Composites formed of magnetic and non magnetic material mixtures are often used in ultra wideband RF applications like electromagnetic suppression; circuit and antenna substrates; filters, phase shifters, medical diagnostics and treatment, magnetic film-mounted transmission lines; or high power radiators. Accurate design of the composites requires that magnetic properties (e.g. saturation magnetization, coercivity, anisotropy field and DC permeability) and/or dielectric material properties be known as functions of frequency, RF signal power and particulate size since at diameters less than ～ 100 nm magnetic and dielectric properties can significantly deviate from the bulk. In this paper, magnetic measurements of Fe and Fe_3O_4 particulates from 1MHz to 10 GHz are made as functions of magnetic particle size (5 nm to 10s of microns). AC magnetic constitutive parameter data are acquired using strip line cavity and coaxial line reflection-transmission. Composite measurements are fit to Lorentzian functional forms which determine DC susceptibility and resonant and relaxation frequencies of the composite. Effective media models are then applied to map the composite susceptibility, resonant and relaxation frequencies to anisotropy, relaxation frequency and DC susceptibility for each particle size and material. Magnetite composites encompass 3 orders of magnitude in particle scale and their measured data yield the most information for scale dependencies of magnetic parameters. Magnetite magnetization and susceptibility decrease with particle dimension, but a significant increase in anisotropy is observed. This increase in anisotropy modifies expectations for particle resonant frequency and paramagnetic onset temperature and therefore impacts the choice of magnetic scale for RF material, biological and medical applications of magnetite. Fe composites were analyzed for particles from ～ 70 nm to greater than 10 micron diameters. In this size range, susceptibility was measured to be ～ 1/6 the bulk value, with small dependence on particulate size. Fe particle magnetic anisotropy shows small increases over the bulk values. A relationship for magnetization of iron as a function of Fe size is proposed that is similar to that relationship empirically derived for magnetite.
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