The electromagnetic properties of nanoparticle colloids at radio and microwave frequencies

The aim of this study is to investigate the electromagnetic properties of nanoparticle colloids in the frequency range 1 MHz - 20 GHz, with focus on the electromagnetic absorption mechanisms at microwave frequencies. A broad range of magnetic and dielectric properties are investigated and a number of mechanisms are highlighted for tailoring the electromagnetic performance. Results from the Co (x) Ni(1- x) series, with particle sizes ranging from 25 to 200 nm, show particle size- related dielectric and magnetic properties, which aid the optimization of the resulting properties, in addition to conventional mechanisms, which are also demonstrated in colloidal form. A further reduction in particle size to below 20 nm leads to single magnetic domain particles, which also exhibit enhanced electromagnetic properties, as demonstrated with broadband magnetic performance achieved for Co ferrofluid with an average particle size of 5 nm.