Effect of unidirectional anisotropy on the ferromagnetic resonance in ferrite nanoparticles

Magnetization oscillations in a spherical ferromagnetic particle with unidi rectional surface and uniaxial bulk anisotropies are considered. Due to the surface spin pinning, the uniform Larmor precession is replaced by a spati ally modulated mode. The corresponding dispersion equation is solved in the weak pinning limit. The result shows that in the particle FMR spectrum, th e surface effect is equivalent to the presence of an additional intrinsic s ize-dependent field coaligned with the constant magnetizing one. The experi mental evidence obtained on maghemite ferrofluids supports this conclusion. [S0163-1829(99)11041-5].