Magnetic losses in metal nanoparticle-insulator nanocomposites

Metal nanoparticle-insulator nanocomposites were synthesized and characterized for their magnetic loss and frequency-stability to understand the role of metal particle size and oxide passivation. Cobalt nanocomposites were synthesized with different matrices such as lead-borosilicate glass, silica and polymers, and fabricated into toroids for characterizing their microstructure and magnetic properties. X-ray Diffraction and Transmission Electron Miscroscopy were used to characterize the microstructure while Vibration Sample Magnetometry and impedance spectroscopy were used to obtain saturation magnetization, coercivity, permeability and magnetic loss. Nanocomposites with particles in the size range of 40–90 nm showed poor frequency stability and high loss beyond 200 MHz while finer particles (25–40 nm) resulted in stable properties beyond 500 MHz. The high coercivity and FMR broadening with oxide-passivated nanoparticle composites, however, degraded the magnetic losses at 500 MHz even with finer particles. The role of particle size and surface effects in suppressing the permeability and enhancing the frequency-stability is discussed.