Magnetic properties of the weak itinerant-electron ferromagnetNi75Al25: II. The effect of compositional disorder

Abstract

The results of a detailed study of the magnetic properties of well-characterized polycrystallineNipAl100−p () alloys are presented and discussed in the light of the existing theories. Extreme care hasbeen exercised in the sample preparation to ensure that the site disorder (invariablypresent in any alloy system) does not interfere with the compositional disorderbrought 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 disordersmears out the sharp features in the density of states (DOS) curve near the Fermi level,EF, and reducesthe DOS at EF, N(EF), andthereby causes a fall (an enhancement) in the values (value) of the spontaneous magnetization at 0 K,M0, the spin-wavestiffness at 0 K, D0, andthe Curie temperature, TC (zero-field differential susceptibility at 0 K,χ0). However, compositional disorder, unlike site disorder, gives rise to smooth variations inN(EF), the inverse Stoner enhancement factor , M0,D0,TC,D0/TC andχ0 withp. These variationsin the case of M0(p), D0(p) and TC(p) are very well described by the power laws , and with p>pc (pc = the percolation threshold for the appearance of long-range ferromagnetic order) predictedby 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 fromthe hydrodynamic (magnon) to critical (fracton) regime at a well-defined temperatureTco*(p). An elaborate analysis of the magnetization data in terms of the percolation modelspermits a reasonably accurate determination of the magnon-to-fracton crossover line in themagnetic 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 analysisalso vindicate the Alexander–Orbach conjecture and the Golden inequality ford = 3 percolating ferromagnetic networks.