Self-assembled arrays of high anisotropy FePt-Au nanoparticles

We have studied the ferromagnetic resonance response of as-made and annealed Fe46Pt46Au8 self-ordered nanoparticles forming a film. As-made particles, which crystallize in a low magnetic anisotropy FCC phase, show a single isotropic resonance line close to g=2.1. Particles annealed at 450-bar C for 30min are composed of a mixture of the FCC and the high anisotropy L10 phases. In this sample the resonant spectra are anisotropic when the external applied field is rotated from the in-plane to the out-of-plane direction. This absorption also seems to be related to the minority FCC particles which have a better degree of surface order due to the surrounding ferromagnetic environment. From the line width angular dependence we obtained a large dispersion in the anisotropy constant, probably arising from the different degree of atomic ordering of particles in the L10 phase