Dispersible SmCo₅ nanoparticles with huge coercivity

It is difficult to obtain dispersed particles of SmCo₅ by calciothermic reduction because of sintering during the higherature reaction. This study presents a new strategy to synthesize dispersible SmCo₅ particles by co-precipitating a precursor containing amorphous Sm(OH)₃ and coherent nanoscale Co(OH)₂ and Ca(OH)₂ crystallites. The Ca(OH)₂ dehydrates into CaO which forms an isolation shell around the SmCo₅ particles that prevents them sintering during the reaction at 860 °C. A magnetization of 90 Am² kg^-1, a remanence ratio of 0.96 and a huge coercivity of 6.6-7.2 T were achieved at room temperature after dissolving the CaO and orienting a dispersion of the particles in epoxy in a 0.8 T external field. Based on its scan-rate dependence in high quasi-static and pulsed magnetic fields, the coercivity mechanism is identified as nucleation and growth of 88 nm³ nucleation volumes in a low-anisotropy surface region about 15 nm thick. The coercivity is the highest yet reported for nanoparticles of any permanent magnet and it opens the prospect of new higherature magnet composites.