Magnetic hysteresis and coercivity mechanism of a permanent magnet have been long discussed, and their understandings have been significantly deepened during the last decade. Magnetization reversal in a permanent magnet proceeds with thermally activated nucleation and domain wall depinning processes followed by magnetic domain expansions. To treat theoretically the thermally activated magnetization reversal process, atomistic spin model and continuum calculations have been performed. Through these multi-scale calculations, the physical meanings of fluctuation field and activation volume which have been used phenomenologically become very clear. Experimental approaches for both microscopic and macroscopic thermally activated magnetization reversal processes and magnetic domain growths have been also performed. It is expected that these theoretical and experimental multi-scale studies will be integrated to fully understand the magnetic hysteresis and coercivity mechanism with the aid of data science technologies in the future.

We review atomistic spin model studies, a new approach for theoretical investigations, on magnetic properties of permanent magnets. In the atomistic modeling, the microscopic details of magnetic parameters and lattice structures are realistically considered, and the temperature effect, including thermal fluctuation, is properly treated based on statistical physics methods: Monte Caro methods and stochastic Landau-Lifshitz-Gilbert equation methods. We introduce how to treat thermal effects for static and dynamical properties using these methods. Focusing especially on neodymium permanent magnets, we discuss features of magnetization, domain wall, coercivity of a grain, nucleation and pinning fields, and dysprosium substitution effect, which were first elucidated with those methods.

The coercivity of permanent magnets is an important factor for the various applications including the electric vehicle motors. Improving the coercivity is highly demanded, however, it is quite sensitive to the microstructure of the magnet. For understanding the relation between the coercivity and the microstructure, the micromagnetic simulations of the permanent magnets have been studied extensively. Recently, as improving the computer performance and applying numerical techniques, such as the energy minimization, we can treat a multi-grain permanent magnet model in sub-micrometer scale. Hence, the effects of the magnetic grain size, the grain orientation and the magnetic properties of the grain boundary have been analyzed by the micromagnetic simulation. In addition, combining the micromagnetic simulation and the path analysis method make the thermal effect analysis enable. In this paper we summarize the foundation of the micromagnetic simulation for the coercivity analysis and review the studies on the coercivity analyses of the permanent magnet by using micromagnetic simulations.

Surface hardening treatment is used to have strength to mechanical parts, and carburizing and quenching are the most widely used. There are reports on various carburizing efforts to deal with recent environmental issues. The authors have proposed an ultra rapid carburizing above the eutectic temperature, due to realize in-line carburizing. Since this is an unprecedented carburizing treatment method, setting the carburizing conditions that are suitable for efficiency has been the future challenge.

In this paper, we investigated a method for predicting the carbon concentration profile in the steel based on the known carburizing reaction mechanism of ultra rapid carburization. In order to predict the carbon concentration profile in the steel, it was calculated by the finite difference method using the carbon penetration rate F, the use of F = 4.04 × 10^-11e^{(1.20 × 10-2 • T)}, which penetrates from the surface, and the carbon diffusion in the steel based on Fick’s law. In addition, among various carbon diffusion coefficients D_c, the use of D_c(T,C) = 4.53 × 10^-7{1+y_c(1-y_c)}•e^{{-(-2.221•10-4)(17767-y_c•26436)}}, which takes into consideration the dependence of carbon concentration, gave a good agreement with the actual measurement results by EPMA. Furthermore, as a result of investigating efficient carburizing conditions using a prediction method, we could minimize the time required to obtain an effective case depth of 0.8 mm. In addition, the amount of carburizing gas used was also reduced. In other words, it suggests that the accumulation of a huge amount of condition data and the condition setting skills are no longer necessary.

Magnesium chips were coated with silicon powder using a binder, and thixomolding was attempted using the silicon-coated chips as the raw material. The microstructures of the products exhibited that all the added silicon was precipitated as Mg₂Si particles. No voids were observed at the interfaces between the Mg₂Si particles and the matrix. The hardness and Young’s modulus of the products increased with the amount of silicon added. The Young’s modulus followed the rule of mixtures.