Effect of site disorder on the asymptotic critical behaviour of Ni75Al25

Semwal, Anita ; Kaul, S. N. (2002) Effect of site disorder on the asymptotic critical behaviour of Ni75Al25 Journal of Physics: Condensed Matter, 14 (23). pp. 5829-5847. ISSN 0953-8984

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Official URL: http://iopscience.iop.org/0953-8984/14/23/313

Related URL: http://dx.doi.org/10.1088/0953-8984/14/23/313

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 Rx = DBδ -1Γ, 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 TC, 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.

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