Magnetic resonance in spherical Co-Ni and Fe-Co-Ni particles

Magnetic resonance is studied in absence of external field on composite mat erials made up with spherical and monodisperse fine Co-Ni and Fe-Co-Ni part icles. The particle size range varies over one order of magnitude from 25 t o 250 nm. In the frequency range studied [0.1-18 GHz] several resonance ban ds are generally observed attributed to nonuniform resonance modes. The res onance frequencies are found to depend on the magnetic particle size, only weakly for the lowest frequency mode, in a more pronounced manner for the f ollowing modes. A theoretical model based on a discrete treatment of the re sonant effect in independent small spherical or cylindrical grains is propo sed. It allows to compute the resonance frequencies, the spin-wave profiles , and the spin-wave intensities. The respective influences of particle geom etry, surface pinning, crystalline anisotropy, and exchange parameter value s are presented. According to this model it is found that the size dependen ce of the resonance frequencies is mainly related to the surface pinning an d to the exchange parameter value. In all cases the size dependence of the lowest frequency mode is found weaker than that of the following modes and for large particles the frequency of the first mode is directly related to the crystalline anisotropy. The particle shape effect on resonance frequenc ies is weak whereas the spin-wave profiles of the first modes are found to depend on the particle geometry. This model enables us to describe the gene ral shape of the experimental spectra and to infer the magnetocrystalline a nisotropy constants (KI) from the experimental data, which are found in goo d agreement with bulk values. Moreover it shows that the weak size dependen ce of the lowest frequency mode is due to a weak pinning at the particle su rface. Nevertheless, the effect of particle size on higher spin-wave modes requires us to account for the magnetic interactions between grains.