Magnetic properties of the weak itinerant-electron ferromagnet Ni₇₅Al₂₅: I. The effect of site disorder

Abstract

Extensive magnetization measurements have been performed over wide ranges of temperature and magnetic field on polycrystalline Ni₇₅Al₂₅ samples 'prepared' in different states of site disorder and thoroughly characterized by X-ray diffraction (XRD), X-ray fluorescence and inductively coupled plasma optical emission spectroscopy. Detailed analysis of the magnetization (XRD) data permits an accurate determination of the spontaneous magnetization, M₀, zero-field differential susceptibility, χ₀, and spin wave stiffness, D₀, at 0 K, Curie temperature, T_C, density of states (DOS) at the Fermi level, N(E_F) (the atomic long-range order parameter, S, which is a direct measure of the site disorder present). The effect of site disorder on the ground-state as well as the finite-temperature magnetic properties of Ni₇₅Al₂₅ is clearly brought out by the observed variations of M₀, χ₀, D₀, T_C and N(E_F) with S. The main findings are as follows. Site disorder smears out the sharp features in the DOS curve near the Fermi level, EF, reduces N(E_F) and thereby promotes both propagating transverse spin fluctuations (spin waves) and non-propagating zero-point as well as thermally excited spin fluctuations. The reduction in N(E_F), in turn, results in diminished (enhanced) values of M₀, D₀ and T_C(χ0) with increased site disorder. Site disorder only affects the magnitude of the suppression of 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 functional dependence on field is √H for spin waves at low temperatures and slows down from a linear variation (~H)at intermediate temperatures to a √H variation at temperatures close to T_C for the thermally excited spin fluctuations. Another important observation is that all the physical quantities of interest such as M₀, χ₀, D₀, T_C and N(E_F) exhibit a sizable change in their magnitudes when point defects such as vacancies play an important role in enhancing the degree of site disorder.