Crystallization and magnetic properties of melt-spun two-phase nanocrystalline Nd-Fe-B alloy

A new type of permanent magnet alloy with composition Nd9.32Fe85.32B5.36 has been prepared by a melt spinning method. After optimized annealing of the initial amorphous alloy, high magnetic properties are obtained in the isotropic specimens. Remanence, energy product and intrinsic coercivity are 1.09 T, 153.6 kJ m(-3) and 400 kA m(-1), respectively. The samples consist of two phases, a matrix of nanometer-sized hard magnetic Nd2Fe14B phase (about 40 nm) together with numerous alpha-iron particles (about 20 nm) embedded in it. The crystallization behavior for amorphous samples has been studied. The results indicate that the Lu-iron initially precipitates from the amorphous matrix, and at higher annealing temperatures the crystallization of the Nd2Fe14B phase occurs. No metastable phase was observed in all heat treatments. Although the volume fraction of the magnetically soft alpha-iron is about 30 vol%, the second quadrant of the J-H loop is typical of a magnetically soft hard material. The role of exchange coupling between the magnetically soft grain and the magnetically hard grain is discussed.