Structural characterization, magnetic and optical properties of perovskite (La1−xLnx)0.67Ca0.33MnO3 (Ln = Nd and Sm; x = 0.0–0.5) nanoparticles synthesized via the sol-gel process

Abstract

Perovskite (La1−xLnx)0.67Ca0.33MnO3 (Ln = Nd and Sm; x = 0.0–0.5) manganite nanoparticles were synthesized via the sol-gel process. Their structural, magnetic and optical properties of were investigated as a function of the Ln-doping concentration. Crystal structural Rietveld refinements on the x-ray diffraction data verified that all the nanoparticles had the orthorhombic structure with space group Pnma. The lattice parameters a, b/√2, and c are almost equal for the respective nanoparticles. With increasing the Ln-doping concentration unit cell volume contracts in a faster rate in the case of Sm3+-doping than the Nd-doping. Nanoparticles also became more agglomerated each other and their morphologies evolved from spherical shape to irregular one. Their single-crystalline nature was confirmed by HRTEM images. Infrared spectra identified the stretching vibration of the Mn–O bond with a frequency of ~600 cm−1, whose intensity was reduced as increasing the Ln-doping concentration whereas its frequency retained. Mn 2p XPS spectra revealed both Mn3+ and Mn4+ ions existed in the nanoparticles with the Mn3+/Mn4+ ratio = 2:1, and O 1 s XPS spectra confirmed the presence of lattice oxygen and chemically absorbed oxygen. The presence of Ca2+, Nd3+(or Sm3+) and La3+ ions was also confirmed by the Ca 2p, Nd 3d (or Sm 3d) and La 3d XPS spectra. The Ln-doped nanoparticles exhibit smaller Ms and higher Hc than the pristine nanoparticles due to the existence of antiferromagnetic (AFM) interactions. At the low Nd-doping level (0.1 ≤ x ≤ 0.2) the MZFC (T) and MFC (T) curves are similar to that of the pristine nanoparticles, whereas at high Nd-doping level (0.3 ≤ x ≤ 0.5) the MZFC begins to reach and even exceed the MFC at low temperature. Such phenomenon can be understood from a hierarchy of energy barriers existing between the ferromagnetic and charged-ordered AFM phases. All the nanoparticles undergo a paramagnetic-ferromagnetic phase transition with decreasing the temperature. Their Curie temperature, freezing temperature and irreversibility temperature moved to low temperature as the Ln-doping concentration increased. Optical band gaps of all the nanoparticles are in the range of 1.50–1.66 eV, which is ascribed to the electronic charge transfer between two eg bands with up- and down-spins that are separated by Hund’s coupling energy.