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    STUDIA PHYSICA - Issue no. 1 / 2007  

Authors:  E. BRÜCK.
  Abstract:  Magnetic refrigeration, based on the magnetocaloric effect (MCE), has recently received increased attention as an alternative to the well-established compression-evaporation cycle for room-temperature applications. Magnetic materials contain two energy reservoirs; the usual phonon excitations connected to lattice degrees of freedom and magnetic excitations connected to spin degrees of freedom. These two reservoirs are generally well coupled by the spin lattice coupling that ensures loss-free energy transfer within millisecond time scales. An externally applied magnetic field can strongly affect the spin degree of freedom that results in the MCE. In the magnetic refrigeration cycle, depicted in fig. 1, initially randomly oriented magnetic moments are aligned by a magnetic field, resulting in heating of the magnetic material. This heat is removed from the material to the ambient by heat transfer. On removing the field, the magnetic moments randomise, which leads to cooling of the material below ambient temperature. Heat from the system to be cooled can then be extracted using a heat-transfer medium. Depending on the operating temperature, the heat-transfer medium may be water (with antifreeze) or air, and for very low temperatures helium. The cycle described here is very similar to the vapour compression refrigeration cycle: on compression the temperature of a gas increases, in the condenser this heat is expelled to the environment and on expansion the gas cools below ambient temperature and can take up heat from the environment. In contrast to a compression cycle the magnetic refrigeration cycle can be performed quasi static which results in the possibility to operate close to Carnot efficiency.  
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