| Ferrites have been used in magnetic recording devices, isolators, circulators and as permanent magnets for a long time. However, the recent developments in semiconductor technology and nanoelectronics have led to a significant growth in high frequency applications. As the device size becomes smaller and applications shift towards millimeter wavelengths, the need for novel materials capable of handling high frequencies has increased. This has generated interest in the study of materials in the nanoscale domain. Nanoferrites are being extensively studied for their potential application as high frequency absorbers, information storage media, circulators, isolators, etc. Other areas of growing interest for these materials include biomedical engineering, alternate energy, aerospace engineering and defense applications.;Nanoferrites consist of metal substituted iron oxide nanopowders that have average particle size below 100 nm. At these dimensions the domain wall resonance can be avoided since materials can exist in single domain state and thus such materials prove useful for high frequency applications. Nano-size materials have unique mechanical, electrical and magnetic properties. The unique properties of nanomaterials could be attributed to their structure which is close to that of an isolated atom or molecule. The properties of nanomaterials may not necessarily be predicted from those observed at larger scales. In fact, the electromagnetic properties of materials are known to change as particle sizes are reduced to the nanoscale. Consequently, it is necessary to characterize these materials in order to understand their behavior and better predict their potential use in high frequency applications.;A waveguide based set-up has been used here to perform transmission and reflection measurements on powdered nano-ferrite samples at microwave frequencies using a vector network analyzer. This measurement set-up is capable of accurately measuring the material properties in terms of s-parameters in the frequency range from 2 GHz to 40 GHz. The electromagnetic properties, namely, magnetic permeability and electric permittivity are derived from these parameters. The algorithm has been specifically written to calculate the real and imaginary parts of permittivity and permeability of the powdered nano-ferrite samples.;The measurements were also performed on micro-size samples to understand the dependence of material properties on particle dimensions. In order to verify the observed difference in the micro- and nano-size samples, the same powders were also analyzed by another technique at higher frequencies. Transmittance measurements were performed in the millimeter wave frequency range from 40 GHz to 120 GHz using a free-space quasi-optical millimeter wave spectrometer. The set-up is equipped with high-power backward wave oscillators. The complex permittivity for both micro- and nano-ferrites has been calculated from the measured transmittance spectrum. It was observed that the constitutive material properties, namely permittivity and permeability, as well as the ferromagnetic resonance frequency of the samples vary with the change in particle dimensions. Based on the results of these measurements, a model for calculating the ferromagnetic resonance frequency of ferrite powders has been derived, which takes into account the size and shape of the particles in the sample. It can be concluded from the size-dependent absorption properties observed in this study that these materials show promise as tunable millimeter wave absorbers. |
