Enhanced magnetocaloric effect by the rare earth polarization due to the exchange with a transition metal. Study of GdCrO4

Abstract

The magnetocaloric effect of a paramagnetic salt has been used long time ago as the physical mechanism for cooling below 1 K. For weak fields or relatively high temperatures the isothermal entropy change ΔST is proportional to the variation of the squared magnetic field. Therefore, a strong preexisting field increases ΔST for a given external field variation ΔB. In magnetic insulators of rare earth R and transition metal M atoms, the R-R exchange interaction is usually much weaker than the R-M exchange and this one is much weaker than the M-M exchange. At intermediate temperatures between the complete ordering of R\'s and M\'s, the R-M exchange acts as an additional constant external field. The effect was observed time ago in antiferromagnetic perovskites. Now, we study the ferromagnetic case of GdCrO4 zircon polymorph, where an exchange field about 7 T adds on to the external field to produce a large magnetocaloric effect |ΔST| > 20 J/kg·K for an increment ΔB from 0 to 9 T, over a wide temperature range 5 K < T < 35 K, reaching a maximum of 29 J/kg·K at 22 K. These values are very useful regarding to applications for liquefying hydrogen or natural gas as they, at least, double the cooling capacity of the materials (GGG, DGAG, ...) used in recent prototypes of magnetic refrigerators.