  The magnetic response of nanostructures plays an important role on biomedical
applications being strongly influenced by the magnetic anisotropy. In this work
we investigate the role of temperature, particle concentration and nanoparticle
arrangement forming aggregates in the effective magnetic anisotropy of Mn-Zn
ferrite-based nanoparticles. Electron magnetic resonance and coercivity
temperature dependence analyses, were critically compared for the estimation of
the anisotropy. We found that the temperature dependence of the anisotropy
follows the Callen-Callen model, while the symmetry depends on the particle
concentration. At low concentration one observes only an uniaxial term, while
increasing a cubic contribution has to be added. The effective anisotropy was
found to increase the higher the particle concentration on magnetic colloids,
as long as the easy axis was at the same direction of the nanoparticle chain.
Increasing even further the concentration up to a highly packed condition
(powder sample) one observes a decrease of the anisotropy, that was attributed
to the random anisotropy axes configuration.
