HYDROGEN INSERTION IN HARD MAGNETIC MATERIALS BASED ON RARE-EARTH AND IRON: THE EXAMPLE OF THE R2FE17 PHASES
Pages 19-28
Olivier Isnard
Abstract
The insertion of hydrogen atoms within the crystal structure of R2Fe17 type inter-metallic compounds has been found to dramatically influence the physical properties of these phases. The crystal structure of these compounds has been investigated by both X-ray and neutron diffraction. The localisation of the hydrogen within the crystal lattice is described. Two different interstitial sites are filled depending upon the hydrogen content. The maximum hydrogen content that can be inserted reversibly in the R2Fe17 crystal structure is found to be 5 atoms per formula unit for the light rare earth values which decreases down to 3 atoms per formula unit for the heaviest rare-earth elements. In situ neutron diffraction investigation has been undertaken in order to determine the relative stability of the octahedral versus tetrahedral sites. At low temperature the dynamic of the hydrogen atoms has been evidenced by 57Fe Mössbauer spectroscopy and more recently analyzed in the light of neutron quasielastic neutron scattering. The emphasis is given on the description of the hydrogen effect on the magnetic properties of these compounds. The magnetic properties are probed by magnetization measurements up to 24T, Mössbauer spectroscopy, X-ray absorption spectroscopy, and neutron scattering. It will be seen that even if hydrogen is a non magnetic element, it influences almost all the magnetic properties of these compounds : magnetisation, ordering temperature, magnetocrystalline anisotropy, exchange interactions.