Magnetic properties and granular structure of CoPt/B films

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Abstract
Granular CoPt/B films, consisting of high anisotropy face-centered-tetragonal (fct) CoPt particles embedded in a boron matrix have been prepared by heat treating cosputtered films. Transmission electron microscopy and x-ray diffraction studies show that the as-deposited films consist of a mixture of an amorphous-like phase and face-centered-cubic CoPt nanocrystallites. After a heat treatment at 700 °C for 5 min a microstructure consisting of interconnected particles of partially ordered fct CoPt with sizes ranging from 20 to 200 nm is formed. These samples have coercivities of 6 kOe and high reduced remanence (Mr/MS=0.9) due to the interparticle interactions. After prolonged annealing the particles become spherical and their size increases to around 400 nm. The coercivities of these overaged samples are higher but the hysteresis loops are constricted and the remanence is reduced. Measurements of irreversible susceptibility and viscosity as a function of temperature and applied field were used to determine the fluctuation field and activation volume in the optimally annealed sample.