Superferromagnetism in mechanically alloyed fcc Fe(23)Cu(77) with bimodal cluster size distribution.

Magnetic measurements, x-ray diffraction and Mössbauer spectroscopy were used to characterize a nanostructured fcc Fe(23)Cu(77) at.% alloy prepared by high-energy ball-milling, addressing in particular the effect of clustering on the nature of the interacting magnetic entities. The interpretation of magnetization measurements leads to the conclusion that grains, whose mean size is ∼16 nm, contain two populations of magnetic Fe-rich nanoclusters with a bimodal size distribution. These two sets of clusters contain about 14 and 400 Fe atoms and have magnetic moments of 30 µ(B) and 860 µ(B), respectively. The inter-cluster ferromagnetic interactions that lead to superferromagnetism with a Curie temperature T(C)∼220 K can be described by a mean field determined by the smaller clusters only, which account for 90% of the magnetization.