Self-Biased Hexagonal Ferrite Thin Film on GaN, AlN, Si Substrates for mm-Wave Devices
To match the next generation wireless systems requirements for multi-gigabit speed communication, researchers are focusing on mm-wave frequency band up to 100GHz for potential solution. Meanwhile hexagonal barium and strontium ferrite material are attracting increasing attention because its FerroMagnetic Resonance (FMR) frequency is at the vicinity of aforementioned frequency band of interest. Other features such as large saturation magnetization (4pMs), high anisotropy field (HA), low dielectric loss and most importantly, its potential of achieving self-bias, paves the way for fabrication of integrated, planar ferrite devices that can play a crucial role in next generation wireless system. BaFe12O19 (BaM) and SrFe12O19 (SrM) are M-type (magnetoplumbite structure) hexaferrites, anisotropic hard magnetic oxides which have great technological importance as low-cost permanent magnets and maintain great scientific interest due to potential use in magnetic recording and mm-wave filters and absorbers. Many electromagnetic properties of materials change as particle sizes are reduced to the nanoscale. Due to high magneto crystalline anisotropy, both BaM and SrM hexaferrites are expected to have narrow (20 Oe) spin resonances in mm-wave frequency range, while YIG and nickel ferrites have rather wide (300 Oe and wider) ferromagnetic resonance characteristics. Measurements conducted confirm the idea of using BaM and SrM ferrite materials as sharp, narrow ferromagnetic resonance-based devices, operating in mm-waves. Our recent study reveal that the grain size of commercially available nano ferrite powders must be greater than single domain dimension (about 80-100 nanometers in diameter) to have any sharp ferromagnetic resonance to appear in mm-waves. These broadband mm-wave transmittance measurements were performed using a quasi-optical mm-wave spectrometer and a series of backwards wave oscillators encompassing the 30–120GHz range. BaM and SrM powders can be further investigated by DC magnetization, electron microscopy, x-ray diffraction, and light scattering to assess magnetic behavior, size, size distribution, and structure.