Magnetic relaxation/stability of Co ferrite nanoparticles embedded in amorphous silica particles

The investigated system consisted of Co ferrite nanoparticles embedded in amorphous SiO2 particles, with ε=1% magnetic volume fraction. The M–H curve (M is the magnetization and H is the external magnetic field) of the particle system, recorded at room temperature using a 50 Hz alternating magnetic field, showed a very wide hysteresis loop indicating a strong deviation from the theoretical Langevin curve. The structural phases and mean diameter of the nanoparticles were determined by X-ray diffraction and transmission electron microscopy. Structural analysis results and theoretical evaluations of the critical diameter under which the particle has a single-domain magnetic structure suggested that the factor accounting for the observed behavior is a high magnetic anisotropy (anisotropy constant around 1.6×105 Jm−3); even the volume of Co ferrite nanoparticles is within the superparamagnetic range. The observed behavior was explained assuming that the magnetic moments of nanoparticles could be blocked even at room temperature, due to the existence of a high (0.64 eV) energy barrier which cannot be exceeded by thermal activation alone (0.03 eV). The magnetic relaxation time (τ=2.5×10-1s) evaluated from experimental data was much longer than the experiment time (tm=2×10-2s), in agreement with the observed magnetic remanence. The Co ferrite nanoparticles embedded in amorphous SiO2 showed stable magnetic single-domain structure in a very wide range of diameters (7.6–162 nm), and the particle shape appeared to be unimportant due to the high magnetic anisotropy. The results reported in the present paper demonstrate the existence of a relatively simple preparation method of interesting magnetic nanomaterials with potential application for magnetic recording media.