Dynamic susceptibility of reentrant Fe-rich inhomogeneous amorphous alloys

Melt-spun amorphous alloys of Fe 91 Zr 9 , Fe 91 Zr 7 B 2 , Fe 90 Zr 7 B 3 and Fe 88 Zr 8 B 4 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 Fe 91 Zr 9 follows a critical slowing down at the reentrant transition, with exponents $z\nu=7.2$ 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. Copyright Springer-Verlag Berlin/Heidelberg 2003