Magnetic properties of the weak itinerant-electron ferromagnetNi75Al25: I. The effect of site disorder

Abstract

Extensive magnetization measurements have been performed overwide ranges of temperature and magnetic field on polycrystallineNi75Al25 samples ‘prepared’ in different states of site disorder and thoroughly characterizedby x-ray diffraction (XRD), x-ray fluorescence and inductively coupled plasmaoptical emission spectroscopy. Detailed analysis of the magnetization (XRD)data permits an accurate determination of the spontaneous magnetization,M0, zero-field differentialsusceptibility, χ0, andspin wave stiffness, D0, at0 K, Curie temperature, TC, density of states (DOS) at the Fermi level,N(EF) (the atomic long-rangeorder parameter, S, which is a direct measure of the site disorder present). The effect of site disorderon the ground-state as well as the finite-temperature magnetic properties ofNi75Al25 is clearly brought out by the observed variations ofM0,χ0,D0,TC andN(EF) withS. Themain findings are as follows. Site disorder smears out the sharp features in the DOS curve near the Fermilevel, EF,reduces N(EF) and thereby promotes both propagating transverse spin fluctuations (spin waves) andnon-propagating zero-point as well as thermally excited spin fluctuations. The reduction inN(EF), in turn, results in diminished (enhanced) values ofM0,D0 and TC(χ0) with increased site disorder. Site disorder only affects the magnitude of the suppressionof spin waves and thermally excited spin fluctuations by the magnetic field(H) but does not alter the functional form of the suppression with field. This functionaldependence on field is for spin waves at low temperatures and slows down from a linear variation at intermediate temperatures to a variation at temperatures close toTC for the thermally excited spin fluctuations. Another importantobservation is that all the physical quantities of interest such asM0,χ0,D0,TC and N(EF) exhibit a sizable change in their magnitudes when point defects such as vacancies play animportant role in enhancing the degree of site disorder.