Ferromagnetic resonance in ϵ -Co magnetic composites

We investigate the electromagnetic properties of assemblies of nanoscale ϵ -cobalt crystals with size range between 5 to 35 nm, embedded in a polystyrene matrix, at microwave (1–12 GHz) frequencies. We investigate the samples by transmission electron microscopy imaging, demonstrating that the particles aggregate and form chains and clusters. By using a broadband coaxial-line method, we extract the magnetic permeability in the frequency range from 1 to 12 GHz, and we study the shift of the ferromagnetic resonance (FMR) with respect to an externally applied magnetic field. We find that the zero-magnetic field ferromagnetic resonant peak shifts towards higher frequencies at finite magnetic fields, and the magnitude of complex permeability is reduced. At fields larger than 2.5 kOe the resonant frequency changes linearly with the applied magnetic field, demonstrating the transition to a state in which the nanoparticles become dynamically decoupled. In this regime, the particles ...

Khattiya Chalapat, Jaakko V I Timonen, Maija Huuppola, Lari Koponen, Christoffer Johans, Robin H A Ras, Olli Ikkala, Markku A Oksanen, Eira Sepp?l? and G S Paraoanu

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