Effect of unidirectional anisotropy on the ferromagnetic resonance in ferrite nanoparticles

Magnetization oscillations in a spherical ferromagnetic particle with unidirectional surface and uniaxial bulk anisotropies are considered. Due to the surface spin pinning, the uniform Larmor precession is replaced by a spatially modulated mode. The corresponding dispersion equation is solved in the weak pinning limit. The result shows that in the particle FMR spectrum, the surface effect is equivalent to the presence of an additional intrinsic size-dependent field coaligned with the constant magnetizing one. The experimental evidence obtained on maghemite ferrofluids supports this conclusion.