Magnetic properties of nanoparticle composites, consisting of aligned ferromagnetic nanoparticles embedded in a nonmagnetic matrix, have been determined using a model based on phenomenological approaches. Input materials parameters for this model include the saturation magnetization (M s ), the crystal anisotropy field (H k ), a damping parameter ͑␣͒ that describes the magnetic losses in the particles, and the conductivity ͑͒ of the particles; all particles are assumed to have identical properties. Control of the physical characteristics of the composite system-such as the particle size, shape, volume fraction, and orientation-is necessary in order to achieve optimal magnetic properties ͑e.g., the magnetic permeability͒ at GHz frequencies. The degree to which the physical attributes need to be controlled has been determined by analysis of the ferromagnetic resonance ͑FMR͒ and eddy current losses at varying particle volume fractions. Composites with approximately spherical particles with radii smaller than 100 nm ͑for the materials parameters chosen here͒, packed to achieve a thin film geometry ͑with the easy magnetization axes of all particles aligned parallel to each other and to the surface of the thin film͒ are expected to have low eddy current losses, and optimal magnetic permeability and FMR behavior.
A 2.7-V sigma-delta modulator with a 6-bit quantizer is fabricated in a 0.18-m CMOS process. The modulator makes use of noise-shaped dynamic element matching (DEM) and quantizer offset chopping to attain high linearity over a wide bandwidth. The DEM algorithm is implemented in such a way as to minimize additional delay within the feedback loop of the modulator, thereby enabling the use of the highest resolution quantizer yet reported in a multibit sigma-delta analog-to-digital converter of this speed. The part achieves 95-dB peak spurious-free dynamic range and 77-dB signal-to-noise ratio over a 625-kHz bandwidth, and consumes 30 mW at a sampling frequency of 23 MHz. The part achieves 70-dB signal-to-noise ratio over a 1.92-MHz bandwidth and dissipates 50 mW when clocked at 46 MHz.
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