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

The longitudinal components of the `zero-field-cooled' (Mzpc) and 'field-cooled' (Mpc) magnetization of amorphous Fe90-xTMxZr10 (TM= Co, Ni) and Fe90+yZr10-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 Tgt (H) and Tat (H), respectively, for fields H H; H depends on x and y. Tgt (H)and Tat (H) follow the relations {1-[Tgt (H)/Tgt (0)]} =Agt(H) and {1-[Tat (H)/Tat (0)]}3 = Aat h2, whereh= gub H/KB To (To 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 Tgt(H) is extremely sensitive to thermal cycling rates (TCR) if they exceed a threshold value. The physical implications of extremely large magnitudes of Agt and Aat, 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