Magnetic states of granular layered CoFe-Al2O3 system

The granular layered magnetic system Co80Fe20 (t)/Al2O3 (3 nm), where the C o80Fe20 layers of nominal thickness t form separate, almost spherical magne tic granules of typical diameter 2-3 mn between the Al2O3 spacers, was stud ied. We discuss measurements of the dc and ac magnetic susceptibility chi f or 1 nm < t < 1.3 nm at temperatures 10 K < T < 300 K and ac frequencies 0. 01 Hz < f < 10(3) Hz, revealing a rich variety of magnetic states, includin g superparamagnetic (SPM), ferromagnetic (FM)-like, and glass (G)-like. In order to describe the ac response in this disordered planar system, a gener alization of the Mel-Brown approach is proposed, including random anisotrop y 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 inter action between granules: usual dipolar (point-like) and FM "exchange" (due to stray dipolar fields from nonspherical granules). In particular, the coi ncidence of maximum low field sensitivity dMR/dH with the SPM-FM transition Hue on this diagram is confirmed.