Anisotropy of exchange stiffness based on atomic-scale magnetic properties in the rare-earth permanent magnet Nd2Fe14B

We examine the anisotropic properties of the exchange stiffness constant A for a rare-earthpermanent magnet, Nd2Fe14B, by connecting analyses with two different scales of length, i.e.,Monte Carlo (MC) method with an atomistic spin model and Landau-Lifshitz-Gilbert (LLG)equation with a continuous magnetic model. The atomistic MC simulations are performed onthe spin model of Nd2Fe14B constructed from ab initio calculations, and the LLGmicromagnetics simulations are performed with the parameters obtained by the MCsimulations. We clarify that the amplitude and the thermal property of A depend on theorientation in the crystal, which are attributed to the layered structure of Nd atoms and weakexchange couplings between Nd and Fe atoms.We also confirm that the anisotropy ofAsignificantly affects the threshold field for themagnetization reversal (coercivity) given by thedepinning process.

 We acknowledge collaboration and fruitful discussions with Taro Fukazawa, Taichi Hinokihara, Shotaro Doi, Munehisa Matsumoto, Hisazumi Akai, and Satoshi Hirosawa. This work was partly supported by Elements Strategy Initiative Center for Magnetic Materials (ESICMM) under the auspices of MEXT; by MEXT as a social and scientific priority issue (Creation of New Functional Devices and High-Performance Materials to Support Next-Generation Industries; CDMSI) to be tackled by using a post-K computer. The computation was performed on Numerical Materials Simulator at NIMS; the facilities of the Supercomputer Center, the Institute for Solid State Physics, the University of Tokyo; the supercomputer of ACCMS, Kyoto University.