Some common misconceptions concerning magnetic refrigerant materials

Abstract

The relationships between both extensive and intensive properties quantifying the magnetocaloric effect, i.e., between the isothermal entropy change and the adiabatic temperature change, respectively, have been analyzed. An extensive measure of the magnetocaloric effect alone, without considering another important and also extensive thermodynamic property, i.e., the heat capacity, may lead to biased conclusions about the size of the magnetocaloric effect and, consequently, about the applicability of a magnetic material as a magnetic refrigerant. The near room temperature magnetocaloric properties of the colossal magnetoresistive manganites $\mathrm{[(}{\mathrm{R}}_{\text{1−x}}{\mathrm{M}}_x{\mathrm{)MnO}}_{\mathrm{3}},$ where $\mathrm{R}=\mathrm{lanthanide}$ metal and M is alkaline earth metal] and the recently discovered Fe-based intermetallic material $({\mathrm{LaFe}}_{\mathrm{11.47}}{\mathrm{Co}}_{\mathrm{0.23}}{\mathrm{Al}}_{\mathrm{1.3}})$ have been reaccessed and correctly compared with those of the metallic Gd prototype. Our analysis has shown that these $\text{3d}$ materials are inferior to Gd by a factor of 2 or more because of the high values of the heat capacity per unit mass. Also a comparison of the volumetric isothermal entropy change, which is a critical parameter for the operation of a refrigeration unit, indicates that Gd is superior to these $\text{3d}$ materials for practical applications.