In this work, we report on the application of fractional calculus to the modeling of the isochronal behavior of complex magnetic susceptibility obtained from polymer-magnetic nanocomposites composed of cobalt-ferrite nanoparticles embedded into a chitosan matrix. From the isochronal measurements of real and imaginary parts of the complex magnetic susceptibility and temperature-dependent static measurements, performed at different applied dc-fields, it was observed that the spins' response is mainly leaded by three contributions, which are attributed to the intrinsic magnetic anisotropy of the particles, the surface-to-core spins exchange within particles and to the dipole-dipole interactions among particles. Accounting these contributions, the proposed magnetic model was capable to describe, in very precise way, the experimental behavior of both, real and imaginary, parts of the complex magnetic susceptibility, at temperatures below to that related to the transition of the polymer-magnetic nanocomposite into the superparamagnetic regime. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 2154-2161, 2012