A permanent magnet is one made from a material that stays magnetized. An example is a magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic. These include iron, nickel, cobalt, some rare earth metals and some of their alloys (e.g Alnico), and some naturally occurring minerals such as lodestone. Permanent magnets are made from "hard" ferromagnetic materials which are designed to stay magnetized, while "soft" ferromagnetic materials like soft iron are attracted to a magnet but do not tend to stay magnetized.

An electromagnet is made from a coil of wire which acts as a magnet when an electric current passes through it, but stops being a magnet when the current stops. Often an electromagnet is wrapped around a core of ferromagnetic material like steel, which enhances the magnetic field produced by the coil.

Although ferromagnetic materials are the only ones strongly enough attracted to a magnet to be commonly considered "magnetic", all other substances respond weakly to a magnetic field, by one of several other types of magnetism. Paramagnetic materials, such as aluminum and oxygen are weakly attracted to a magnet. Diamagnetic materials, such as carbon and water, which include all substances not having another type of magnetism, are weakly repelled by a magnet.

The overall strength of a magnet is measured by its magnetic moment, while the local strength of the magnetism in a material is measured by its magnetization.

Background on the physics of magnetism and magnets

Magnetic field

main: Magnetic field The magnetic field (usually denoted B) is called a field because it has a value at every point in space. The magnetic field (at a given point) is specified by two properties: (1) its direction, which is along the orientation of a compass needle; and (2) its magnitude (also called strength), which is proportional to how strongly the compass needle orients along that direction. Direction and magnitude makes B a vector, so B is a vector field. (B can also depend on time.) In SI units the strength of the magnetic field is given in tesla.

Magnetic moment

main: Magnetic moment A magnet's magnetic moment (also called magnetic dipole moment, and usually denoted μ) is a vector that characterizes the magnet's overall magnetic properties. For a bar magnet, the direction of the magnetic moment points from the magnet's south pole to its north pole, and the magnitude relates to how strong and how far apart these poles are. In SI units the magnetic moment is specified in terms of A·m².