Structural, magnetic and electrical transport properties of the sol-gel derived La1-xCaxMnO3 (0≤x≤0.3) nanoparticles

Abstract

Sol-gel (SG) prepared calcium doped La1-xCaxMnO3 (0 ≤ x ≤ 0.3) nanoparticles (>35 nm) show a phase transformation from rhombohedral-R3c to orthorhombic-Pnma. The dopant concentration influences the bond length and lattice parameters and their variations as observed from the rietveld refinement. The phase stability and chemical compositions are confirmed using the Thermogravimetric and differential thermal analysis (TG-DTA) and Fourier transform infrared (FTIR) spectroscopy measurements. The latter confirms the presence of dangling bonds from the changes in the characteristic vibrational frequencies and intensities. Field emission scanning electron microscope-energy dispersive X-ray analysis (FESEM-EDAX) confirms the nano-dimension of the particles and good chemical homogeneity of the samples. The Curie temperature Tc, estimated from the field cooled magnetization curve, shifts towards room temperature (Tc = 160–270 K) with the increase of dopant concentration. The samples exhibit a clear metal to insulator transition, the temperature of which shifts to higher values (169–267 K) with increase in dopant content. With the application of an external magnetic field, local ordering of magnetic moments occurs and the evolution of a ferromagnetic metallic phase suppresses the existing paramagnetic insulating phase, decreasing the value of resistivity. Three different temperature ranges demarked in the resistivity curves are studied using different theoretical models. The observed low temperature resistivity upturn is explained on the basis of the Kondo-like model while the ferromagnetic metallic region is studied using different scattering processes like electron – electron, electron – magnon etc. The variable range and small polaron hopping (VRH and SPH) models are used in the high-temperature region data and with the aid of Holstein's relation the non-adiabatic SPH model it is found to be most suitable to understand the conductivity behavior. The temperature dependent evolution of the Griffith's like phase can explain and correlate the electric transport and magnetic properties exhibited by the samples. This highlights the evolution of Mn3+-O2--Mn4+ FM oriented networks, which form conduction channels with the lowering of temperatures. These channels mediate the enhanced conduction and helps set in long-range FM ordering in the samples/