Interplay of Superstructural Ordering and Magnetic Properties of the Sr2FeMoO6-delta Double Perovskite

Abstract

Strontium ferromolybdate (Sr2FeMoO6-delta, SFMO) is a material exhibiting promising magnetoresistive properties. We have synthesized SFMO samples out of simple oxides (SrCO3, Fe2O3, and MoO3) or partially reduced SrFeO3-x (SFO) and SrMoO4-y (SMO) precursors. The samples have been experimentally investigated using X-ray diffraction, temperature-dependent magnetization, Mossbauer effect and ferromagnetic resonance measurements. Samples of the first type contain a high density of defects, especially [Fe-Mo], [Mo-Fe] antisites, and do not exhibit any superstructural ordering of the iron and molybdenum ions (P=0%). These samples comprise iron cations in a mixed valence state, Fe2+/3+, and are characterized by a higher magnetic inhomogeneity than those synthesized out of precursors. The use of the latter increases the sample density and brings about a growth acceleration, synthesis temperature reduction, as well as the appearance of a superstructural ordering of the Fe3+ and Mo5+ cations with P=64%. The samples exhibit magnetic anisotropy and consist of nanosize grains. Zero-field-cooling measurements of the temperature dependences of the magnetization reveal a sudden leap of the magnetization at low temperatures (below 23 K) that witnesses the existence of magnetic regions with a low coercivity, in which a superparamagnetic state exists. The obtained results are important for the optimization of the synthesis technology of SFMO for device applications.