Magnetic and magnetocaloric performances of frustrated fcc lanthanide oxides Fiamma Berardi 1 EliseAnne C. Koskelo 2 , Liam A. V. Nagle-Cocco 1 , Camilla Tacconis 1 , Xiaotian Zhang 1 , Cheng Liu 1 , Sian E. Dutton 1 1 Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE 2 Now at Department of Physics, Harvard University, Cambridge, MA 02138, USA It is crucial to explore magnetic refrigeration as an alternative to vapour-compression refrigeration because the latter requires greenhouse gases, ozone depletors and/or gases from a finite source, such as helium. Nowadays, liquid He and 3-He are most commonly used to achieve cryogenic temperatures. Magnetically frustrated compounds are promising candidates for solid-state refrigeration because their magnetocaloric performances are enhanced due to the suppressed ordering temperatures, high magnetic moments, large ground state entropy, and minimal nearest-neighbour ( nn ) spin interactions. Gadolinium oxides with a double perovskite structure in frustrated fcc lattices remain a relatively new field of research and are less investigated than the garnet and pyrochlore lattice analogues. Here we aim to investigate the role of d 0 and d 10 ions on the lanthanide super- exchange by comparing the performances of two families of fcc gadolinium oxides: A 2 GdNbO 6 and A 2 GdSbO 6 ( A = {Ba, Sr, Ca}). Zero field cooled (ZFC) magnetic susceptibility measurements on A 2 GdNbO 6 compounds show that there is no evidence of magnetic ordering down to 1.8 K, indicating minimal short-range correlations. The compounds adhere well to the predicted Curie-Weiss behaviour for free uncoupled spins, with small ferromagnetic (Ba 2 GdNbO 6 ) and antiferromagnetic (Sr 2 GdNbO 6 and Ca 2 GdNbO 6 ) deviations. The experimental effective magnetic moments of all compounds are in good agreement with the theoretical value for Gd 3+ ions. Ca 2 GdNbO 6 is found to be site-disordered, and the tuneability of its disorder is investigated in order to minimise the degree of correlations. The overall results suggest that fcc gadolinium oxides are ideal candidates for the design of next generation magnetocaloric materials. References 1. E. C. Koskelo, C. Liu, P. Mukherjee, N. D. Kelly and S. E. Dutton, Chemistry of Materials , 2022, 34 , 3440-3450. 2. P. Xu, Z. Ma, P. Wang, H. Wang and L. Li, Materials Today Physics ,2021, 20 , 100470.A. H. Olafsdottir and H. U. Sverdrup, Biophys Econ Sust , 2020, 5 , 1-18.
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