Tuning of normal and inverse magnetocaloric effect in Sm_0.35Pr_0.15Sr_0.5MnO₃ phase separated manganites

Graphical abstract: ΔS_M vs. T plots of nano (left) and bulk (right) samples at different magnetic fields. - Highlights: • Bulk to nano show first order FM → PM phase transition at low magnetic field. • Bulk sample exhibits normal and inverse MCE around T_C and after T_g, respectively. • The value of ΔS_M at T_C is almost three times larger than at T_g. • The value of ΔS_M also decreases with reduction of particles sizes. • The bulk sample also exhibits a large RCP of 43.5 J/kg for a magnetic field of 1 T. - Abstract: Magnetic and magnetocaloric properties of Sm_0.35Pr_0.15Sr_0.5MnO₃ polycrystalline manganite (bulk and nanometric samples) are investigated in detail. It has been observed that all the particle sizes (bulk to nano) show first order ferromagnetic → paramagnetic phase transition at low magnetic field. Ferromagnetic transition temperature also decreases with decreasing the particle size. This suggests that ferromagnetism is weakened and the first order magnetic phase transition is softened. We have investigated the magnetocaloric effect (MCE) of both bulk and nanometric samples around their spin glass-like transition temperature, T_g and Curie temperature, T_C. It has been found that bulk sample exhibits both normal (i.e., negative ΔS_M) and inverse (i.e., positive ΔS_M) MCE around T_C and after T_g, respectively. The value of ΔS_M (+3.17 J kg⁻¹ K⁻¹) at T_C is almost three times larger than at T_g (ΔS_M = −0.52 J kg⁻¹ K⁻¹) for a magnetic field change of 7 T. The bulk sample also exhibits a large relative cooling power (RCP) of 43.5 J/kg for a magnetic field of 1 T. The corresponding adiabatic temperature change of bulk sample is observed to be ∼1.5 K for a magnetic field change of 3 T. The value of ΔS_M also decreases with reduction of particles sizes. The temperature width of ΔS_M broadens with decreasing particle size. The value of ΔS_M and the adiabatic temperature change vary with temperature and it is also found to be significant at all applied magnetic fields. Our results suggest that Pr³⁺ doping induces ferromagnetic clusters in the charge ordered matrix exhibiting large normal MCE at T_C and the antiferromagnetic coupling between the Sm 4f and Mn 3d moments is most likely responsible for the inverse MCE below T_g. The co-existence of normal and inverse MCEs is technologically potential and interesting as the sample can be cooled by adiabatic magnetization and demagnetization in different temperature regions enhancing the refrigeration capacity