Controlled magnetic properties of Ni nanoparticles embedded in polyimide films

We have studied the magnetic properties of Ni nanoparticles incorporated in polyimide films using dc- and ac-magnetization measurements. Transmission electron microscopy images reveal that both the mean particle diameter (dave) and the volume fraction of particles (ηave) in the composite layer can be controlled independently. In a series of samples with fixed dave, the system with the lowest ηave behaves magnetically as an ensemble of noninteracting superparamagnets. With increasing ηave, typical signatures of the dipole-dipole interparticle interaction, such as an enhanced blocking temperature, are observed. In dense systems, with a particle concentration above the threshold for geometrical percolation, the magnetic response is ferromagnetic due to the exchange interaction between particles in direct contact.