Mechanically alloyed nanocomposite magnets (invited)

Nanocomposites, consisting of a hard magnetic rare earth-transition metal p hase exchange coupled to soft magnetic alpha-Fe or alpha-(Fe,Co), exhibit e nhancement of the remanent magnetization due to exchange coupling across in terfaces between grains. Modeling studies have shown that crystallite sizes of less than 20 nm are generally required for significant remanence enhanc ement and values of remanent magnetization equal to 70%-80% of saturation m agnetization have been reported in mechanically alloyed two phase mixtures of alpha-Fe and a hard magnetic phase, such as Nd2Fe14B. Studies of microst ructural evolution during mechanical alloying have shown that as-milled str uctures consist of a magnetically soft two phase mixture of alpha-Fe and an amorphous phase. Similar microstructures are observed regardless of whethe r mechanical milling or mechanical alloying has been carried out. Heat trea tment 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 ph ase. It is found that the crystallization temperature is an important param eter determining the grain size of the soft magnetic phase and, hence, magn etic properties. Recent measurements of the reversible and irreversible mag netization behavior of this novel class of permanent magnet are also discus sed. (C) 1998 American Institute of Physics. [S0021-8979(98)42911-6].