Nanocrystalline hard magnets

in daily-life applications and are key components of numerous electronic, data and medical devices as well as of many applications in the automotive sector. The combination of properties of the R sublattice (4f) and the T sublattice (3d) led to the dramatic improvement in the energy density (BH)max of hard magnetic materials during the last two decades. In the case of nanocrystalline R-T compounds, it is the grain size and the presence or absence of intergranular phases which give rise to unusual magnetic properties because of surface/interface effects different from those of bulk or microcrystalline materials. Large coercivities can be obtained once the grain size is below a certain threshold where the crystallites become single domain, here usually a fraction of a micron. Assuming idealized microstructures, three prototypes of R-T magnets can be distinguished on the basis of the phase diagram and the variation of the R content: Type (I) is R rich and the individual crystallites are separated by a thin paramagnetic layer, the R-rich intergranular phase. This structure leads to a decoupling of the hard magnetic grains resulting in high coercivities. Type (II) is obtained using a stoichiometric composition and the hard magnetic grains are in direct contact with each other ( ́single-phase exchange coupled magnets ́). Type (III) nanocomposite magnets [1] are R deficient and the coupling occurs between the hard magnetic grains (to provide high coercivity) and soft magnetic T rich grains (to provide high polarisation; e.g. Js(a-Fe)=2.16T). The randomly oriented nanograin structure results in magnetically isotropic magnets, with the remanent polarisation, Jr, and (BH)max limited to 0.5 and 0.25, respectively, of the values obtainable for ideal microstructures consisting of single domain grains and with full crystallographic alignment ((BH)max = Jr / 4μ0, Jr is half of the saturation polarisation Js for an assembly of Highlights 2000