Dependence of the magnetocaloric effect on the A-site ionic radius in isoelectronic manganites

In this work, we explore the magnetocaloric and magnetic properties of isoelectronic manganites R0.6Sr0.4MnO3 (R = La, Pr, Nd, and Sm). Upon substitution of La3+ by smaller rare-earth ions, the average ionic radius 〈rA〉 of the A-site (A = (R, Sr)) elements systematically decreases. It is found that, with decreasing 〈rA〉, the magnetic-ordering temperature decreases from 341 K for La0.6Sr0.4MnO3 to 126 K for Sm0.6Sr0.4MnO3. Interestingly, the magnetic-entropy change increases with decreasing 〈rA〉, reaching ΔSM=−8.4 J/kg K for ΔH=0−20 kOe for Sm0.6Sr0.4MnO3. For manganites, this is a high value of ΔSM, and it is related to the fact that the compound exhibits first-order magnetic transition. In contrast, the three other compounds exhibit a second order transition. The results indicate that the structural distortions caused by the decreasing 〈rA〉 couple the spin subsystem to the lattice, thus, inducing a first-order magnetic transition.