Polarized neutron study of the compounds Y2Fe14B and Nd2Fe14B

Abstract

Outstanding permanent magnet properties were recently observed in a Nd-Fe-B compound which was shown to crystallize in a new phase, R2Fe14B. Polarized neutron measurements are reported on Y2Fe14B and Nd2Fe14B single crystals. The 3d Fe moment on different sites in Y2Fe14B at 4.2 K is closely related to local environment. Its value is maximum for atoms in σ-like layers at the center of an Fe antiprism. On the contrary, it is reduced to 1.95 μB by 4d-3d electron transfer and hybridization for Fe atoms which have the largest coordinance number of Y atoms, 4. The measurements at 250 K reveal a larger thermal decrease of the 3d moment for Fe atoms which exhibit shortest Fe-Fe interatomic distances. This property reveals a reduction of 3d magnetic interactions for short distances as was previously observed in R2Fe17 compounds. In Nd2Fe14B, the low values obtained at 4.2 K for Nd magnetic moments suggest that the magnetic structure, determined by a competition between 3d-4f exchange interactions and crystal-field interactions, is not collinear. Thus only the projection of the Nd moment along the direction of the spontaneous magnetization is measured. Additional contributions to the 3d moments induced by 4f interactions are evidenced, especially for those Fe atoms surrounded by 4 Nd neighboring atoms.