Ahas just gained international attention through the journal Nature Materials, from the Nature Publishing Group, a discovery that contributes to enabling the replacement of gases used in domestic and industrial refrigeration by active magnetic regenerators (solid materials), which is particularly important in the case of refrigerators that need to absorb large heat capacity. The article published in October reports on the colossal magneto-caloric effect at ambient pressure (Ambient pressure colossal magnetocaloric effect tuned by composition in Mn1-xFex As), which makes it possible to increase the efficiency of magnetic refrigerators. A group of researchers coordinated by professors Sérgio Gama, Flávio Gandra and Lisandro Cardoso, from the Gleb Wataghin Physics Institute (IFGW) at Unicamp, signed the article. The work included theoretical input from another group coordinated by professors Pedro J Von Ranke and Nilson A. de Oliveira, from the Physics Institute at UERJ. Doctoral and post-doctoral students participate in research.

The objective is to replace gases with solid materials

About two years ago, the same group from Unicamp had already discovered the colossal magneto-caloric effect, which manifests itself when the material is subjected to high pressures in an appropriate magnetic field. The study that merited an article in Physical Review Letters of December 2004, involved the intermetallic compound manganese-arsenic, whose capacity to remove heat from the environment is twenty times greater than gadolinium, a substance commonly used in prototypes of magnetic refrigerators.

The effect now reported in Nature Materials, observed at ambient pressure, has theoretical and practical importance. From a theoretical point of view, the colossal effect at ambient pressure allows us to question the origin of the magneto-caloric effect as resulting from a purely magnetic phenomenon. This is because it shows that the material's crystalline network also contributes to the amount of energy (entropy) removed from it by the action of the magnetic field. Regarding the application, as suggested at the beginning of the text, the discovery makes it possible to increase the efficiency of magnetic refrigerators.

Professor Sérgio Gama sees multiple advantages in replacing gases with solid materials (most likely metallic alloys) in conventional refrigeration. “The current process requires large volumes of gases to promote large refrigeration loads, with performance being significantly reduced by friction in the pistons. Furthermore, gases harm the environment because they destroy the ozone layer, worsening the greenhouse effect. In magnetic refrigeration, the volumes occupied by solid materials are relatively small and the friction losses are significantly smaller, which results in much greater efficiency. They also do not cause environmental problems. There are many benefits, which justify the commitment to making this technology viable”, states the scientific coordinator of the project.

The effect - Regarding the magneto-caloric effect, Sérgio Gama explains that a metallic alloy, for example, when subjected to a magnetic field without exchanging heat with the environment (in an adiabatic manner), suffers an increase in temperature, which returns to its original value when the magnetic field is removed. “This reversible effect is applicable in what is called magnetic refrigeration.”

According to professor Flavio Gandra, another coordinator of the IFGW group, to explain the energetic variations (in entropy) that occur within the magnetic material, two distinct parts must be considered: that relating to electrons (which is affecting the electronic distribution) and another relating to the crystalline network. Gandra clarifies that the magnetic field acts on these electrons – which constitute the magnetic part – and can guide them to a decrease in entropy. The entropy of the system, however, needs to be constant. Thus, once the entropy of the magnetic part is lowered, that of the crystal lattice increases, in order to keep the total entropy constant.

This variation, according to Flavio Gandra, manifests itself in the form of heat and the system heats up. This happens while the solid material is subjected to the magnetic field. Once the action of the magnetic field ceases, the entropy of the electronic part increases again and, consequently, the entropy of the crystal lattice decreases – that is, the material cools. Thus, it can be used to remove heat from an environment, such as the inside of a refrigerator. As the cycle repeats itself, the material serves to replace the gases used in the conventional refrigeration system.

Simplifying the language, one can understand the change in entropy as a variation in the amount of heat in the system, either by its removal or by its addition. Or, even, that the increase in temperature means that the magnetic field adds heat, which is removed when this action stops. “The novelty demonstrated with the discovery of the colossal effect is that heat (entropy) is extracted not only from the magnetic part, but also from the crystalline lattice. This fact made an important contribution to the understanding of the effect. From a practical point of view, the greater the amount of heat extracted, the greater and more efficient the application; If there is the same temperature variation, the greater the amount of heat that can be extracted”, adds Sérgio Gama.

Discoveries – The magneto-caloric effect was discovered in 1881. In 1926, it was suggested the use of magnetization of paramagnetic material to obtain very low temperatures. In 1932, the Nobel Prize in Physics was awarded for work that led to the construction of a magnetic refrigerator, which allowed temperatures close to zero Kelvin to be reached, unfeasible to obtain with gas refrigerators.

Until 1997, says professor Sérgio Gama, the magneto-caloric effect was determined only in materials that presented the so-called second-order magnetic transition, such as elements such as gadolinium, iron and nickel, in which the effect is relatively small, although there was already attempts to use them in refrigerators. It was in the United States, in 1997, that they discovered materials that registered first-order transitions, such as the intermetallic compound Gd5Ge2Si2. The associated, much larger effect was then called giant magneto-caloric. The discovery triggered a great movement of research in the area, around the world, as it became clear that these new materials could compete, in terms of energy and price, with conventional gas refrigeration.

“That was when we discovered this effect in the compound MnAs (manganese arsenide) and its derivatives. We call the effect colossal because it far exceeds the giant effect and also the magnetic limit expected for these materials. Until then, in none of the materials studied, the effect had exceeded the magnetic limit. Therefore, theoretical modeling was limited only to the limits of the magnetic contribution”, recalls Sérgio Gama. However, the colossal effect, by exceeding these limits, introduced a new theoretical challenge. “The challenge is to correctly model the experimental result and, which will certainly happen, contribute to obtaining an optimized effect, opening up possibilities for the discovery of other materials presenting the colossal effect.”

The professor seeks to show the variation in entropy differences observed in the materials used over time, since the discovery of the effect. He remembers that in the conventional magneto-caloric effect this variation was 10 joule/kg.K; in the giant magnetic-caloric effect it reached 40/50 joule/kg.K at ambient pressure; and in the colossal the order is 300 joule/kg.K at a pressure of 2300 atm, an order of magnitude that is also observed in the recent discovery in which doping of manganese arsenide with iron and ambient pressure was used. In this case, the iron atoms caused a decrease in the volume of the crystalline lattice, as if it were subjected to external pressure.