Ferromagnetic resonance in arrays of highly anisotropic nanoparticles

We present in this study computational simulations of the ferromagnetic resonance response of magnetic nanoparticles with a uniaxial anisotropy considerably larger than the microwave excitation frequency (in field units). The particles are assumed to be randomly oriented in a two dimensional lattice, and are coupled by dipolar interactions through an effective demagnetization field, which is proportional to the packing fraction. We have included in the model fluctuations in the anisotropy field (H-K) and allowed variations in the demagnetizing field. We then analyzed the line shape and line intensity as a function of both fields. We have found that when H-K is increased the line shape changes drastically, with a structure of two lines appearing at high fields. The line intensity has a maximum when H-K equals the frequency gap and decreases considerably for larger values of the anisotropy. The effects of fluctuations in H-K and variations in the packing fraction have been also studied. Comparison with experimental data shows that the overall observed behavior is dominated by the particles with lower anisotropy.