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: Long Phan (ѱܴ ڹа)
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Magnetocaloric effect:Physics, Application, and Material


Magnetocaloric (MC) effect discovered by Warburg in 1881 is the thermal response of a magnetic material when it is subjected to a magnetic-field change. This effect can be quantified as the reversible change in temperature (DTad) of the material when there is a field change (DH) taking place in an adiabatic process or the reversible change of the magnetic entropy (DSm) if DH is brought about in an isothermal process. The relationship between these two parameters is illustrated in the TS schematic diagram, Fig. 1. The MC effect at T0 is generated from an adiabatic temperature change (DTad = T1 - T0) or an isothermal change of magnetic entropy (DSm = S1 - S2). The ordering of magnetic moments increases with increasing DH (i.e., Sm is decreased), leading to a temperature increase. The inverse process causes a temperature decrease [2].
 Based on the MC effect, one can fabricate magnetic refrigerators working at any temperature ranges. For the research purpose in laboratory, one has fabricated magnetic refrigerators operating at ultra-low temperatures below 1 K by using paramagnetic salts. These devices are also used for the liquefaction of nitrogen and helium gases. In recent years, due to the rising concerns about the global warming, environment pollution and energy shortage, one has focussed on designing and developing next-generation refrigerators that can work in the temperature range of 270~320 K. Magnetic materials used for this technology are ferromagnets, namely refrigerants. Comparing with the conventional technology based on gaseous compression/expansion processes, the magnetic-refrigeration technology has a potential energy saving, low-noise, and more reduced environmental impact (due to the use of recyclable solid-state refrigerants) that is being expected to replace the traditional one.
 To realize the above idea, it is necessary to seek for refrigerants showing the giant MC effect at temperatures T = 270~320 K. This can be found around the ferromagnetic-paramagnetic transition temperature of ferromagnets, where magnetic moments vary from the order state to the disorder one. This work will present recent advances in finding giant MC materials for developing next-generation refrigerators. The physical parameters related to the MC effect and application aspect are also analysed and discussed.