Diamagnetism and Paramagnetism
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Only a few materials found in nature – iron, nickel, cobalt and gadolinium* – are ferromagnetic, meaning they exhibit a strong response to a magnetic field by aligning with it. All materials, however, experience some sort of reaction to magnetic fields, though the reaction may be so slight that it is imperceptible to the human eye. These reactions are described in the text and interactive tutorial below.
These forces can be hard to understand because they occur at the atomic level, due to the presence of electrons – negatively charged particles – in atoms. These electrons react to the external magnetic field, orienting toward one direction or the opposite direction. This orientation is known as magnetic moment.
In ferromagnetic substances, electrons align readily (in regions called magnetic domains) with outside magnetic fields, such as the Earth’s magnetic field, and stay that way. Paramagnetic substances also align with outside magnetic fields, but the effect is both weaker and more fleeting. Unlike ferromagnetic materials, which retain their alignment even after they leave the external magnetic field, in paramagnetic materials electrons return to their original orientations, some pointing one way, some the other. (Ferromagnetic materials stay magnetized because of the unique arrangement of their electrons).
In diamagnetism, another magnetic phenomenon, electrons within a substance respond to the outside magnetic field by, essentially, spinning faster. All this spinning generates magnetic forces that resist the outside field. Because all atoms possess electrons, all materials are diamagnetic to some degree. But if present, the stronger forces of paramagnetism or ferromagnetism will easily overshadow the diamagnetism.
Observe the responses exhibited by a variety of common paramagnetic and diamagnetic materials exposed to a strong magnet with this interactive tutorial. Make a selection from the Choose a Material pull-down menu, and a disk of that material will appear between the poles of a strong variable gap electromagnet. Then click on and drag the disk to change its position; when released it will naturally realign itself either with the magnetic field of the magnet (paramagnetic), or against it (diamagnetic). Paramagnetic materials characteristically align with and strengthen an external magnetic field, while diamagnetic substances partially expel an applied field and always align themselves so that they are perpendicular to its lines of magnetic force. (The well-known Meissner Effect is a special example of diamagnetism involving superconductivity).
As you play with these different elements, you’ll notice that, when displaced, they return to their paramagnetic or diamagnetic alignments at varying speeds. Though not a literal representation, this illustrates the varying degrees of magnetic susceptibility that these materials possess.
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*A fifth element, dysprosium, also becomes ferromagnetic, but only at low temperatures.