Some common misconceptions concerning magnetic refrigerant materials

The relationships between both extensive and intensive properties quantifyi ng the magnetocaloric effect, i.e., between the isothermal entropy change a nd the adiabatic temperature change, respectively, have been analyzed. An e xtensive measure of the magnetocaloric effect alone, without considering an other important and also extensive thermodynamic property, i.e., the heat c apacity, may lead to biased conclusions about the size of the magnetocalori c effect and, consequently, about the applicability of a magnetic material as a magnetic refrigerant. The near room temperature magnetocaloric propert ies of the colossal magnetoresistive manganites [(R1-xMx)MnO3, where R=lant hanide metal and M is alkaline earth metal] and the recently discovered Fe- based intermetallic material (LaFe11.47Co0.23Al1.3) have been reaccessed an d correctly compared with those of the metallic Gd prototype. Our analysis has shown that these 3d materials are inferior to Gd by a factor of 2 or mo re because of the high values of the heat capacity per unit mass. Also a co mparison of the volumetric isothermal entropy change, which is a critical p arameter for the operation of a refrigeration unit, indicates that Gd is su perior to these 3d materials for practical applications. (C) 2001 American Institute of Physics.