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Permanent magnets

The magnetic project described on this homepages has been funded by the Danish Council for Independent Research (DFF) through a Sapere Aude research leader grant.

Permanent magnets are everywhere in our daily life, typically hidden, but they are the heart in electromotors converting electricity to kinetic energy e.g. the motor in electric razor and toothbrush, the pump for the refrigerator compressor.

The magnetic material consists of differently oriented magnetic domains. By applying an external field it is possible to align the magnetic domains. The performance of a permanent magnetic material is given by the hysteresis curve.

The left figure shows the hysteresis curve for different magnets. The blue curve is a soft magnetic material like iron, while the green curve is a highly aligned hard magnetic material. The red curve is the magnetization of a realistic permanent magnetic material. The colored area under the green and the red curve corresponds to the energy product i.e. the energy stored in magnetic field outside the magnet. The right figure shows a powder of very fine maghemite particles aligned by an external magnetic field.

 

For permanent magnets the goal is to have highly aligned magnetic domains with large coercivity and remanence magnetization. Magnetic materials with these properties are often anisotropic, i.e. the magnetization is different along directions.