Dynamic susceptibility of reentrant Fe-rich inhomogeneous amorphous alloys

Melt-spun amorphous alloys of Fe91Zr9, Fe91Zr7B2, Fe90Zr7B3 and Fe88Zr8B4 have been characterized by AC susceptibility at frequencies between 30 Hz and 10 KHz. The measurements reflect the existence of reentrant spin glass transitions below 40 K for all cases. The transition shift per frequency decade is large compared to the observed in conventional spin-glass alloys. The shift value increases with the boron content and becomes closer to values in fine-particle systems. We find that the relaxation in Fe91Zr9 follows a critical slowing down at the reentrant transition, with exponents zv = 7.2 and beta = 0.7. The non-linear susceptibility displays a peak at the transition in this alloy, but wider than in canonical spin glasses. A Vogel-Fulcher (VF) activation process can explain the frequency variation in all the Fe-Zr-B alloys. The reduction of the ideal glass temperature in the VF approach, found in the higher Boron content alloys. is an indication of a superparamagnetic-like behavior. The behavior shown by these alloys is intermediate between a collective freezing and superparamagnetic-like relaxation. We propose that this is arising due to a heterogeneous spin structure. The inclusion of a reduced amount of B, affects the magnetic spin structure.