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

The effect of dipolar and exchange interactions on the magnetic behaviour o f sputtered CoFe-Ag(Cu) granular alloys is examined through the study of re manence curves and Delta M plots as a function of the ferromagnetic volume content, x(nu), and annealing temperature. As-deposited samples, with eithe r a random distribution of weakly interacting fine magnetic particles (3 nm in size) at low x(nu) or with strong dipolar and exchange interactions (le ading to a long range out-of-plane stripe-like domain structure) at x(nu) g reater than or equal to 0.25, display negative Delta M values. In the forme r, this is attributed to dipolar interactions being dominant in a random di stribution of well separated particles. In the latter, Delta M < 0 is attri buted 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 increa se of the direct exchange through the surface of neighbouring grains in the same particle clusters, at the expense of dipolar interactions, which decr ease as the clusters become more separated. Consequently, at high x(nu), an nealed samples display positive Delta M values.