Magnetic behavior of an amorphous ferromagnet for temperatures close to, and above, the Curie point: structural relaxation effects

Abstract

Detailed bulk magnetization (ac susceptibility) measurements on the amorphous Fe₁₆Ni₆₄B₁₉Si alloy have been performed in the temperature region 0.95T_C-1.5T_C (0.95T_C≤ T≤ 1.05T_C) before and after it had undergone isothermal annealing at 400 K for 60 and 120 min. An elaborate analysis of the data so obtained, besides yielding accurate values for the asymptotic and leading "correction-to-scaling" (CTS) critical exponents and amplitudes, reveals that the structural relaxation consequent upon isothermal annealing does not have any influence on the asymptotic and CTS critical exponents and on the universal amplitude ratios m₀/M_s(0) and Dm₀^δ/h₀, whose values are close to those theoretically predicted for a three-dimensional Heisenberg ferromagnet, but has profound effect on the ratio μ₀h₀/k_BT_C and the Curie temperature, T_C. The fraction of spins 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 scaling fields) in the critical region but the reverse is true for T» T_C. Regardless of whether the sample is annealed or not, the initial susceptibility follows the generalized Curie-Weiss law for as wide a temperature range as T_C ≤ T≤ 1.5T_C. The present results provide a strong evidence for weak itinerant ferromagnetism in the glassy alloy in question.