Study of the effect of doping and processing conditions on the temperature coefficient of resistivity of perovskite manganite doped at A-site: A1-xRxMnO3

Abstract

Manganite based perovskite materials are extremely intriguing from both a technological and fundamental standpoint due to the substantial correlations observed among their structural, magnetic, and transport properties. The primary aim of this thesis is to synthesize La0.7Ca0.18Ba0.12Mn0.95Sn0.05O3 (serie 1) and La0.7Ca0.18-xSrxBa0.12Mn0.95Sn0.05O3(0.00≤x≤0.06) (serie 2) perovskite manganite compounds with different quenching processes and strontium content and to investigate their structural, electrical, magneto-transport and magnetic properties. These two sets of samples were prepared by the well-known solid-state method and characterized by using X-ray diffraction, SEM, 4- Point probe method and SQUID magnetometer. The detailed structural study of the both series confirmed that all samples crystallized in the orthorhombic system with Pnma , Pbnmspace-groups and in the rhombohedral system with R3 ̅c space-group. The micrometric granular nature of the samples with distinct grain boundariesare revealed by a scanning electron microscope. The magnetic measurements carried out for all the samples showed a Ferromagnetic-paramagnetic transition at TC temperature and demonstrated the existence of Griffiths phase in the nitrogen quenched sample (serie 1). Doping with Sr ions (serie 2) conducted to an increase in the TC temperature. Moreover, the electrical and magneto-transport findings indicated that all samples undergo a transition from metal to insulator at TMI temperature and demonstrated a magnetoresistive character when subjected to a 1 Tesla external magnetic field. Quenching in nitrogen showed a decrease in the electrical resistivity of the sample, an increase in the TMI, and an increase in the magnetoresistance value (best magnetoresistance value 33.31%), while the maximum value of the temperature coefficient of resistivity (TCR) is recorded for the sample quenched in air. The Sr doped samples displayed a decrease in the electrical resistivity, which affected the magnetoresistive properties. The maximum temperature coefficient of resistivity value 1.63% was obtained for the sample with x=0.02. According to the resistivity fitting results, the ρ (T)curves for all samples in the metallic state are governed by a combination of various interactions: weak localization, residual resistivity, electron-phonon, electron-electron and electron-magnon interactions. Additionally, the fitting indicated that within the insulating state, the conduction process is regulated by variable range hopping mechanism above the half Debye's temperature 〖(θ〗_D/2) and by polaronic mechanism below it. The mean hopping distance (R_h) and the mean hopping energy (E_h) expended by the electron during its hops were calculated within the suitable temperature range and discussed in the light of the theory of VRH.