Magnetic materials fulfill crucial functions in various high-technology devices, including high density memories, recording media, and passive components. With the ever increasing speed of signal processing and propagation, the high-frequency limits of traditional magnetic materials has been long surpassed. However, soft magnetic materials in the form of nanostructured materials sputter deposited as thin films and magnetic nanoparticle composites are promising choices for high frequency (~ GHz) applications, with nanoparticle composites being the focus of the present study.

In an attempt to identify the theoretical limits of the nanoparticle composite approach, a phenomenological or empirical model has been developed. The model will be shown to yield relationships between key magnetic properties of the composite (e.g., the low frequency effective permeability and the ferro-magnetic resonance (FMR) frequency) and the materials and physical attributes of the nanoparticles (such as saturation magnetization, crystal anisotropy, shape, volume fraction and packing type). The composite material is assumed here to consist of identical ideal ellipsoidal or cylindrical metallic ferromagnetic nanoparticles embedded in a non-magnetic matrix. At the heart of this model is an iterative extension to the effective medium theory. One of the reasons magnetic nanoparticle composites are preferred over sputter deposited thin films is the low eddy current loss displayed by the former. Larger the typical dimension of a system, more is the eddy current loss, and larger is the permeability degradation. Even when the particles are small enough to result in negligible eddy current losses at GHz frequencies, the statistical effect of particle clustering at high particle volume fractions could result in effective particles much larger than the actual particles. This effect has been included in the effective medium theory, thereby extending the latter further. Critical particle sizes beyond which eddy current losses become prohibitive at GHz frequencies will also be addressed.