Large-Scale Micromagnetics Simulation of Magnetization Dynamics in a Permanent Magnet during the Initial Magnetization Process

Large-scale micromagnetics simulations based on the Landau-Lifshitz-Gilbert equation are performed to clarify the initial magnetization process of a Nd2Fe14B hot-deformed permanent magnet. A simulation model of the permanent magnet having 5302 grains is capable of elucidating the effects of networks among the grains. The calculated initial magnetization curve of the permanent magnet reproduces the two-step structure found in experiments. In the demagnetized state obtained from random magnetic configuration, multi- and single-magnetic-domain structures are formed in tabular grains that constitute the hot-deformed permanent magnet. The external magnetic field displaces the domain wall during the first step in the initial magnetization curve, and the multidomain grains disappear after the first step. In the second step, the magnetization reversal of the single domain occurs in grains with magnetization opposite the external field, and the magnetization reversal tends to occur in the grains with the easy axis tilted from the bulk easy magnetization axis. The average magnetic switching field of these single-domain grains is slightly higher than that of the bulk hot-deformed permanent magnet, and the domain-wall displacement across the grain boundary reduces the coercivity by 3.57 kOe in our simulation model. This discrepancy enables us to estimate the effects of networks among the grains through the dipolar interaction.