Critical phenomena and giant superparamagnetic moments above Curie point of the amorphous Fe₂₀Ni₆₀P₁₄B₆ alloy

Abstract

Results of the magnetization measurements performed on the amorphous Fe₂₀Ni₆₀P₁₄B₆ alloy in the temperature range 210 to 255 K in fields up to 10 kOe are reported. Magnetic data taken in the critical region, when analyzed with caution, give the values for the Curie temperature and critical exponents β, γ, and δ as 227.7 ± 0.5 K, 0.39 ± 0.02, 1.33 ± 0.05, and 4.45 ± 0.07, respectively. A close agreement between the above exponent values and those derived from the theories based on a three-dimensional Heisenberg model suggestive of the dominant short-range forces in the critical region (long-range forces, e.g., dipolar forces are shown to have negligible effect on the critical fluctuations of magnetization) has been observed. The data satisfy the magnetic equation of state characteristic of a second-order phase transition over the entire temperature range of the present investigation. A detailed analysis of the magnetization versus field isotherms taken above T_c reveals the existence of giant superparamagnetic clusters in the present alloy for temperatures above T_c. A small peak, which shifts to higher temperatures when the field strength is increased, observed in the magnetization versus temperature curves above T_c for fields ≤ 100 Oe has been shown to arise from the freezing of the ferromagnetic order within the giant clusters. Finally, a physical basis is provided for observing different values for the ferromagnetic ordering temperature as determined by a local measurement like the Mossbauer effect, on one hand, and resistivity and the present bulk magnetic measurements, on the other.