Magnetic Materials30 Dec 2011
A material is ferromagnetic if its atoms or molecules are magnets and they tend to group into domains. When a ferromagnetic material is placed in a magnetic ?eld, the lines of the ?eld ?nd it easier to pass through the material than the space
around the material. Ferromagnetic materials have a large magnetic permeability. When the magnetic lines of force pass through the material, they act on the domains within the material and cause them to become aligned. At this point, the ferromagnetic material is a magnet in its own right and has its own magnetic ?eld that is aligned with and enhances the original magnetic ?eld. Ferromagnetic materials are so permeable to magnetic lines of force that they are able to react well with even very weak external magnetic ?elds. Materials such as iron, nickel, cobalt and steel are ferromagnetic.
The atoms or molecules of paramagnetic materials are magnetic but the material is not very permeable to magnetic ?elds. Thus, if the external magnetic ?eld is weak, the material shows very little or no magnetism. The magnetic effects become noticeable at room temperature, and only if the external ?eld is very strong. With a weak ?eld, the permeability is so small that it cannot overcome the thermal activity of the atoms in order to align the domains. As soon as the external ?eld is gone, the small amount of magnetism that was induced in the material disappears. However, if the temperature is very low, the internal thermal activity of the material is reduced. Any external ?eld can align with the domains more easily and the material exhibits magnetic behaviour similar to a ferromagnetic material. Aluminum is a paramagnetic material as are many gases.
A third type of reaction of a magnetic ?eld is called diamagnetism. Diamagnetic materials are less permeable than air. They never show ferromagnetic properties but become very weakly magnetized in the opposite direction to the applied ?eld. Like paramagnetic materials, they need very strong magnetic ?elds or very cold temperatures before they exhibit any of their characteristic behaviour. The atoms of diamagnetic materials are not magnetic and thus do not form domains. The diamagnetism results from the motion of the electrons within the atoms of the material. Materials such as copper, rubber and glass are diamagnetic.