Nanocrystalline structures and the enhancement of remanence and energy product in melt spun iron-rare earth-boron alloys for permanent magnets

The influence of alloy composition and processing conditions on the magnetic properties of melt spun hard magnetic iron-rare earth-boron-based alloys is discussed. It is shown that, under carefully controlled conditions, a nanocrystalline, randomly oriented grain structure with mean grain size d_g≤30 nm can be obtained in the as-spun state and that as d_g is decreased below ∼30 nm the remanence J_r is progressively increased above the value of 0.8 T expected from the Stoner-Wohlfarth theory. This results from exchange coupling between Fe₁₄Nd₂B grains. The effect is shown to occur not only in alloys having Si additions but also in ternary FeNdB, including alloys containing finely dispersed αFe phase and in Fe-Mischmetal-B alloys with remanence enhancement up to ∼1.2 T and 0.78 T, respectively. This leads to substantial improvement in maximum energy product, for instance up to > 160 kJ m^-3 (20 MGOe) for some ternary FeNdB alloys. A roughly linear relation is shown to exist between J_r and intrinsic coercivity in the nanocrystalline regime.