Ferromagnetic resonance in ferrite nanoparticles with uniaxial surface anisotropy

Magnetization oscillations in a single-domain spherical ferromagnetic parti cle with uniaxial surface and bulk anisotropies are studied. In a linear ap proximation we consider a weakly nonuniform precession mode and derive the dispersion relationships for the cases of the applied field either parallel or normal to the particle easy axis. It is shown that in both situations t he surface can produce a considerable shift of the precession frequency as observed with the conventional ferromagnetic resonance (FMR) technique. The data obtained for fine particle assemblies (frozen magnetic fluids based o n gamma-Fe2O3 nanosize grains) support the conclusion. Moreover, in the sys tems under study the surface anisotropy contribution to the FMR resonance f ield appears to entirely dominate the bulk one. (C) 1999 American Institute of Physics. [S0021-8979(99)03508-2].