ScienceWeek

4. PARAMAGNETISM

ON MAGNETIC REFRIGERANTS

In general, the term "magnetocaloric effect" (thermomagnetic effect) refers to a reversible change of temperature resulting from a change in the magnetization of a ferromagnetic or paramagnetic substance. The effect increases as the initial temperature of the substance is lowered, so that the effect has been used for the production of temperatures approaching absolute zero. At very low temperatures, paramagnetic substances become antiferromagnetic, restricting further cooling. (A "ferromagnetic substance" is a material (e.g., iron) in which there may be a permanent magnetic moment, and in which the spins of the atoms are aligned parallel to each other. In general, "paramagnetic substances" and paramagnetic chemical groups have a capability to be magnetized which is slightly greater than that of a vacuum and much less than that of iron. The paramagnetism is due to the presence of permanent magnetic dipoles caused by unpaired electron spins.)

The following points are made by O. Tegus et al (Nature 2002 425:150):

1) Magnetic refrigeration techniques based on the magnetocaloric effect have recently been demonstrated as a promising alternative to conventional vapor-cycle refrigeration (1). In a material displaying the magnetocaloric effect, the alignment of randomly oriented magnetic moments by an external magnetic field results in heating. This heat can then be removed from the magnetocaloric effect material to the ambient atmosphere by heat transfer. If the magnetic field is subsequently turned off, the magnetic moments randomize again, which leads to cooling of the material below the ambient temperature.

2) Magnetic refrigeration is an environmentally friendly cooling technology. It does not use ozone-depleting chemicals (such as chlorofluorocarbons), hazardous chemicals (such as ammonia), or greenhouse gases (hydrochlorofluorocarbons and hydrofluorocarbons). Another important difference between vapor-cycle refrigerators and magnetic refrigerators is the amount of energy loss incurred during the refrigeration cycle. The cooling efficiency of magnetic refrigerators working with gadolinium (Gd) has been shown4 to reach 60% of the theoretical limit, compared to only about 40% in the best gas-compression refrigerators. The use of magnetic refrigerators with such high energy efficiency will result in a reduced consumption of fossil fuels, in this way contributing to a reduced carbon dioxide release. However, with the currently available magnetic materials, this high efficiency is only realized in high magnetic fields of 5 T.

3) The heating and cooling that occurs in the magnetic refrigeration technique is proportional to the size of the magnetic moments and to the applied magnetic field. This is why research in magnetic refrigeration is at present almost exclusively conducted on super-paramagnetic materials and on rare-earth compounds (5). For room-temperature applications like refrigerators and air-conditioners, compounds containing manganese should be a good alternative. Manganese is a transition metal of high abundance. Also, there exist (in contrast to rare-earth compounds) an almost unlimited number of manganese compounds with magnetic ordering temperatures near room temperature. However, the magnetic moment of manganese is generally only about half that of heavy rare-earth elements. Enhancement of the caloric effects associated with magnetic moment alignment may be achieved through the induction of a first-order phase transition, which will result in much higher efficiency of the magnetic refrigerator. In combination with currently available permanent magnets, this should open the way to the development of small-scale magnetic refrigerators that no longer rely on rather costly and service-intensive superconducting magnets. Another advantage of magnetocaloric refrigerators is that the cooling power can be varied by scaling from milliwatts to a few hundred watts.

References (abridged):

1. Glanz, J. Making a bigger chill with magnets. Science 279, 2045 (1998).

2. Pecharsky, V. K. & Gschneidner, K. A.Jr Giant magnetocaloric effect in Gd5(Si2Ge2). Phys. Rev. Lett. 78, 4494-4497 (1997).

3. Gschneidner, K. A.Jr et al. Recent developments in magnetic refrigeration. Mater. Sci. Forum 315-317, 69-76 (1999).

4. Zimm, C. et al. Description and performance of a near-room temperature magnetic refrigerator. Adv. Cryogen. Eng. 43, 1759-1766 (1998).

5. Tishin, A. M. in Handbook of Magnetic Materials Vol. 12 (ed. Buschow, K. H. J.) 395-524 (North Holland, Amsterdam, 1999).

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