Magnetic properties and microstructural characterisation of isotropic nanocrystalline Fe-Nd-B based alloys

The effects of a nanocrystalline structure on the magnetic properties of isotropic melt spun Fe-Nd-B alloy ribbon are discussed. Three classes of alloys are considered: a) low Nd (8-10 at.%) which also contain α-Fe; b) near-stoichiometric Nd contents (11-13 at.%) which are single phase Fe₁₄Nd₂B; c) high Nd (16-20 at.%) which also contain a Nd-rich phase. For types a) and b), as the scale of the nanostructure decreases, the remanence J_r is increasingly above J_s/2 while the coercivity H_cj is decreased, with a linear J_r-H_cj relationship. No J_r enhancement occurs for type c) alloys, even for a nanocrystalline structure, but rather a decrease due to volume dilution of the Fe₁₄Nd₂B phase. These effects are discussed in terms of inter-grain exchange coupling and de-coupling and the implications with respect to permanent magnets with improved properties are considered. In particular, it is demonstrated that the benefits to enhanced J_r in terms of improved energy product are limited by corresponding attenuation of the coercivity.