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 dg≤30 nm can be obtained in the as-spun state and that as dg is decreased below ∼30 nm the remanence Jr 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 Jr and intrinsic coercivity in the nanocrystalline regime.