Coercivity of Magnetic Nanoparticles and Nanocomposites

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9900550MajetichThe magnetization reversal, or coercivity, of nanoparticles and nanocomposites is investigated on a nanometer scale using the Foucault method of Lorentz microscopy. Nanoparticles with extremely large or very small values of magnetocrystalline anisotropy are studied to extend existing models of coercivity. In high anisotropy ball milled SmCo5 nanoparticles the emphasis is on understanding the roles of strain, grain boundaries, and particle sizes in order to maximize the switching field. In low anisotropy Fe50Co50 nanoparticles minimal coercivity is desired, but it is unclear whether the best precursors to soft magnetic nanocomposites are superparamagnetic, or merely have low coercivity. The processing methods are varied to optimize the coercivities of the nanoparticles, whereupon they are compacted into nanocomposites. Cold isostatic pressing, magnetic compaction, and plasma pressure compaction are examined to determine the best method to yield the highest density without significant grain growth. Two types of nanocomposites with potential applications are examined, exchange spring magnets made by compaction of SmCo5 and Fe50Co50 nanoparticles and soft magnetic nanocomposites made by compaction of Fe50Co50 nanoparticles that have very thin carbon-rich coatings. Lorentz microscopy techniques are used to observe magnetization reversals on a submicron length scale. The microscopy results, in combination with standard methods to analyze the microstructure and chemical composition, identify the weak links that reduce the coercivity of a permanent magnet, or increase it in a soft magnetic material. The feedback guides the processing modifications required to optimize the magnetic properties of the nanocomposites.%%%This program examines exchange spring magnets, which could have higher energy products at lower cost than current high performance permanent magnets, and soft magnetic nanocomposites, which could have lower power losses, higher permeabilities, and greater stability than existing materials in niche applications at high temperatures or high frequencies.***