Phase separation, inverse magnetocaloric effect and signature of Griffiths phase in the Alkali-ion doped Sm0.5Ca0.4A0.1MnO3 (A = Na+, K+) manganites

Undoped Sm0.5Ca0.5MnO3 and doped Sm(0.5)Ca(0.4)A(0.1)MnO(3) (A = Na and K) compounds were prepared via the sol-gel chemistry route and the composition was verified using X-ray and EDAX measurements. Results from X-ray powder diffraction reveal an orthorhombic structure with Pnma space group. Particle size calculated using the SEM images confirmed a successful synthesis of nanomaterials in the range of 76 and 136 nm. The temperature dependence magnetization curve of the Sm0.5Ca0.5MnO3 compound shows three distinct points at 45 K, 65 K and 270 K corresponding to the nonhomogeneous magnetic clusters, weak ferromagnetic and charged ordered antiferromagnetic transitions, respectively. The experimental results confirm that Na substituted sample undergoes a charge ordering transition, while K substitution suppresses the charge ordering phase and leads to Griffiths phase formation. In addition, zero-field-cooled and field-cooled magnetization curves bifurcate and the complex coexistence of magnetic phases at the lower temperature is supposed. Such phase coexistence at low temperature may be responsible for the exchange bias effect. Our study also highlights the efficacy of magnetocaloric effect as a tool to discern between the different magnetic states. Undoped and Na doped samples show both normal and inverse magnetocaloric effect which is interesting for magnetic refrigeration applications. The maximum value of magnetic entropy change was found to be -Delta S-M = 1.1 J.kg(-1).K-1 for K doped sample upon an applied magnetic field change of 5 T. Meanwhile, this sample exhibits a net relative cooling power of 55.75 J.kg(-1) at mu(0)Delta H = 5 T.