Magnetic properties of the weak itinerant-electron ferromagnet Ni₇₅Al₂₅: II. The effect of compositional disorder

Abstract

The results of a detailed study of the magnetic properties of well-characterized polycrystalline Ni_pAl_100-p (73.5 at. % ≤ p ≤76 at.%) alloys are presented and discussed in the light of the existing theories. Extreme care has been exercised in the sample preparation to ensure that the site disorder (invariably present in any alloy system) does not interfere with the compositional disorder brought about by the reduction in the concentration of the magnetic (Ni) atoms. Thus, the observed variation in the magnetic properties with Ni concentration (p) is solely controlled by the compositional disorder. Like site disorder, compositional disorder smears out the sharp features in the density of states (DOS) curve near the Fermi level, E_F, and reduces the DOS at E_F, N(E_F), and thereby causes a fall (an enhancement) in the values (value) of the spontaneous magnetization at 0 K, M₀, the spin-wave stiffness at 0 K, D₀, and the Curie temperature, T_C (zero-field differential susceptibility at 0 K, χ₀). However, compositional disorder, unlike site disorder, gives rise to smooth variations in N(E_F), the inverse Stoner enhancement factor S^-1 = 1N (E_F)-1, M₀, D₀, T_C, D₀/T_C and χ₀ with p. These variations in the case of M₀(p), D₀(p) and T_C(p) are very well described by the power laws M_o(p) ~(p-p_c)^β_p, Do(p)~(p-p_c)^θ_p, and T_C(p) ~(p-p_c)^Φ with p>p_c (p_c = the percolation threshold for the appearance of long-range ferromagnetic order) predicted by the percolation theories for these quantities on a regular three-dimensional (d = 3) percolating network. The alloys in question exhibit a crossover in the spin dynamics from the hydrodynamic (magnon) to critical (fracton) regime at a well-defined temperature T_co^∗ (p). An elaborate analysis of the magnetization data in terms of the percolation models permits a reasonably accurate determination of the magnon-to-fracton crossover line in the magnetic phase diagram, the percolation-to-thermal crossover exponent, fractal dimension, fracton dimensionality, the percolation critical exponents for spontaneous magnetization, spin-wave stiffness, correlation length and conductivity. The results of this analysis also vindicate the Alexander-Orbach conjecture and the Golden inequality for d = 3 percolating ferromagnetic networks.