Magnetic coupling in metallic granular systems

The coupling between magnetic clusters in granular metals is investigated and the relative importance of the Ruderman-Kittel-Kasuya-Yosida and classical dipolar interactions is discussed. It is shown that beyond a certain critical cluster size, the coupling is entirely dominated by the dipolar term, leading to an antiferromagnetic bias. Numerical results for Cu1-xCox alloys indicate that such a critical value corresponds to clusters of about N = 100 Co atoms. The importance of those results to the understanding of the giant-magnetoresistance effect in granular materials is pointed out.