MAGNETIC-BEHAVIOR OF AN AMORPHOUS FERROMAGNET FOR TEMPERATURES CLOSE TO, AND ABOVE, THE CURIE-POINT - STRUCTURAL RELAXATION EFFECTS

Detailed bulk magnetization (ac susceptibility) measurements on the am orphous Fe16Ni64B19Si alloy have been performed in the temperature reg ion 0.95T(C)-1.5T(C) (0.95T(C) less-than-or-equal-to T less-than-or-eq ual-to 1.05T(C)) before and after it had undergone isothermal annealin g at 400 K for 60 and 120 min. An elaborate analysis of the data so ob tained, besides yielding accurate values for the asymptotic and leadin g ''correction-to-scaling'' (CTS) critical exponents and amplitudes, r eveals that the structural relaxation consequent upon isothermal annea ling does not have any influence on the asymptotic and CTS critical ex ponents and on the universal amplitude ratios m0/M(s)(0) and Dm0delta/ h0, whose values are close to those theoretically predicted for a thre e-dimensional Heisenberg ferromagnet, but has profound effect on the r atio mu0h0/k(B)T(C) and the Curie temperature, T(C). The fraction of s pins actually involved in the transition at T(C) is small and reduces slowly with increasing annealing time. Consistent with the theoretical expectations, nonanalytic corrections (originating from the nonlinear irrelevant scaling fields) to the singular behavior at T(C) dominate over the analytic ones (arising on account of the nonlinear relevant s caling fields) in the critical region but the reverse is true for T mu ch greater than T(C). Regardless of whether the sample is annealed or not, the initial susceptibility follows the generalized Curie-Weiss la w for as wide a temperature range as T(C) less-than-or-equal-to T less -than-or-equal-to 1.5 T(C). The present results provide a strong evide nce for weak itinerant ferromagnetism in the glassy alloy in question.