Effect of the size factor on the magnetic properties of manganite La0.50Ba0.50MnO3

Nanocrystalline manganite La_0.50Ba_0.50MnO₃ was synthesized by an optimized sol-gel method. The initial sample was subjected to step-by-step heat treatment under air atmosphere. The ion stoichiometry, the morphology of crystallites of ceramics, and the magnetic properties were studied. It is established that the average crystallite size D increases from ～30 nm to ～7 μm with increasing annealing temperature. All of the samples studied are characterized by a perovskite-like cubic structure, with the unit cell parameter a increasing continuously from ～3.787 to ～3.904 Å with the average crystallite size. The most significant lattice compression (≈3%) occurs in the sample with an average crystallite size of ～30 nm. The increase in the average crystallite size causes a nonmonotonic increase in the Curic temperature T _C from ～264 to ～331 K and in the spontaneous magnetic moment σ _S from ～1.52 to ～3.31 μ_B/f.u. The anomalous behavior of the magnetic properties of the manganite La_0.50Ba_0.50MnO₃ obtained is explained by the competition between two size effects, namely, the frustration of the indirect exchange interactions Mn³⁺-O-Mn⁴⁺ on the nanocrystallite surface and the crystal lattice compression due to the crystallite surface tension.