Research Papers in Physics and Astronomy
Date of this Version
July 2001
Abstract
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.
Comments
Published by IEEE. ©2001 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE. IEEE Trans. Magn. 37, 2200 (2001). http://www.ieee.org/.