what is magnetic refrigeration?
Magnetic refrigeration involves a substance that can be magnetized in a magnetic field (not a permanent magnet). The substance is placed in a magnetic field large enough to align the magnetic moments. This causes heating. The material is cooled back to its starting temperature by removing the heat of magnetization. Then the magnetic field is reduced, causing the magnetic moments that were aligned to become random again. This causes the substance to cool dramatically and to absorb heat from any sample connected to it. The substance used can show magnetic behavior in its electrons, in which case temperatures can be decreased by factors of 5 - 20. Depending on the particular material, this cooling can be in the range of 10s of degees kelvin or as low as hundredths of a degree kelvin. Other materials, such as copper, show magnetic behavior in their nuclei. In this case the magnetic fields must be MUCH larger to align the nuclear moments, but the temperatures that can be achieved are in the thousandths of a degree kelvin. After the sample is cold, inevitable heat leaks warm the sample until the cooling must be repeated. It is possible with multiple stages of cooling to provide a nearly constant cold temperature to samples for extended periods of time.
