Effect of site disorder on the asymptotic critical behaviour of Ni₇₅Al₂₅

Abstract

Extensive bulk magnetization and ac susceptibility measurements have been performed over a wide temperature range on well characterized polycrystalline Ni₇₅Al₂₅ samples 'prepared' in different states of site disorder. A detailed data analysis unambiguously establishes (i) the existence of multiplicative logarithmic corrections to the mean-field (MF) power laws in the asymptotic critical region near the ferromagnetic-paramagnetic phase transition and (ii) a gradual crossover to the Gaussian fixed point at temperatures outside the critical regime, irrespective of the degree of site disorder present. The latter crossover is followed by yet another crossover from Gaussian to pure MF regime in all the samples. Accurate determination of the universal amplitude ratio R_x = DB^{δ -1}Γ, the asymptotic critical exponents β , γ and δ and the logarithmic correction exponents x^-, x⁺ and x⁰ for spontaneous magnetization, initial susceptibility and the magnetization versus field isotherm at the Curie temperature T_C, coupled with the observations made on the same system previously, not only rules out completely the possibility of isotropic short-range Heisenberg or isotropic long-range dipolar or uniaxial dipolar asymptotic critical behaviour in Ni₇₅Al₂₅ but also indicates strongly that, in the asymptotic critical region, the weak itinerant-electron ferromagnet Ni₇₅Al₂₅ behaves as an isotropic d = 3, n = 3 ferromagnet in which the attractive interactions between magnetic moments decay with intermoment distance (r) as J(r)~ 1/r^(3/2)d, and that site disorder is irrelevant in the renormalization group sense.