ψ1 of FexNi80−xB18Si2 alloys near the percolation threshold

Accurate measurements of the static magnetic susceptibility of amorphous Fe_xNi_80?xB₁₈Si₂ alloys with a concentration just above the critical one for the onset of ferromagnetism are reported. The effective critical exponent ψ¹ is found to vary non-monotonically with temperature. Shallow maxima in ψ¹ occur at increasing reduced temperatures (t=(T?T_c)/T_c) for decreasing Fe content (x). At elevated temperatures ψ¹ decreases with t but it does not reach the mean field value even for t=13. The results are consistent with the predictions of recent theoretical models.