Effect of Potassium Substitution on Structural, Magnetic, Magnetocaloric, and Critical Properties of La 0.65 Sr 0.35-X K X MnO 3 ( X=0.075 and 0.15) Manganites

We have investigated the effect of potassium (K) doping at Sr-sites on the structural, magnetic, magnetocaloric, and critical behaviors of La0.65Sr0.35-xKxMnO3(x = 0.075 and 0.15) perovskite manganite. These samples were successfully prepared using the sol-gel method. X-ray powder diffraction (XRD) showed that our samples crystallize in the rhombohedral structure within space group R3(& horbar;)c (No.K167). As the temperature increases, the synthesized compounds undergo a phase transition from ferromagnetic (FM) to paramagnetic (PM) material. The magnetocaloric effect has been evaluated by assessing the isothermal magnetization curve at different temperatures close to the Curie temperature (T-C), employing the Maxwell relationship. At an applied field with a magnitude of 5 T, the maximum magnetic entropy change (-Delta S-M(max)) and relative cooling power (RCP) are 4.25 J/kg K and 288 J/kg for x = 0.075 and 3.67 J/kg K and 267 J/kg for x = 0.15, respectively. The important magnetic entropy change, along with the adaptable Curie temperature and high relative cooling power (CP), renders these oxides beneficial for magnetic refrigeration across extensive temperature ranges, including near-room temperatures. Critical phenomena have been studied using different theoretical models: modified Arrott plots (MAP), Kouvel-Fisher (KF) method and critical isotherm (CI) analysis. The obtained critical exponents (beta, gamma and delta) revealed that the simple x = 0.075 is consistent with the 3D Ising model, whereas the critical exponents determined for x = 0.15 were close to the 3D Heisenberg model. The obtained values of the critical exponents were verified by the Widom relationship and scaling hypothesis, which confirmed the associated models.