Irreversibility lines in the H- T phase diagram of re-entrant amorphous ferromagnets

Abstract

The longitudinal components of the `zero-field-cooled' (Mzpc) and 'field-cooled' (Mpc) magnetization of amorphous Fe_90-xTM_xZr₁₀ (TM= Co, Ni) and Fe_90+yZr_10-y re-entrant ferromagnetic alloys have been measured in the static mode and at different thermal cycling rates varying from 0.01Kmin^-1 to 2 Kmin^-1in constant magnetic fields (H) ranging between 1.5 Oe and 15 kOe. The difference, , is taken to be the direct measure of irreversibility in magnetization. The onset of weak irreversibility and a crossover from weak to strong irreversibility are observed at the temperatures T_gt (H) and T_at (H), respectively, for fields H H; H depends on x and y. T_gt (H)and T_at (H) follow the relations {1-[Tgt (H)/Tgt (0)]} =Agt(H) and {1-[Tat (H)/Tat (0)]}³ = Aat h², whereh= gub H/KB T^o (T^o stands for Tgt(o)or Tat(o) ), which have the same form as those predicted by the modified versions of the Gabay-Toulouse (GT) and de Almeida-Thouless (AT) mean-field (MF) theories that include non-vanishing spontaneous magnetization, but the observed values of the coefficients Agt and Aat are several orders of magnitude larger than the MF estimates. Tat(H) is relatively insensitive while T_gt(H) is extremely sensitive to thermal cycling rates (TCR) if they exceed a threshold value. The physical implications of extremely large magnitudes of A_gt and A_at, and of the observed TCR-induced shifts in the GT and AT irreversibility lines in the H-T plane, have been brought out clearly while discussing these results in terms of the existing theoretical models.