Magnetic refrigerator–heat pump

For almost half a century low‐temperature physicists have been exploiting the adiabatic demagnetization of paramagnetic materials to achieve temperatures near absolute zero. Until recently the potential utility of the magnetic cycle for refrigeration and heat pumping near room temperature has not been taken seriously for two reasons: First of all, the degree of magnetic order achievable in paramagnetic materials at practical field strength becomes very small at room temperature. Thus the entropy change that pumps heat when magnetic fields are applied and withdrawn becomes too small to be useful. Furthermore, by 20 K the lattice entropy changes of typical paramagnetic salts and the magnetic entropy changes from which they must be subtracted in a Carnot refrigeration cycle are comparable.