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

Abstract

The longitudinal components of the `zero-field-cooled' and `field-cooled' magnetization of amorphous ( TM= Co, Ni) and re-entrant ferromagnetic alloys have been measured in the static mode and at different thermal cycling rates varying from to in 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 and , respectively, for fields ; depends on x and y. and follow the relations and , where ( stands for or ), 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 and are several orders of magnitude larger than the MF estimates. is relatively insensitive while is extremely sensitive to thermal cycling rates (TCR) if they exceed a threshold value. The physical implications of extremely large magnitudes of and , 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.