The effects of Bi substitution on the structural, microstructural, magnetic, and magnetocaloric properties of \({\text{La}}_{0.65}{\text{Ba}}_{0.35-\text{x}}{\text{Bi}}_{\text{x}}{\text{MnO}}_{3-\delta }\) (where x = 0.00, 0.05, 0.10, and 0.15) are reported in this study. The X-ray diffraction pattern indicates that the crystal structure of the samples is in a rhombohedral structure with \(R\overline{3 }c\) space group. The microstructural analysis reveals that the grain growth of the samples strongly depends on the amount of Bi content. The average grain size \({(D}_{avg})\) increases from 0.112 ± 0.04 to 1.443 ± 0.83 \(\mu m\) as the content of Bi increases from x = 0.0 to 0.10. However, the value of \({D}_{avg}\) is reduced for x = 0.15. Temperature-dependent magnetization has been investigated under ZFC and FC conditions. The degree of bifurcation in ZFC and FC moods is relatively smaller for x = 0.10, which indicates the lowering of the canted nature of spin and magnetic inhomogeneity. The Curie temperature ( \({T}_{C}\) ) has decreased from 298 to 214 K with increasing Bi content, which is attributed to the weakening of the double exchange (DE) interaction. Interestingly, above \({T}_{C}\) , the samples do not exhibit a fully paramagnetic (PM) nature, but a short-range ferromagnetic (FM) cluster, known as the Griffith phase (GP), has been observed. The value of magnetic susceptibility exponent in GP region, \({\lambda }_{GP}\) (= 0.739 ± 0.04) and temperature range of GP (= 44.86%) are higher for x = 0.15, indicating a stronger A-site disorder in the sample. The magnetocaloric effect has been observed in terms of maximum magnetic entropy change ( \(-\Delta {S}_{M}\) )max and relative cooling power (RCP) factor. The values of ( \(-\Delta {S}_{M}\) )max and RCP are found to be 1.234 \({\text{J kg}}^{-1}{\text{K}}^{-1}\) and 124.61 \({\text{J kg}}^{-1}\) for x = 0.10, respectively, at \(\Delta H\) = 2 T. This high value of RCP is desirable for magnetic refrigeration over a wide range of temperatures.
