Detailed magnetization study of an amorphous ferromagnet

Abstract

Exhaustive magnetization measurements have been carried out on the amorphous ferromagnet Fe₄₀Ni₄₀P₁₄B₆ in the temperature range 4.2 to 700 K in fields up to 16 kOe. Magnetization at low temperatures is found to decrease in accordance with the predictions of an isotropic nearest-neighbor Heisenberg model. It is observed that in this alloy the rms range of exchange interaction equals roughly the mean nearest-neighbor distance. A theoretical interpretation has been provided for the observation that the spin-wave stiffness constant D is smaller in amorphous than in crystalline ferromagnets. Magnetization data taken in the critical region, besides yielding the values for the Curie temperature and critical exponents as T_C=519.9± 0.2 K, β =0.38± 0.01, γ =1.31± 0.02, and δ =4.46± 0.04, satisfy the magnetic equation of state characteristic of a second-order phase transition. The observed exponent values not only obey the static scaling relation but also present a reasonable agreement with the theoretical values deduced from a three-dimensional Heisenberg model. They do, however, exhibit a slight but systematic shift towards the molecular-field values. This observation has been shown to imply an increased effective range of the exchange interaction in the critical region. Long-range forces are found to have a negligible influence on the critical fluctuations of magnetization. Critical amplitude values suggest that the elementary moments involved in the magnetic order-disorder transition are those forming a part of the giant superparamagnetic moments which are found to exist for temperatures well above T_C. Above 575 K, the susceptibility obeys the Curie-Weiss law. A careful analysis of the susceptibility data reveals that the spin polarization of conduction electrons gives a negligible contribution to the distribution of the magnetic moment and that the moments on Ni and Fe atoms remain localized even in the paramagnetic state. No evidence has been found for the indirect exchange between spins on magnetic atoms mediated by conduction-electron spin, as conjectured by Kaneyoshi in amorphous ferromagnets, in the present glassy alloy.