Electromagnetic properties of superparamagnetic nanocomposites

We report here new experimental results on superparamagnetic nanocomposites . We compare experimental transmission measurements with calculations based on the Onsager local field, taking into account the permanent magnetic mom ent of the particles. Optical measurements are performed on a Fourier trans form spectrometer in the far infrared range, and on a microwave setup. Usua lly, superparamagnetic relaxation is understood by comparing the relaxation time tau of the magnetic moment of the particle with the measuring time ta u(m). According to Ni el's model tau is temperature dependent. In our case, tau(m) is given by the electromagnetic wave period. As this experimental t echnique has been proved to be relevant to investigate superparamagnetism r elaxation, we check it with two kinds of nanocomposites: Ni (ferromagnetic) embedded in alumina or aluminum nitride matrix and gamma-Fe2O3 particles ( ferrimagnetic) in a polymer. The dominant anisotropy is magnetocrystalline for Ni and shape and/or surface for gamma-Fe2O3. This difference leads to d ifferent transmission behaviors while changing sample temperature from 5 to 300 K. The magnetocrystalline anisotropy shows up temperature dependence w hile the shape or surface anisotropies do not. (C) 2000 Elsevier Science B. V. All rights reserved.