Coercivity and its thermal stability of Nd–Fe–B hot-deformed magnets enhanced by the eutectic grain boundary diffusion process

In this chapter, we review recent advances in Nd-Fe-B based permanent magnets that have been made in the last 10 years with a historical review of the early developments of Nd-Fe-B magnets in the 1980s. The basic technology in the commercial production of Nd-Fe-B permanent magnets was established within a decade after its invention in 1983. The overwhelmingly high energy density of Nd-Fe-B magnets at that time led to major applications in small motors and actuators in many electric devices such as hard disk drives and cell phones, where the miniaturization of motors and actuators is of priority over their relatively high cost. However, the current major applications have shifted to the traction mortars of hybrid and pure electric vehicles, for which higher coercivity is required against thermal demagnetization. The merit of permanent magnet generators for large scale wind turbines has also been established, and these new applications will boost the usage of Nd-Fe-B PM in the near future. Recent government policies in Europe, India and China have forced major automotive manufactures to accelerate the development of electric vehicles and the emerging area such as robotic, drones and future electric planes will further boost the demand for high performance permanent magnets. To achieve the required coercivity value for these applications, the partial substitution of Dy for Nd has been employed for the past three decades. However, recently emerging issue on the limited natural resources of heavy rare earth elements created a new technological challenge to develop high coercivity Nd-Fe-B magnets without using heavy rare earth elements for future mass supply of Nd-Fe-B permanent magnets. With this background, the researches on the structure-coercivity relationships of Nd-Fe-B permanent magnet have been revived recently mainly in Japan, Europe and China. With new analytical tools employed in recent studies, many new findings on the microstructural features of Nd-Fe-B permanent magnets have been made in the last five years and the ways to increase the coercivity substantially higher than the conventional values in commercial magnets have been proposed. This chapter will update these recent progresses made in Nd-Fe-B based permanent magnets.

Advances in Nd-Fe-B Based Permanent Magnets

Microstructure of a Dy-free Nd-Fe-B sintered magnet with 2 T coercivity

The microscopic mechanism of coercivity at finite temperature is a crucial issue for permanent magnets. Here we present the temperature dependence of the coercivity of an atomistic spin model for the highest-performance magnet Nd2Fe14B. For quantitative analysis of the magnetization reversal with thermal fluctuations, we focus on the free energy landscape as a function of the magnetization. The free energy is calculated by the replica-exchange Wang–Landau method. This approach allows us to address a slow nucleation problem, i.e., thermal activation effects, in the magnetization reversal. We concretely observed that the thermal fluctuations lead to a downward convexity in the coercivity concerning the temperature. Additionally, through analyzing the microscopic process of the thermal activation (nucleation), we discover the activation volume is insensitive to a magnetic field around the coercivity. The insensitivity explains the linear reduction of the free energy barrier by the magnetic field in the nucleation process.

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E. Mehofer

Papers

Searching the weakest link: Demagnetizing fields and magnetization reversal in permanent magnets