Magnetic+Materials

=Magnetic Materials=

There are three main types of magnetic materials: diamagnetic, paramagnetic, and ferromagnetic.

Diamagnetic materials
Diamagnetism occurs when current loops created by the movement of electrons about the nucleus of an atom line up in opposition to an applied magnetic field. All diamagetnic materials have only //paired// electrons. These forces are very weak, millions of times less than those of ferromagnets. When the magnetic field is removed, the magnetic properties of the diamagetic substances are not retained. Although all substances have diamagetic properties, only materials that have //only// these magnetic properties are classified as diamagnetic. Some common substances may not appear magnetic on the surface level but are, in fact, weakly diamagnetic. Examples of these are wood, water, diamonds and most elements in the periodic table. Stronger diamagetic materials are graphite and bismuth. Diamagetism is an induced repulsion and is used in magnetic levitation. In magnetic levitation, objects "float" above a strong magnetic force. Researchers at the University of Nijmegen in the Netherlands demonstrated levitating a small frog in a powerful magnetic field. More information on magnetic levitation can be found in [|Magnet levitation at your fingertips]from the 2 July 1999 issue of Nature.

Paramagnetic materials
Paramagnetic materials are metals with //unpaired// electrons that are realigned when subjected to an external magnetic field. This effect is slight and is extinguished once the external field is removed. Examples of paramagnetic materials are copper, aluminum, and magnesium. In these substances, poles of the molecule align parallel to the applied magnetic field and enforce it. At room temperature these effects are slight because molecular vibrations disrupt the magnetic moments. Liquid oxygen at -183°C, can be pulled by a strong magnet.

When placed in very low temperatures, paramagnetic materials become more magnetic. Like diamagnetic materials, the magnetic force of paramagnets is weak, about 1 million times less than a ferromagnet. Generally speaking, since the attractive force is so small, paramagnetic materials are typically considered nonmagnetic. __Honey bees.__ Researchers at National Tsing Hua University in Taiwan, honeybees have tiny paramagnetic particles in their stomachs. These paramagnets can increase or decrease in size when exposed to external magnetic fields and act as compasses. When the particles change shape so do the cell walls which send signals to the bee's brain. Using the magnetic field created by iron oxides in the Earth, the bees can navigate home. For more information, see [|Magnetic Bees].

Ferromagnetic materials
Ferromagnetic materials are made up of small regions called "magnetic domains." In each of these, the dipoles of the atoms -- 10^12 to 10^15 of them! -- line up in parallel which gives a specific domain a strong magnetic field.



However, the orientation of the domains within the material are random the overall material may not appear to be magnetic. When an external magnetic force or electrical current is applied, the domains align with each other and, unlike diamagnetic and paramagnetic materials, the effect remains when the external force is removed.

Iron is the most common example of ferromagnetic material, although nickel and cobalt are also common.

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