Low-Dimensional Hard Magnetic Materials

Ferromagnetism is a size-dependent physical phenomenon. Although the size dependence has been studied in theory for decades, nanoscale ferromagnetic materials especially hard magnetic nanostructures and materials with controllable size and shape are not available till recently. We have worked at bottom-up approaches to preparation of low-dimensional hard magnetic materials including nanoparticles and nanowires/nanorods. By adopting newly developed “salt-matrix annealing”, “surfactant-assisted milling” and improved chemical solution techniques, we have successfully synthesized monodisperse hard magnetic FePt and SmCo nanoparticles as well as FeCo, CoNi, CoCx and Co based nanowires/nanorods. These first-ever-available nanoscale ferromagnets display various hard and soft magnetic properties at room temperature which are found to be strongly size and shape dependent. A systematic study on size dependent Curie temperature of the L1₀ structured FePt ferromagnetic nanoparticles with size from 2 to 15 nanometers reveals the finite size effect in the tiny ferromagnets. In case of the Co nanowires with controlled diameter and length, coercivity up to the theoretical limit has been achieved that renewed our understanding of the Brown’s Paradox. The ferromagnetic nanocrystals can be used as building blocks for advanced bulk and thin film magnets, and can be also applied in biomedicine and ferrofluid technologies.