HYDROGENATED HARD MAGNETIC-ALLOYS FROM FUNDAMENTAL TO APPLICATIONS

The powerful magnet materials based on rare earth (R) and magnetic 3d metals (M = Fe,Co) exhibit a large affinity to absorb hydrogen. Hydrides of the anisotropic materials R2M17 and R2M14X with X = B, C, and RM12 with M = Fe(1-x)M(x)', have been systematically investigated. First, the impact on the fundamental properties (structure, magnetization, anisotropy, easy axis, etc.) have been analyzed by several techniques (x-ray and neutron scattering, magnetization, Curie temperature, etc.), and in many cases the hard magnetic characteristics are shown to be dramatically reduced. These effects are discussed in terms of M-M and M-R exchange interactions, d-f overlaps, and 3d and 4f (CEF) anisotropies. Since magnetoelastic effects upon hydrogenation are large, isotropic and anisotropic cell expansion terms are analyzed. Second, the role of microstructure is of great importance, since the coercivity is very large. Hydrogenation effects at a macroscopic scale have also been investigated. Further restoring heat treatments are needed in order to improve the hydrogen decrepitation route used for magnet processing.