Comparative evaluation of magnetocaloric effect in Ba−substituted La−Sr−Mn−O compounds by using comprehensive TEM analysis

This study has investigated the intricate interplay between Barium doping, structural changes, and magnetic properties in perovskite compounds. Notably, Ba ions, characterized by their large ionic radii compared to Strontium ions, were explored for their impact on crystal structure and magnetic properties of the compounds. The research unveiled significant structural alterations in Ba−doped compounds, leading to noteworthy outcomes. X−Ray Diffraction (XRD) analyses confirmed rhombohedral structure with ̅ R 3 ̅ c symmetry in all compounds, absent of secondary phases. Ba incorporation in the La 0.67 Sr 0.33 MnO 3 resulted in increased crystal lattice parameters, Mn−O bond lengths, and Mn−O−Mn bond angles. Scanning Electron Microscopy (SEM) revealed reduced particle sizes (the average grain size from 322.5 nm to 141.5 nm) and increased inter-particle spacing with rising Ba ion concentrations, indicating lattice strain effects. Energy−Dispersive X−ray Spectroscopy (EDS) analyses confirmed the chemical formulas closely matched the intended compositions. Transmission Electron Microscopy (TEM) provided invaluable insights into unit cell deformations caused by increasing Ba concentrations, as well as revealing a more diverse and non-uniform crystal order behavior within Ba−doped compounds. TEM analyses revealed a slight reduction in average crystallite size with increasing Ba content, calculated as 78 nm, 76 nm, and 78 nm for La 0.67 Sr 0.33 MnO 3 , La 0.67 Sr 0.15 Ba 0.18 MnO 3 , and La 0.67 Sr 0.10 Ba 0.23 MnO 3 , respectively. Magnetization-temperature (M−T) analyses have revealed that the Curie temperature ( T C ) decreases with increasing Ba ion concentrations ( from 370 K to 350 K). Particularly, despite the theoretically identical Mn 3+ /Mn 4+ ratios in all compounds, XRD and TEM analyses have showcased severe structural alterations within Ba−doped compounds, leading to adverse effects on ferromagnetic exchange interactions. The increased Ba ion content has introduced structural distortions that significantly impact magnetic interactions, magnetic moments' less cooperative alignment, and overall magnetization behavior. Magnetization hysteresis curves (M−H) in Ba-doped compounds showed incomplete saturation, attributed to domain wall mobility, magnetic anisotropy, and structural disruptions. Maximum magnetic entropy changes ( − ∆ S M max ) decreased with increasing Ba ion concentration, primarily due to structural distortions and under a magnetic field of 1 T were determined as 1.41 j/kgK, 1.32 j/kgK and 1.03 j/kgK, respectively.