The following sections address state-of-the-art magnetic materials and briefly describe research to improve their performance.
DEFINITION OF TERMS
Two parameters determine magnetic performance: saturation magnetization (“magnetization”) and coercive field (“coercivity”). Magnetization is the density of magnetic moments in a ferromagnetic material. A material with higher magnetization can produce larger external magnetic fields than a same-sized material with lower magnetization, and by the same token requires less material to achieve the same magnetic field.
In addition to the level of saturation magnetization, some magnetic materials, called “soft” magnets, require the application of an external magnetic field to align their magnetic moments and others, called “hard” or permanent magnets, produce significant magnetic field without an applied field. The distinguishing characteristic between these classes of materials is their coercivity, the intensity of the magnetic field required to reduce the magnetization to zero.
Hard and soft magnetic materials were refined during the 20th century to provide optimal performance for applications in which magnetization is either very resistant to switching when a magnetic field is applied (i.e., hard or permanent magnets) or easily switched when a magnetic field is applied (i.e., soft magnets) (Gutfleisch et al. 2011). Hard magnetic materials are characterized by large coercivities (more than ~10 kiloamperes per meter, or kA/m) and greater energy storage, making them useful for motor and generator applications. Soft magnetic materials, which have a low value of coercivity (less than ~400 amperes per meter, or A/m), are used in applications that require easy switching, such as induction motors, inverters, and power electronics (Emadi et al. 2008).
Coercivities available today span 8 orders of magnitude between the softest and hardest magnetic materials (Figure 1). Progress in the development of magnetic materials has been accomplished with jumps in performance when new materials are introduced, followed by incremental steps as compositions and processing steps are refined to provide the best microstructures and phase combinations.2
The following sections focus on each class of magnetic material and some of the current technological issues being addressed by researchers.
Rare Earths and Their Challenges
High-performance permanent magnets typically used in EVs and HEVs are made of rare earth elements (largely neodymium [Nd] and dysprosium [Dy]), a
2A phase is a chemically distinct region in a material that possesses uniform physical properties.