Text: Upon the removal of a magnetic field from a material, the resulting reduction in magnetic spin alignment represents an increase in the material's spin entropy (delta S). If the field reduction is performed adiabatically so that the total entropy change is zero, then the increased spin entropy is offset by an equal decrease in lattice entropy, as reflected by a decrease in the temperature of the material. This delta T is called the magnetocaloric effect, and it is a property of the material and its magnetic state. http://www.aps.org/BAPSMAR98/abs/S3220.html Discovered in 1881 by Warburg. What is the magnetocaloric effect and what materials exhibit this effect the most? Asked by: Tim Michnick Answer Some magnetic materials heat up when they are placed in a magnetic field and cool down when they are removed from a magnetic field. This is known as the magnetocaloric effect. This effect was discovered by E. Warburg in 1881 in pure iron. The size of the effect has been around .5 to 2°C per Tesla change in magnetic field. One Tesla is about 20,000 times the earth's magnetic field. Recently, alloys of gadolinium, germanium and silicon have produces a much larger effect size of 3 to 4°C per Tesla change. The general equation for this material is; Gd_5(Si_xGe_1-x)_4, where x=0.5. Experimental refrigerators based on the magnetocaloric effect have been tested in laboratories using magnetic fields of around 5T produced by superconducting magnets. http://www.cfs.me.uvic.ca/PAGES/amr.html

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