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 contains α-Fe; (b) near-stoichiometric Nd contents (11-13 at.%) which are single phase Fe₁₄Nd₂B; (c) high Nd (16-20 at.%) which also contains a Nd-rich phase. For types (a) and (b), as the scale of the nanostructive 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 decoupling, and the implications with respect to permanent magnets with improved properties are considered. It is demonstrated that the benefits of enhanced J_r in terms of improved energy product are limited by corresponding attenuation of the coercivity