Large magnetocaloric effect due to spin-glass behavior in the (Mn_1/3R_2/3)₂(Mn_1/3Sb_2/3 )₂O₇ pyrochlore series

The synthesis, structural characterization and magnetic properties of the (Mn1/3R2/3)(2)(Mn1/3Sb2/3)(2)O-7 pyrochlore series are detailed herein. The R3+ (Rare earth) and Sb5+ cations occupy the A and B sites respectively, due to their charge/size ratio. Mn is found to be located in a 1/3 ratio in both positions with selective oxidation state due to size restrictions. Mixed valence in Mn2+/Mn3+ was confirmed from EELS spectroscopy. According to their geometric frustration and R3+-Mn2+/Sb5+-Mn3+ disorder, spin glass behavior is observed from dc and ac magnetic susceptibility data and no long-range magnetic order could be observed from NPD data down to 1.5 K. The magnetic entropy variation calculated from field dependent magnetization curves reveals large magnetocaloric effect (MCE) for these compounds, suggesting its relation to the dynamics of the frozen state. These values reach up to Delta SM similar to 20.7 J/kgK for the R=Gd sample, which is comparable to other materials with large MCE induced by FM order. This renders these materials competitive in magnetic refrigeration and opens the path for new enhanced magnetocaloric materials exploiting this mechanism.