Charge ordering and magnetic properties in nanosized Sr 2 FeMoO 6– δ powders

Nanosized single‐phase powders of the ferrimagnetic double perovskite Sr 2 FeMoO 6– δ (SFMO) with various degrees of the superstructural ordering of the Fe 3+ and Mo 5+ cations have been synthesized by the citrate–gel method at pH = 4, 6, 9 (samples SFMO 4, 6, and 9). According to the results of the X‐ray photoelectron spectroscopy, a mixed valence state of the iron and molybdenum cations is realized in the obtained SFMO powders, whereby the concentration of Fe 2+ increases and that of Fe 3+ decreases with increasing pH. The charge state of iron and the redistribution of the electron density in SFMO has been studied by means of the Mössbauer spectroscopy. The charge disproportionation for the powders obtained at pH = 6 and 9 facilitates the segregation of α‐Fe. The temperature dependences of the magnetization measured in the field cooling and zero‐field cooling regimes indicate the existence of superparamagnetism in the nanosized grains of SFMO 4, 6, and 9 at T  < 19 K. The superparamagnetism is metastable up to a temperature Т S . The applied magnetic field stimulates the transition in nanosized grains from the metastable superparamagnetic state into the stable one, above some value of the magnetic field induction exceeding the minimal anisotropy field. 57 Fe Mössbauer spectra taken at 20 K of Sr 2 FeMoO 6– δ prepared at pH = 6.