Amorphous RE–Fe–B–Na colloidal nanoparticles: High temperature solution synthesis and magnetic properties

RE–Fe–B–Na (RE   =   Nd–Er) colloidal nanoparticles are prepared by high-temperature solution synthesis. These nanoparticles are ultra-small monodisperse, air-stable and amorphous, whose particle size and magnetic property are characterized by transmission electron microscope and superconducting quantum interference device. Taking Nd–Fe–B–Na nanoparticle as an example, it is found that the particle size can be controlled in less than 7   nm. Besides, the magnetic properties of RE–Fe–B–Na colloidal nanoparticles can be compared for different rare earth elements. Based on the bulk ferromagnetic coupling, other possible magnetic coupling mechanism is discussed.