Structure of nanocrystalline Nd0.5R0.5FeO3 (R=La, Pr, and Sm) intercorrelated with optical, magnetic and thermal properties

Abstract

Polycrystalline samples of Nd0.5R0.5FeO3 (R = La, Pr and Sm) synthesized by sol-gel combustion method have been investigated for their structural, optical, thermal and magnetic properties. X-ray diffraction in combination with Fourier transform infrared (FTIR) spectra confirm the single-phase orthorhombic structure with Pbnm space group of all the samples. Rietveld refinement employed to determine the bond lengths (Fe–O, R–O) and bond angles (Fe–O–Fe) show a decrease in the bond angle with Sm doping. Transmission electron microscopy (TEM) micrographs reveal the nano-size particles and selected area electron diffraction (SAED) patterns indicate good crystallinity with a uniform distribution of particles throughout the samples. Raman spectra clearly indicate the compressive strain in the Sm doped sample that has been correlated with the structural parameters obtained from Rietveld refinement. The shift in wavenumber is the main cause of change in bond length (Fe–O) and tilt of octahedra due to rare earth ions. The optical bandgap is determined using Tauc's relation from the diffuse reflectance spectra. The variation of specific heat has been investigated with temperature in a broad temperature range. The endothermic peak in specific heat has been correlated with magnetic order. The value of Néel temperature increases in the case of La-doped sample and reduces for the Pr doping. However, the decrease of specific heat in Sm doped sample around 300 °C may be attributed due to the spin reorientation. Magnetic studies at room temperature reveal antiferromagnetic behavior with weak ferromagnetic component due to the canted ordering of Fe3+ spins. Magnetic properties are strongly dependent on the type of rare earth ion as the maximum magnetization (Mm) is enhanced from 0.259 to 0.434 emu/g with the doping of rare earth ions (La, Pr and Sm). The increase in magnetization has been correlated with increased distortion which is evident from the decrease in the bond angle. Exchange bias (EB) effect is observed for all samples as single-phase orthorhombic structure is composed of antiferromagnetic (AFM) and ferromagnetic (FM) phases.