The Institute for Solid State Physics, The University of Tokyo
Abstract:
In order to meet the ever-increasing demand for better permanent magnets, to design novel magnetic materials that have both the high saturation magnetization and strong magnetic anisotropy enough to be served as ingredients of permanent magnets is necessary. Fort this, to make clear of the specific role of electron states of each element in permanent magnets is crucial. Most high performance permanent magnets now used are Fe-based rare-earth magnets such as Nd2Fe14B and Sm2Fe17N3. It is known that the high saturation magnetization of those materials originates from Fe and the strong magnetic anisotropy stems from the 4f states of rare-earth elements. For the latter, it is commonly accepted that the 4f states of rare-earth elements are more or less atomic-like, their charge distribution being fixed by the lattice through electrostatic crystal fields. Thus oriented 4f states provide a magnetic anisotropy field through the 4f spin-orbit and 4f-5d intra-atomic exchange couplings. According to this scenario most electronic structure calculations bases on the density functional theory treat 4f states as open-cores, i.e. localized atomic states, despite of their positive orbital energies. Such scenarios, however, sometimes severely contradict the experimental observations. For example, treating 4f states as semi-cores cannot reproduce the experimental cohesive properties of rare-earth metals. Also, unfortunately, treating 4f states as extended, as is naturally expected in the conventional LDA/GGA framework, fails. It is clear that 4f states never participate to the chemical bonding in the same way as s, p or d states do. In order to reveal the role of the 4f states in rare earth magnet compounds, all-electron calculations where the 4f states are treated as “not completely localized” are performed [1]. The results show that the chemical bonding between 4f states and surrounding ligands sometimes plays important roles in determining the magnetic anisotropy. This means the magnetic anisotropy that originates from rare-earth elements can be controlled not only by the crystal structure but also the chemical environment 4f states.
[1] Masako Ogura, Ayaka Mashiyama, and Hisazumi Akai, J. Phys. Soc. Jpn, in press (2015).
[1] Masako Ogura, Ayaka Mashiyama, and Hisazumi Akai, J. Phys. Soc. Jpn, in press (2015).