Mössbauer study of the superspin glass transition in nanogranular Al49Fe30Cu21

The spin-glass-like behavior observed in mechanically alloyed Al49Fe30Cu21 is attributed to the collective freezing of ferromagnetic nanoclusters, i.e., the appearance of a superspin glass phase. The presence of the clusters and their estimated size (;1 nm) were inferred from magnetization measurements. The transition critical exponent zn513.860.4, extracted from ac-susceptibility data, is similar to those obtained in other superspin glasses. Mössbauer spectroscopy offered more direct evidence: the component related to the clusters undergoes a transition from a high-temperature quadrupolar doublet to a distribution of sextets, whose mean hyperfine splitting grows with decreasing temperature in a similar fashion to spin glasses. The relation between these magnetic dynamics and the sample structure is addressed.