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The magnetic properties of materials are classified in a number of different ways.
One classification of magnetic materials—into diamagnetic, paramagnetic, and ferromagnetic—is based on how the material reacts to a magnetic field. Diamagnetic materials, when placed in a magnetic field, have a magnetic moment induced in them that opposes the direction of the magnetic field. This property is now understood to be a result of electric currents that are induced in individual atoms and molecules. These currents, according to Ampere's law, produce magnetic moments in opposition to the applied field. Many materials are diamagnetic; the strongest ones are metallic bismuth and organic molecules, such as benzene, that have a cyclic structure, enabling the easy establishment of electric currents.
Paramagnetic behavior results when the applied magnetic field lines up all the existing magnetic moments of the individual atoms or molecules that make up the material. This results in an overall magnetic moment that adds to the magnetic field. Paramagnetic materials usually contain transition metals or rare earth elements that possess unpaired electrons. Paramagnetism in nonmetallic substances is usually characterized by temperature dependence; that is, the size of an induced magnetic moment varies inversely to the temperature. This is a result of the increasing difficulty of ordering the magnetic moments of the individual atoms along the direction of the magnetic field as the temperature is raised.
A ferromagnetic substance is one that, like iron, retains a magnetic moment even when the external magnetic field is reduced to zero. This effect is a result of a strong interaction between the magnetic moments of the individual atoms or electrons in the magnetic substance that causes them to line up parallel to one another. In ordinary circumstances these ferromagnetic materials are divided into regions called domains; in each domain, the atomic moments are aligned parallel to one another. Separate domains have total moments that do not necessarily point in the same direction. Thus, although an ordinary piece of iron might not have an overall magnetic moment, magnetization can be induced in it by placing the iron in a magnetic field, thereby aligning the moments of all the individual domains. The energy expended in reorienting the domains from the magnetized back to the demagnetized state manifests itself in a lag in response, known as hysteresis.
Ferromagnetic materials, when heated, eventually lose their magnetic properties. This loss becomes complete above the Curie temperature, named after the French physicist Pierre Curie, who discovered it in 1895. (The Curie temperature of metallic iron is about 770° C/1300° F.)