NANOCRYSTALLINITY AND MAGNETIC PROPERTY ENHANCEMENT IN MELT-SPUN IRON-RARE EARTH-BASE HARD MAGNETIC-ALLOYS

Refinement of the grain size below approximately 35 nm mean diameter in melt-spun FeNdB-base alloys leads to enhancement of remanent polarization, J(r), above the level predicted by the Stoner-Wohlfarth theory for an aggregate of independent, randomly oriented, and uniaxial magnetic particles. This article summarizes the results of the recent systematic research on this phenomenon, including the influence of alloy composition and processing conditions on the crystallite size, degree of enhancement of J(r), and maximum energy product (BH)max. It has been shown that the effect can also occur in ternary FeNdB alloys, without the addition of silicon or aluminum, which was originally thought necessary, providing the nanocrystallites are not magnetically decoupled by a paramagnetic second phase. Values of (BH)max above 160 kJ . m-3 have been achieved. The relationship between grain size, J(r), intrinsic coercivity, (J)H(C), and (BH)max are discussed in terms of magnetic exchange coupling, anisotropy, and other parameters. Recent extension of this work to the enhancement of properties in Fe-Mischmetal-Boron-base alloys and to bonded magnets with a nanocrystalline structure is also described.