Magnetic nanogranularity and spin-glass behavior in mechanically alloyed Fe35Al50B15

The combination of Mossbauer spectroscopy and magnetization measurements is demonstrated to be efficient in the determination of the phases responsible for the glassy magnetic dynamics of the highly disordered alloy Fe35Al50B15 produced by mechanical alloying. Its spin-glass behavior is apparent from the FC-ZFC magnetization irreversibility, the thermoremanent magnetization, and the slightly frequency-dependent peak in the temperature dependence of the ac susceptibility. Furthermore, the maximum at T(p)approximate to22 K in the real component of the ac susceptibility is accompanied by a sudden onset of the imaginary component. The fit of the frequency dependence of T-p to the critical slowing down law yielded a critical exponent znu=8+/-1. However, Mossbauer spectroscopy uncovered the existence of two different Fe environments, of which only the minor one freezes at low temperatures into a distribution of magnetic sextets. This result, combined with the superparamagnetic behavior detected at moderate fields, allowed us to ascribe the spin glass behavior to the presence of interacting, very fine, ferromagnetic clusters (estimated size similar to1 nm). The origin of this nanoscale compositional heterogeneity is argued to be related to regions of partially unalloyed Fe. The conclusions are contrasted with other mechanically alloyed systems with reported spin glass behavior or compositional inhomogeneity. (C) 2002 American Institute of Physics.