Ferromagnetic interactions in nanostructured systems with two different Curie temperatures

We study the exchange interactions between ferromagnetic grains dispersed in a paramagnetic amorphous matrix. This interaction becomes very important for high-density packing of the grains and also when the amorphous matrix is near its Curie temperature. We obtain analytical results based on a mean-field approximation and also perform a Monte Carlo simulation. We use both procedures to estimate the ordering temperature as a function of the effective intergrain separation. The theoretical predictions are compared with experimental results for the ${\mathrm{Fe}}_{77}$ ${\mathrm{B}}_{18}$ ${\mathrm{Nb}}_4$Cu nanocrystalline alloy. © 1996 The American Physical Society.