Rare-earth permanent magnets
How it works
The strength of rare-earth magnets stems from an intermetallic phase with high uniaxial magnetocrystalline anisotropy: the crystal lattice (Nd2Fe14B or SmCo5/Sm2Co17) strongly prefers a single magnetization axis and resists reversal, yielding high coercivity. During manufacture the powder is pressed and aligned in a strong magnetic field so the grains' easy axes are parallel, then either sintered (sintered magnets) or bonded with a binder (bonded magnets), and finally magnetized. In NdFeB, heavy rare-earth additions such as dysprosium and terbium are commonly used to raise coercivity and temperature resistance. SmCo, through its bond with cobalt, retains its magnetic properties at much higher temperatures. Mounted on the rotor of a PMSM/BLDC motor, the magnet provides a constant field that interacts with the stator's rotating field to produce torque.
Problem solved
High-torque-density motors and actuators require a magnetic field with the highest possible energy density in the smallest possible volume and mass. Conventional ferrite and AlNiCo magnets have too low an energy product and coercivity to build a compact, lightweight, high-efficiency drive. Rare-earth magnets solve this by providing the strongest available permanent field.
Components
Tetragonal intermetallic phase of neodymium, iron and boron. The strongest commercial permanent magnet: BHmax up to ~512 kJ/m3 (64 MGOe), saturation magnetization ~1.6 T, remanence ~1.3 T. Lower temperature tolerance than SmCo; usually needs an anti-corrosion coating.
Magnet of samarium and cobalt in the 1:5 (SmCo5) and 2:17 (Sm2Co17) series. BHmax ~112-264 kJ/m3 (14-33 MGOe), Curie temperature 700-800 degrees Celsius, operating temperature 250-550 degrees Celsius. Very high coercivity, excellent corrosion resistance; brittle and more expensive than NdFeB.
Evolution
Karl Strnat (Wright-Patterson Air Force Base) and Alden Ray (University of Dayton) develop the first SmCo5 formulation, launching the rare-earth magnet era.
General Motors (melt-spun, nanocrystalline) and Sumitomo Special Metals (full-density sintered) independently discover Nd2Fe14B, the strongest commercial permanent magnet and cheaper than SmCo.
General Motors founds Magnequench, supplying melt-spun Nd2Fe14B powder to bonded-magnet makers and scaling NdFeB production.
Hyperparameters (configurable axes)
Choice between NdFeB (highest energy density, lower operating temperature) and SmCo (lower BHmax, higher operating temperature and corrosion resistance).
Maximum operating temperature bounded by Curie temperature and coercivity; verified 250-550 degrees Celsius for SmCo, substantially lower for NdFeB and raised by Dy/Tb additions.
Sintered magnets have higher energy density; bonded magnets allow complex shapes and isotropy at the cost of lower BHmax.
Hardware requirements
Rare-earth magnets are a physical construction material for motors and actuators, independent of any compute platform.