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Why are neodymium-iron-boron magnets used in electric motors and speakers?

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A line graph showing electron probability distributions for gadolinium, with separate curves for 4f, 5s, 5p, 5d, and 6s orbitals. The x-axis shows distance from the nucleus in atomic units and the y-axis shows probability P2(r). — rare-earth element Electron probabilities, P²*(r)*, for the 4f, 5s, 5p, 5d, and 6s electrons of gadolinium.

Neodymium-iron-boron (Nd $_{2}$ Fe $_{14}$ B) magnets are widely used in electric motors and speakers because they are the hardest and strongest permanent magnetic materials known.1

Their effectiveness in these applications is due to specific magnetic properties:

High Remanence ( $B_{r}$ ): These magnets possess a large spontaneous magnetization in zero field, meaning they retain a strong magnetic flux density after the magnetizing field is removed.23
High Coercive Force ( $H_{c}$ ): They exhibit a high resistance to demagnetization, allowing them to maintain their magnetic strength even when exposed to external fields.23
Energy Product ( $(B H)_{m a x}$ ): They are characterized by a high maximum energy product, which is a measure of the material's quality. This allows for the creation of high-performance, compact components, such as the small, powerful motors found in modern automobiles and the speakers used in portable media players.13

By combining these properties, Nd $_{2}$ Fe $_{14}$ B magnets provide the necessary magnetic intensity in a minimal volume, making them ideal for high-performance electric motors, generators, and various electronics applications.14