diverges, thus indicating the absence of a well-defined critical concentration.