Role of iron in the ferromagnetic-antiferromagnetic boundary of La0.5Ca0.5Mn1-xFexO3+δ (0 ≤ x ≤ 0.5) manganites

Samples of La0.5Ca0.5Mn1-xFexO3+delta (0 <= x <= 0.5) were synthesized using a solid-state reaction method involving a milling process and thermal treatment up to 1200 degrees C in an air atmosphere. Samples were characterized structurally with X-ray diffraction analysis and Rietveld refinement with morphology characterization using scanning electron microscopy. Magnetic properties were investigated using a physical property measurement system to obtain zero field cooling and the associated curves to plot hysteresis loops. Our results revealed the interplay between the structural and magnetic properties as Fe ions attached to the crystalline structure. A mechanism based on the substitution of Mn3+ and Mn4+ by low-spin Fe3+ and Fe4+ ions, respectively, was hypothesized to interpret the experimental data. More specifically, the temperature at which the transition from ferromagnetic to paramagnetic occurred increased with increasing Fe content as a result of a greater density of oxygen-mediated ferromagnetic bonds. Conversely, the magnetization weakened because the t(2g) electrons were distributed in the respective d orbitals by adopting a low-spin configuration. Such a configuration is preferred as a result of the unit cell distortion in the milling process where the greater ionic radius of the Fe4+ ions leads to an elongated c-axis tetragonal symmetry and a greater unit cell volume. Finally, low-temperature magnetic behavior revealed the occurrence of a reentrant spin-glass type state within the ferromagnetic matrix favored by a milling-driven structural disorder and the existence of competitive superexchange interactions. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.