Tailored magnetic transitions and magnetocaloric properties towards room temperature in La _{0.67− <i>x</i>} Bi _<i>x</i> Sr _0.33 MnO ₃ ( <i>x</i> = 0.05–0.25) manganites

A series of Bi-doped manganites, <i>i.e.</i>, La_{0.67-<i>x</i>}Bi_<i>x</i>Sr_0.33MnO₃ (<i>x</i> = 0.05, 0.1, 0.15, 0.2, and 0.25), were synthesized using the conventional solid-state method. X-ray diffraction analysis confirmed a mixed-phase structure, with La_{0.67-<i>x</i>}Bi_<i>x</i>Sr_0.33MnO₃ in rhombohedral (<i>R</i>3̄<i>c</i>) and Mn₃O₄ in tetragonal (<i>I</i>4₁/<i>amd</i>) phases coexisting and the fraction of the tetragonal phase increasing with an increase in the Bi content. Scanning electron microscopy revealed surface homogeneity with polygonal morphology. X-ray photoelectron spectroscopy confirmed the mixed oxidation states of Mn, showing a systematic decrease in the Mn³⁺/Mn⁴⁺ ratio with an increase in the Bi content. Magnetic measurements using a vibrating sample magnetometer demonstrated a decline in the Curie temperature with an increase in Bi substitution, linked to lattice distortions and weakening of Mn³⁺-O-Mn⁴⁺ ferromagnetic interactions. The magnetocaloric properties were determined from both isothermal magnetization studies and direct measurement of adiabatic change studies. The study provides insights into the effect of Bi in La_0.67Sr_0.33MnO₃ on transition temperatures and magnetocaloric properties, highlighting its potential impacts on magnetic refrigeration applications towards room temperatures.