The nature of magnetic interactions in CoFe-Ag(Cu) granular thin films

The effect of dipolar and exchange interactions on the magnetic behaviour of sputtered CoFe-Ag(Cu) granular alloys is examined through the study of remanence curves and Delta M plots as a function of the ferromagnetic volume content, x(nu), and annealing temperature. As-deposited samples, with either a random distribution of weakly interacting fine magnetic particles (3 nm in size) at low x(nu) or with strong dipolar and exchange interactions (leading to a long range out-of-plane stripe-like domain structure) at x(nu) greater than or equal to 0.25, display negative Delta M values. In the former, this is attributed to dipolar interactions being dominant in a random distribution of well separated particles. In the latter, Delta M < 0 is attributed to the flux closure between antiparallel neighbouring domains, which is strongly demagnetizing. Annealing causes phase segregation and particle growth, while particle clusterization occurs. These facts lead to an increase of the direct exchange through the surface of neighbouring grains in the same particle clusters, at the expense of dipolar interactions, which decrease as the clusters become more separated. Consequently, at high x(nu), annealed samples display positive Delta M values.