aimants

Aimants, or magnets, are materials or objects that produce a magnetic field and can exert force on other magnets or ferromagnetic materials. Permanent magnets retain a fixed magnetization, while temporary magnets or electromagnets generate a magnetic field only when stimulated by an external source, such as an electric current.

Magnetic properties arise from the alignment of atomic moments within ferromagnetic materials. When domains align, the object behaves as a magnet. Common permanent-magnet materials include ferrites (ceramic magnets), alnico, neodymium-iron-boron (NdFeB), and samarium-cobalt (SmCo). NdFeB magnets have very high strength but are more sensitive to heat and corrosion; SmCo offers good temperature stability and resistance to corrosion; ferrites are inexpensive and corrosion resistant but weaker; alnico provides stable magnetism over a wide temperature range but lower overall strength.

Electromagnets consist of coils of wire around a magnetic core; the magnetic field strength depends on the current and the number of turns. Electromagnets allow controllable magnetism and are essential in devices such as relays, cranes for lifting heavy objects, and many electrical machines. Magnetic fields are described by principles of electromagnetism; for permanent magnets, material properties such as residual flux density and coercivity are important indicators of performance.

Temperature and external fields influence magnet performance. Most magnets lose some magnetization above certain temperatures (their Curie or working temperatures) and can be demagnetized by strong opposing fields, shocks, or mechanical stress.

Historically, lodestones were among the first magnets used, with later theoretical advancements by scientists such as William Gilbert and explorations of electromagnetism by Ørsted, Faraday, and others. Today aimants are used in motors, generators, transformers, speakers, data storage, medical imaging, and a wide range of consumer and industrial applications.