The magnetic phase transition in ordered and disordered ferromagnets

Abstract

The magnetic phase transition is investigated for the structurally disordered alloys FeNi 3, Fe 3Pt, FePd 3 and for the ordered alloys Fe 3Pt and FePd 3. The asymptotic critical exponents β, γ, γ' and δ are determined by several methods: modified Arrott plot, method of Kouvel and Fisher, scaling plot and ln M vs. ln H plot. No influence of structural disorder on the critical exponents is found. Furthermore the temperature dependence of the Kouvel-Fisher exponent γ( T) is discussed, for which a decisive difference between ordered and disordered materials is expected. The values of the exponents for the alloy FeNi 3 are consistent with the predictions for the 3d Heisenberg model. In contrast, for Fe 3Pt and FePd 3 the exponent values are modified towards the mean field values, presumably because of long-range exchange interactions.