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The granular layered magnetic system Co80Fe20(t)/Al2 O3 (3 nm), where the Co80Fe20 layers of nominal thickness t form separate, almost spherical magnetic granules of typical diameter 2-3 nm between the Al2O3 spacers, was studied. We discuss measurements of the dc and ac magnetic susceptibility χ for 1 nm 3 Hz, revealing a rich variety of magnetic states, including superparamagnetic (SPM), ferromagnetic (FM)-like, and glass (G)-like. In order to describe the ac response in this disordered planar system, a generalization of the Neel-Brown approach is proposed, including random anisotropy and random molecular fields acting on the granules. The observed magnetic states are summarized in a phase diagram in the variables t-T, which is explained through the competition of thermal energy with two kinds of interaction between granules: usual dipolar (point-like) and FM “exchange” (due to stray dipolar fields from nonspherical granules). In particular the coincidence of maximum low field sensitivity dMR/dH with the SPM-FM transition line on this diagram is confirmed.