Critical phenomena and giant superparamagnetic moments above Curie point of the amorphous Fe<SUB>20</SUB>Ni<SUB>60</SUB>P<SUB>14</SUB>B<SUB>6</SUB> alloy

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 &#946;, &#947;, and &#948; as 227.7 &#177; 0.5 K, 0.39 &#177; 0.02, 1.33 &#177; 0.05, and 4.45 &#177; 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 Tc reveals the existence of giant superparamagnetic clusters in the present alloy for temperatures above Tc. A small peak, which shifts to higher temperatures when the field strength is increased, observed in the magnetization versus temperature curves above Tc for fields &#8804; 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