Electron magnetic resonance in interacting ferromagnetic-metal nanoparticle systems: experiment and numerical simulation

We have studied electron magnetic resonance (EMR) in ferromagnetic-metal nanoparticle systems which show promise as a component of left-handed metamaterials. Metallic Ni nanoparticles of about 8 nm in diameter are embedded in polymer films. When the average distance between the nanoparticles is decreased, we observed that the EMR signal shifts and broadens. Theoretical analyses based on micromagnetics simulation confirm that the shift of the signal is traced back to an increase in the magnetic dipole field in the nanoparticle systems due to the decrease in interparticle distance. Moreover, the simulation reveals that the perpendicular component of the dipolar field causes the broadening of the signal. The present study demonstrates that a dynamic analysis of the magnetization, with an explicit treatment of the magnetic dipole interactions, is necessary for a thorough understanding of the EMR and magnetic permeability of interacting nanoparticle systems.