Magnetic states of granular layered CoFe-Al2O3

The granular layered magnetic system Co80Fe20( )/Al2O3(3 nm), where the Co80Fe20 layers of nominal thickness 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 for 1 nm 1.3 nm at temperatures 10 K 300 K and ac frequencies 0.01 Hz 10 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 Néel–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 , 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 with the SPM–FM transition line on this diagram is confirmed.