Effect of local chemical disordering on magnetic properties in high entropy manganite of variable hole concentration

Abstract

A series of high entropy manganite perovskites (La0.2Pr0.2Nd0.2Bi0·2Ba0.2)MnO3, (La1/6Pr1/6Gd1/6Bi1/6Sr1/6Ba1/6)MnO3, (La0.2Nd0.2Gd0.2Sr0·2Ba0.2)MnO3 and (La1/6Pr1/6Nd1/6Ca1/6Sr1/6Ba1/6)MnO3 with variable hole concentration at fixed A-site ionic radius (<rA> = 1.253 Å) have prepared by sol-gel technique. Single phase samples were characterized by the powder X-ray diffraction (PXRD), Field Emission Scanning Electron Microscopy (FE-SEM), Energy Dispersive X-ray (EDX) analysis and elemental mapping which confirm the phase purity of the samples, surface morphology, elemental ratio and chemical homogeneity. X-ray photoelectron spectroscopic (XPS) studies reveal the increase of Mn4+ ions systematically with the substitution of hole concentration at A-site. Magnetization study in this high entropy sample with variable hole concentration at fixed <rA> = 1.253 Å exhibits several contrasting behaviour compare to the conventional hole doped samples. In these high entropy manganites the variation in critical temperature (TC) does not follow expected trend neither with the hole concentration nor to the cationic size disorder parameter, σ2. Though the <rA> is fixed, there is a slight increase in tolerance factor (f) due to hole variation in B-site. Interestingly, TC variation with f shows similar trend to the hole concentration, which is expected to be increased linearly. The effect of f on TC is rather weaker in comparison to σ2. The large drop in TC for high entropy system with respect to the conventional manganite is related to the local structural distortion which severely hampers the Mn3+—O—Mn4+ double exchange interaction. The competing magnetic interactions lead to magnetic phase separation as well as Griffith like phase. Spin pinning is noticed for the samples containing six elements in A-site which is independent on hole concentration. Possible spin clustering is also realized above ferromagnetic TC. Careful analysis of the magnetization data suggests that observed anomaly in magnetic behaviour can be ascribed to the local chemical disordering apart from the size disorder parameter. The possible role on magnetic properties of the number of elements occupying in a crystallographic site is emphasized.