Neodymium magnet

A neodymium magnet is the strongest kind of permanent magnet you can buy. A piece the size of a coin can hold far more than its own weight, which lets speakers, motors and headphones be small and light. Its main crystal is Nd₂Fe₁₄B: two atoms of neodymium for every fourteen of iron and one of boron.

Iron makes up about two thirds of its weight, neodymium most of the rest, with about 1 percent boron. Some neodymium is swapped for dysprosium in magnets that must stay strong when hot. The metals are melted together and cast into thin flakes, which are broken up with hydrogen and milled into a powder a few micrometers fine. The powder is pressed inside a strong magnetic field, which lines up every grain, then sintered (baked until the grains fuse) and heat treated. The blocks are cut to shape, coated with nickel because the metal rusts very easily, and finally magnetized with a powerful pulse.

In a speaker, it is the fixed magnet the voice coil pushes against. These magnets were invented in the early 1980s, independently by General Motors and Sumitomo Special Metals. They are brittle and chip if they snap together.