The search for the most suitable material for the magnetocaloric technology will be most efficient when done in parallel with the study of fundamental physics underlying the phenomenon [1,2], which in part will lead to the better understanding of the phase transition in this group of magnetic systems. The fundamental principle behind the use of the technology is the transfer of entropy between a magnetic system (magnetic entropy) and an environment (thermal entropy). Magnetic field is applied to align magnetic moments in the magnetocaloric materials. In this state, due to the ordering of the magnetic moments, the magnetic entropy is low. The materials are then transferred to the region with no magnetic field, where they absorbed heat from the environment, destroying the magnetic order, increasing the magnetic entropy, and at the same time lowering the thermal entropy. Hence the environment becomes cooler. The physical quantities that are used to indicate the desiring properties for the magnetocaloric materials are magnetic entropy change ΔSm and adiabatic temperature change ΔTad, where both of these quantities must be high. In addition, the magnetocaloric materials must have the Curie temperature (Tc), at which ΔSm attains the maximum value, close to room temperature, and they should require a magnetic field less than 2 tesla, which is the maximum field generated from current permanent magnets, to align the magnetic moments. Currently, only few materials can meet all these requirements, which leads to the delay in the adopting of the magnetocaloric technology. Our goal is to accelerate the field by searching for a more suitable magnetocaloric materials.
In the past, the magnetocaloric technology has been used to cool down a system below 1 K using paramagnetic salts. Later, gadolinium (Gd) was discovered and studied to show high magnetocaloric effect near room temperature (Tc= 294 K) . The first break-through happens in 1997, when giant magnetocaloric effect was discovered in Gd2Si2Ge2 . This discovery renewed the interest in the field and rapidly fueled the development of a prototype of a magnetic refrigeration system. However, due to the high price of gadolinium (~US$ 4,000 per one kilogram), there have been a lot of efforts to replace it with cheaper materials, such as Ni-Mn-Ga alloy , MnAs alloy , Mn-Fe-P-As alloy , LaFe11.57Si1.43 H 1.3 , MnCoGeB0.02 , and manganites (LaMnO3) . All of these materials are rare-earth-free, and hence cheaper, but they cannot compete with the Gd-based materials in terms of efficiency.
One of the most important factors why this project chooses to study a group of materials with transition metals is that they are rare-earth-free. Rare-earths are costly to purify and their mining is not environmental-friendly . Due to its cost, the rare-earth materials, especially gadolinium, are not economically suitable to be used in the magnetic refrigeration technology. Many groups around the world are racing to find the next break-through magnetocaloric materials in order to once again reignite the field after the discovery of Gd2Si2Ge2 in 1997. Our group aims to join this race and hopefully our contribution will advance the field and accelerate the search to meet the environmental challenge of the 21st century.
Principal Investigator: Assistant Professor Dr. Kittiwit Matan 1)