Structural, magnetic and magnetocaloric effect studies of Nd_0.6Sr_0.4A_xMn_1-xO₃ (A=Co, Ni, Zn) perovskite manganites

The effect of B-site substitution by other transition elements on the structural, magnetic, and magnetocaloric effect properties of Nd_0.6Sr_0.4A_xMn_1-xO₃ (x = 0.1 and x = 0.2, A=Co, Ni, and Zn) nanopowders has been reported in this study. Auto-combustion sol-gel method was used to synthesize all the nanopowder samples. Room temperature X-ray diffraction shows that all the nanopowders have orthorhombic structure of Pnma space group crystallography, confirming the purity of the single phase. Also, the average nanosized scale of these powder samples is ~ 43 nm, revealing a spherical shape with a packed and homogenous structure. The Mn⁴⁺/Mn³⁺ ratio in these nanopowder compounds is highly dependent on the dopants concentration (x), which shows a significant diminishes as the dopants introduce into the parent compound (Nd_0.6Sr_0.4MnO₃). The decrease in the Mn⁴⁺/Mn³⁺ ratio enhances in the antiferromagnetic (AFM) super exchange interaction (SE) due to the lack of Mn³⁺— O—Mn⁴⁺ ferromagnetic (FM) double exchange (DE). The magnetization measurements indicate that all samples exhibit a ferromagnetic (FM) to paramagnetic (PM) transition with increasing temperature. The Curie temperature (T_C) is also affected by the dopants concentrations where considerable decrease has been noticed for all compounds compared to the parent compound. The M-H curves for all compounds reveal PM behavior at 300 K, whereas FM behavior characterized the magnetic hysteresis at low temperature (2 K) which also shows the presence of exchange bias effect. The magnetic entropy change |−∆S_M^Max| has shown bigger effect for Co dopant (5.92 J/kg.K) compared to Ni and Zn dopants with values of 5.19 J/kg.K, and 4.28 J/kg.K respectively at an applied magnetic field of 9 T. Despite substituting at Mn-site by other transition elements lower the Tc, |−∆S_M^Max| and RCP, the obtained results are promising for magnetic cooling materials and further investigation.