B-Site Fe/Re Cation-Ordering Control and Its Influence on the Magnetic Properties of Sr₂FeReO₆ Oxide Powders

Double-perovskite oxide Sr₂FeReO₆ (SFRO) powders have promising applications in spintronics due to their half-metallicity and high Curie temperature. However, their magnetic properties suffer from the existence of anti-site defects (ASDs). Here, we report on the synthesis of SFRO powders by the sol-gel process. The B-site cationic ordering degree (η) and its influence on magnetic properties are investigated. The results demonstrate that the η value is well controlled by the annealing temperature, which is as high as 85% when annealing at 1100 °C. However, the annealing atmospheres (e.g., N₂ or Ar) have little effect on the η value. At room temperature, the SFRO powders crystallize in a tetragonal crystal structure (space group I4/m). They have a relatively uniform morphology and the molar ratios of Sr, Fe, and Re elements are close to 2:1:1. XPS spectra identified that Sr, Fe, and Re elements presented as Sr²⁺, Fe³⁺, and Re⁵⁺ ions, respectively, and the O element presented as O^2-. The SFRO samples annealed at 1100 °C in N₂, exhibiting the highest saturation magnetization (M_S = 2.61 μ_B/f.u. at 2 K), which was ascribed to their smallest ASD content (7.45%) with an anti-phase boundary-like morphology compared to those annealed at 1000 °C (ASDs = 10.7%) or 1200 °C (ASDs = 10.95%).