Elaboration and study of the physical properties of a chromite with perovskite structure and/or derivatives

Abstract

In this thesis, we prepared and examined in depth the structural, microstructural, optical, and magnetic properties of some lanthanum-based perovskite chromites. We are interested in the effect of rare earth substitution in the 𝐿𝑎0.8𝐶𝑎0.2𝐶𝑟𝑂3 and 𝐿𝑎0.8𝐵𝑎0.2𝐶𝑟𝑂3, in which the RE are Nd, Sm, and Eu, so that we got the series: 𝐿𝑎0.7𝑅𝐸0.1𝐶𝑎0.2𝐶𝑟𝑂3, as well as 𝐿𝑎0.7𝑅𝐸0.1𝐵𝑎0.2𝐶𝑟𝑂3. We have prepared the samples by the solid-state reaction technique under identical conditions. The structural investigation revealed that the samples crystallized in an orthorhombic perovskite structure with the Pbnm or Pnma space group with slight differences in the structural parameters values. The FTIR analysis confirms the perovskite structure, while EDS checks and confirms the purity of our components. The SEM images revealed the granular and the porous characters of the samples surfaces. The grains have a micrometer size which appears to be influenced by the RE substitution. The optical absorption of the samples is found to be influenced by the A-site composition, where the LCCO sample exhibits a higher optical absorption than the LBCO. The reduction in the RE ionic radius improves the optical absorption in the 𝐿𝑎0.7𝑅𝐸0.1𝐶𝑎0.2𝐶𝑟𝑂3 samples; however, it has no significant effect on the 𝐿𝑎0.7𝑅𝐸0.1𝐵𝑎0.2𝐶𝑟𝑂3 ones. In the 𝐿𝑎0.7𝑅𝐸0.1𝐶𝑎0.2𝐶𝑟𝑂3 system, the estimated Eg values show an increase by decreasing the RE ionic radii from 2.96 to 3.27 eV, while the inversion trend is obtained for 𝐿𝑎0.7𝑅𝐸0.1𝐵𝑎0.2𝐶𝑟𝑂3. For pristine samples, the lowest value of Eg=3.2 eV is achieved for LBCO sample. The structural parameters: Cr- O band distance, Cr-O-Cr average bond angles, and octahedral tilting, present an obvious effect on the magnetic properties of the samples. The magnetic investigation of samples showed a reduction of the antiferromagnetic transition temperature (TN), except in the case of 𝐿𝑎0.7𝑅𝐸0.1𝐵𝑎0.2𝐶𝑟𝑂3 system, where it presents a slight rise which could be consider as stable. By the cooling below TN, the samples exhibit a weak ferromagnetic explained via the Cr3+ ions antiferromagnetic canting behavior. A particular magnetic signature is seen in the samples substituted with 𝑁𝑑 and 𝑆𝑚, traduced by the occurrence of a spin reorientation transition. The samples doped with Nd exhibit negative magnetization in their FC curves, which is may be caused by the formation of an opposite internal field to an applied one and confirmed by the fitting of M(T). In its ZFC curve, the sample 𝐿𝐶𝐸𝑢𝑂 exhibits spin glasses-like behavior at low temperatures. The hysteresis loops of all compounds indicate the presence of AFM and Weak FM states. It is found to be varied with the selected temperature measurement and with the RE elements. The interaction of these two states in the interface gives rise to the exchange bias effect. This effect is most noticeable at low temperatures (T=1.8), and its field takes opposite sign in some samples. According to the finding values of 𝐸𝑔, we can suggest that our materials can be used in optoelectronic devices. In thermomagnetic switch and spin-valve devices, Nd-doped samples can be a useful material. Because of the EB effect, the samples are suitable for magnetic sensors, magnetic recording devices, nonvolatile magnetic random-access memory (NMRAMs), and other spintronics devices.