Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)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 < r(A)> of the A-site (A = (R, Sr)) elements systematically decreases. It is found that, with decreasing < r(A)>, the magnetic-ordering temperature decreases from 341K for La0.6Sr0.4MnO3 to 126K for Sm0.6Sr0.4MnO3. Interestingly, the magnetic-entropy change increases with decreasing < r(A)>, reaching Delta S-M = - 8: 4 J/kg K for Delta H = 0 - 20 kOe for Sm0.6Sr0.4MnO3. For manganites, this is a high value of DSM, 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 < r(A)> couple the spin subsystem to the lattice, thus, inducing a first-order magnetic transition. VC 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4795769]11311PROPPI-UFFConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)