Effect of intrinsic vacancies on the electromagnetic properties of half-doped Sm0.5Ca0.5MnO3 manganites studied by positron annihilation

Abstract

Mixed valence manganites are potential functional materials due to their unique electromagnetic properties. In this work, half-doped ceramics with the perovskite structure Sm0.5Ca0.5MnO3 polycrystalline samples are synthesized by the solid-state reaction method in open air at different final sintering temperatures. Structures and particle sizes are studied by x-ray diffraction and scanning electron microscopy, respectively. Positron annihilation spectroscopy and density-functional theory calculations are used to characterize the intrinsic vacancies in the bulk. Thereafter, the effect of vacancies on the magnetic and magnetoresistance (MR) properties is investigated. We find that Mn monovacancies (VMn) exist in the bulk among all the samples, and the concentration of VMn is different. We suggest a possible defect model that can be well applied to explain the phenomena of their electromagnetic properties. The transition temperature of the charge-order state (TCO) and ferromagnetic and anti-ferromagnetic (TN) are most likely related to the concentration of VMn and the particle sizes or pore sizes, respectively. The glass spin state transition temperature seems to be independent of the defect concentration and type. The metal conductive behavior does not appear in a high magnetic field and at low temperatures, which may be caused by the presence of a high concentration of VMn in the bulk and oxygen-related defects in the boundary, and the double exchange interaction is suppressed. At temperatures below TN and under a weak magnetic field, the MR is related to the total defect concentration. In addition, the high concentration of VMn is unfavorable for obtaining a high MR value.