Reentrant spin-glass behavior in Fe–Zr–B amorphous alloys

Fe91Zr9 and Fe–Zr–B (Fe-rich >80%) amorphous alloys show a reentrant spin-glass behavior at low-temperatures that has been characterized by AC susceptibility measurements. Frequencies between 10 Hz and 10 kHz have been used to define the dynamic behavior. The reentrant spin-glass temperature TRSG is below 40 K for all cases, while Curie temperatures, Tc range between 200 and 350 K. For the Fe91Zr9 the TRSG shift as a function of frequency is large compared to canonical spin-glass. The dynamic scaling shows a reentrant transition in Fe91Zr9 with a critical exponent zν=8.2. The effect of boron additions is to increase the TRSG frequency shift. This value becomes close to the values expected for superparamagnetic fine-particle systems. The relaxation kinetics are explained by means of a Vogel–Fulcher equation. The observed behavior should arise from a very inhomogeneous spin structure, in the form of magnetic clusters either of different composition or just of different density, which result in different exchange coupling values. The inclusion of boron atoms, even in reduced quantities, clearly affects the magnetic spin structure.