Role of potassium substitution in the magnetic properties and magnetocaloric effect in La_0.8−xK_xBa_0.05Sr_0.15MnO₃ (0 ≤ x ≤ 0.20)

The magnetic and magnetocaloric effects of potassium-substituted La_0.8-xK_xBa_0.05Sr_0.15MnO₃ (0 ≤ x ≤ 0.20) manganite were explored. The samples in polycrystalline form were synthesized by the sol–gel method, with a final sintering temperature of 1100 °C. Powder X-ray diffraction (XRD) patterns refined by Rietveld refinement show that all samples crystallized in rhombohedral structure with R-3c space group. The unit cell volume of the samples decreases with increasing potassium concentration. In addition, small changes in average bond length and bond angle are also observed in the samples. Scanning electron microscope (SEM) images reveal that the largest average grain size was observed for x = 0.10. Field-cooled (FC) magnetization measurements show that the Curie temperature (T_C) of the samples increases from 320 K for x = 0 to 360 K for x = 0.2. The largest magnetocaloric (MCE) effect, which is represented by maximum magnetic entropy change (−ΔS_M,MAX), reaches its greatest value for the x = 0.10 sample. The monotonous increase in T_C suggests that T_C is mainly governed by the ferromagnetic coupling between Mn ions induced by the changes on average bond length and bond angle. The obtained −ΔS_M,MAX value suggests that MCE property is more sensitive to Zener theory of double exchange, which is strongly related to the Mn³⁺/Mn⁴⁺ ratio of the samples.