Free-spin dominated magnetocaloric effect in dense Gd${}^{3+}$ double perovskites

Abstract

Frustrated lanthanide oxides with dense magnetic lattices are of fundamental interest for their potential in cryogenic refrigeration due to a large ground state entropy and suppressed ordering temperatures, but can often be limited by short-range correlations. Here, we present examples of frustrated fcc oxides, Ba${}_2$GdSbO${}_6$ and Sr${}_2$GdSbO${}_6$ and the new site-disordered analog Ca${}_2$GdSbO${}_6$ ([CaGd]${}_A$[CaSb]${}_B$O${}_6$), in which the magnetocaloric effect is influenced by minimal superexchange ($J_1\sim 10$ mK). We report on the crystal structures using powder x-ray diffraction and the bulk magnetic properties through low-field susceptibility and isothermal magnetization measurements. The Gd compounds exhibit a magnetic entropy change of up to -15.8 J/K/mol${}_{\text{Gd}}$ in a field of 7 T at 2 K, a 20 % excess compared to the value of -13.0 J/K/mol${}_{\text{Gd}}$ for a standard in magnetic refrigeration, Gd${}_3$Ga${}_5$O${}_{12}$. Heat capacity measurements indicate a lack of magnetic ordering down to 0.4 K for Ba${}_2$GdSbO${}_6$ and Sr${}_2$GdSbO${}_6$, suggesting cooling down through the liquid 4-He regime. A mean-field model is used to elucidate the role of primarily free spin behavior in the magnetocaloric performance of these compounds in comparison to other top-performing Gd-based oxides. The chemical flexibility of the double perovskites raises the possibility of further enhancement of the magnetocaloric effect in the Gd${}^{3+}$ fcc lattices.

Accepted in Chemistry of Materials