The role of lanthanum in the structural, magnetic and electronic properties of nanosized mixed manganese ferrites

Abstract

Manganese ferrite nanoparticles with either Fe3+ or Mn2+ substitution by La3+ (MnFe2O4, La0.125MnFe1.875O4 and La0.125Mn0.875Fe2O4) were obtained by sol–gel method. A single phase corresponding to inverse spinel crystal structure was detected. Lattice parameter increases by incorporating La3+. Microstrain and crystallite size (∼9 nm) were obtained by Williamson-Hall (W-H) plots and field emission scanning electron microscopy (FE-SEM), respectively. Lanthanum ions promote a superparamagnetic behavior at 300 K, while at low temperature ferrites acquire a typical ferromagnetic behavior according to the blocking temperature (TB) detected by zero-field-cooled and field-cooled (ZFC/FC) magnetization measurement. Lanthanum modifies slightly the chemical environment and the cation distribution, promoting the Fe partial reduction and the subsequent Mn oxidation. Experimental results were compared with those obtained by Density Functional Theory (DFT) calculations. The band gap energy (Eg) of MnFe2O4 at 0 K is 1.109 eV (DFT) and no significant change was observed by incorporating lanthanum, while at 300 K (UV–visible diffuse reflectance spectroscopy, UV–vis), Eg is 1.796 eV and increases up to 1.835 and 1.842 eV due to the substitution, by La3+, of Fe3+ and Mn2+, respectively. The role of lanthanum in the Mn-ferrite is the decrease in the magnetic properties by substituting either Fe3+ or Mn2+ due to the electronic transition from the high spin to low spin configuration by the presence of this rare earth element located at the octahedral sites.