"A magnetic refrigerator? I already have many magnets on my fridge!". That is what we - as experts in the field of magnetism - are answered when we tell people the subject of our research. It is true that it is magnetism that allows you to stick nice pictures and letters on the refrigerator door. But now the question is if magnets may have a much more important place in the fridge than just being on the door.
Refrigeration is a technology of very large impact in everyday life and is not just limited to home fridges. Applications includes air-conditioning systems of cars, houses and offices, and the cooling systems of food industry. Just to give one example, the green salad in supermarkets travels at a well defined fresh temperature through a long sequence of passages, in order to arrive fresh at our tables. Then, if we like to buy a salad which is nice looking green and healthy, so that we have nothing to discard, we have to imagine the same thing for refrigeration technology of the future. It should be green - i.e. energy efficient - and nothing should be discarded - i.e. we will have to recycle all its components at the end of its life cycle. This is the reason why scientists are thinking of alternatives to gas compression systems.
Magnetic refrigeration achieved by the cyclic magnetization/demagnetization of a magnetic solid instead of the compression/expansion of a gas of normal refrigerators, is a classic technique employed in laboratories to reach extremely low temperatures. Adiabatic demagnetization owes its development to W. Giauque, who won the Nobel Prize for this in 1949. Replacing the gas with a magnetic material may facilitate recycling because a solid is easy to recycle. However, to become a competitive alternative cooling technology for every-day-life applications, magnetic cooling must prove to be better than gas compression in the room temperature range. And this is a true cutting-edge scientific challenge for physicists and engineers.
At the INRIM Electromagnetism Division we are convinced that the right strategy to win the scientific challenges a ahead of us lies in a combination of deep physical knowledge of the mechanisms governing the behaviour of magnetic materials and excellent measurements. We have been studying magnetic refrigeration for the last few years following two main lines of investigation.
On the one hand, we gained physical insight into coupled magnetic and structural phase transformations. Such coupling is displayed by a few special magnetic alloys and is the basic ingredient for having a large absorption of heat when the magnetic field is removed. The problem is that for these special materials the text-book thermodynamics used by engineers falls short of our needs. It is here that physicists have to take their own chances. In our research work we have extended the out-of-equilibrium models for magnetic hysteresis and we have been able to predict the part of heat reversibly released or absorbed by the magnet and the part of heat which is irreversibly lost. This last quantity is what in physics is termed entropy production: it is a measure of how far the material is from the equal sign appearing in the second principle of thermodynamics. Unfortunately the presence of this entropy production decreases the efficiency of magnetic cooling. Here we have demonstrated that, for the alloys relevant to magnetic refrigeration, like Gd-Si-Ge, entropy production is not too detrimental [1].
On the other hand we have been developing our own measuring systems to characterize magnetic materials from the thermodynamic viewpoint. The entropy is the physical quantity to be measured and the magnetocaloric effect is the name we give to the property a body can have of changing its entropy as a function of the applied magnetic field. Magneto-caloric measurements cannot be simply performed by Differential Scanning Calorimetry (DSC), the routine laboratory method for phase transformations. In fact, the DSC setup cannot be easily placed inside a high magnetic field, which requires special, large devices to be generated.

Therefore, we have designed and tested a magnetic-field-compatible heat flow meter made of thermo-electric active devices. Our active sensor dislikes temperature changes and, as soon as the temperature of the investigated material begins to change, it stops the change by delivering the appropriate quantity of heat. The entropy of the sample as a function of the applied magnetic field is obtained by measuring the heat delivered by the sensor and dividing it by the absolute temperature of the sample. The fact that the sample is kept at the same temperature by the active sensor increases the performance of the instrument [2].




EU has recently decided to fund a scientific research project in the field of magnetic cooling under the Sev-enth Framework Programme. The project is for three years, starting in 2008, and is coordinated by the University of Cambridge. It involves INRIM and six other partners among leading European research centers and universities and private companies. The aim is to discover materials that, with an outstanding figure-of-merit, would promote a relevant development of the technology.

[1] V. Basso, C.P. Sasso, G. Bertotti, M. LoBue, L. Morellon, C. Magen: "Predictions of AMR refrigeration cycles on Gd-Si-Ge alloys", Refrigeration Science and technology proceedings (A. Poredos and A. Sarlah, eds.) IIR/IIF, N.2007-1, pp. 263-270 (2007).
[2] M. Kuepferling, C.P. Sasso, V. Basso, L. Giudici: "An isothermal Peltier cell calorimeter for measuring the magnetocaloric effect", IEEE Trans. Magn. vol. 43, no. 6, pp. 2764-2766 (2007).