Mechanically alloyed nanocomposite magnets (invited)

Nanocomposites, consisting of a hard magnetic rare earth–transition metal phase exchange coupled to soft magnetic α-Fe or α-(Fe,Co), exhibit enhancement of the remanent magnetization due to exchange coupling across interfaces between grains. Modeling studies have shown that crystallite sizes of less than 20 nm are generally required for significant remanence enhancement and values of remanent magnetization equal to 70%–80% of saturation magnetization have been reported in mechanically alloyed two phase mixtures of α-Fe and a hard magnetic phase, such as ${\mathrm{Nd}}_{\mathrm{2}}{\mathrm{Fe}}_{\mathrm{14}}\mathrm{B}.$ Studies of microstructural evolution during mechanical alloying have shown that as-milled structures consist of a magnetically soft two phase mixture of α-Fe and an amorphous phase. Similar microstructures are observed regardless of whether mechanical milling or mechanical alloying has been carried out. Heat treatment above a critical temperature is required to crystallize grains of the hard magnetic phase. The formation of metastable intermediate phases with interesting magnetic properties may precede formation of the equilibrium phase. It is found that the crystallization temperature is an important parameter determining the grain size of the soft magnetic phase and, hence, magnetic properties. Recent measurements of the reversible and irreversible magnetization behavior of this novel class of permanent magnet are also discussed.