Magnets

Magnets are objects that generate magnetic fields which exert forces on certain materials and on other magnets. The magnetic field surrounds a magnet and is strongest near its ends, known as the poles. Permanent magnets retain most or all of their magnetization without an external power source, whereas other materials can become temporary magnets when placed in a magnetic field.

Permanent magnets are made from ferromagnetic materials such as iron, nickel and cobalt, and from alloys such as neodymium-iron-boron (NdFeB), samarium-cobalt (SmCo), ferrites, and aluminum-nickel-cobalt (Alnico). These materials exhibit magnetic ordering at room temperature, but their magnetization can decrease when heated above the Curie temperature or subjected to strong opposing fields, a process called demagnetization. Coercivity measures a material’s resistance to demagnetization.

Electric currents can also produce magnetic fields in electromagnets, where a coil of wire creates a controllable field, often enhanced by a magnetic core. The magnetization of ferromagnetic materials arises from aligned electron spins within microscopic regions called domains; when domains align, they produce a net magnetic moment. Heating near the Curie point or applying reverse fields can disrupt alignment and reduce magnetization.

Applications range from simple fridge magnets and compasses to electric machines such as motors and generators, transformers, and loudspeakers. Magnetic storage relies on magnetized regions in materials such as hard disks. In medicine, magnetic fields are used in MRI. Safety considerations include attraction to ferromagnetic objects and potential risks for people with implanted medical devices.