Static critical phenomena in ferromagnets with quenched disorder

Abstract

A critical review of the experimental data reported to date on the critical behaviour near the ferromagnetic-paramagnetic phase transition in amorphous ferromagnetic alloys is presented. Extensive magnetization measurements have been performed on several metallic glasses over a fairly wide temperature range around T_C (Curie temperature) which embraces the narrow temperature region, |T-T_C|/T_C≈ 0.1, covered in our earlier studies on some of these materials. The results deduced from a detailed analysis (re-analysis) of the present data (published data) are compared with those previously reported on crystalline ferromagnets and with the predictions of the existing theories. Such efforts culminate in a number of important findings that include: (i) critical properties of the pure (ordered) system are preserved so long as the disorder present is mainly of topological character; marked deviations from the pure behavior occur when the compositional disorder dominantly contributes to the quenched disorder present, (ii) in the weak and intermediate disorder regimes, quenched disorder does not significantly alter the width of the pure asymptotic critical region, and (iii) for compositions in the close proximity to the percolation threshold, only a small fraction of spins participates in the magnetic order-disorder transition. A simple model is proposed to account for the diversity of experimental results, a task which the available theories fail to accomplish.