Magnetic states of granular layered CoFe-Al2O3 system

The granular layered magnetic system Co₈₀Fe₂₀(t)/Al₂O₃ (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 <t<1.3 nm at temperatures 10 K<T<300 K and ac frequencies 0.01 Hz<f<10³ 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