What is the magnetocaloric effect and what materials exhibit this effect the most?
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
Scott Wilber, President, ComScire - Quantum World Corporation
There is a small mistake in the answer above.
The magnetocaloric effect was not discovered by Emil Warburg in 1881. This statement has been made by many authors and is the result of a mis-citation in 1999 that has since percolated through the literature.
The observation of the magnetocaloric effect can instead be traced to Weiss and Piccard in 1917. This matter was resolved recently in a very nice article by Anders Smith, called "Who discovered the magnetocaloric effect".
CUNY-Brooklyn College and Imperial College London