Effect of off-stoichiometry and site disorder on the properties ofNi3Al: II. Magnetics

Abstract

A detailed comparison between the magnetic behaviours of the ‘as-prepared’ap-NixAl100−x alloyswith x = 74.3, 74.8, 75.1 and 76.1 at.% (that have both compositional disorder and site disorder) and ‘annealed’counterparts (that have only compositional disorder) over a wide range of temperatures and magneticfields (H) permits us to draw the following conclusions about the role ofdisorder. Regardless of the type of disorder, Curie temperature,TC, and spontaneousmagnetization at 0 K, M0, decrease in accordance with the power lawsTC(x) = tx(x−xc)τ and M0(x) = mx(x−xc)ψ as (the threshold Ni concentration below which the long-rangeferromagnetic order ceases to exist). Site disorder lowers the value ofxc by nearly 1 at.%Ni, enhances TC for a given composition (more so as ) by increasing the number of Ni nearest neighbours for a given Ni atom, and leavesM0 essentially unaltered because site disorder has essentially no effect on the density of states,N(EF), at theFermi level, EF, and the shape of the density-of-states curve nearEF (exceptfor x≈xc, where site disorder tends to primarily enhanceN(EF) and thereby stabilize long-range ferromagnetic order for Ni concentrations below thethreshold concentration, at.%, dictated by compositional disorder). At low and intermediate temperatures, spontaneous magnetization,M(T,H = 0), as well as the ‘in-field’magnetization, M(T,H), exhibit non-Fermi liquid behaviour in the samplesap-Ni74.3 andap-Ni74.8.As xc is approached from above, i.e. as the compositional disorder increases, strongerdeviations from the Fermi liquid behaviour occur and the temperature rangeover which the non-Fermi liquid behaviour persists widens. In contrast, theap-Ni75.1 and ap-Ni76.1 alloys follow the behaviour that the self-consistent spin-fluctuation theory predicts for aweak itinerant-electron ferromagnet with no disorder. Both compositional disorderand site disorder have no effect on the critical behaviour of the alloys near theferromagnetic-to-paramagnetic phase transition.