Charge ordering and magnetic properties in nanosized Sr₂FeMoO_6–δ powders

Abstract

Phone: þ351 234 378 117, Fax: þ351 234 378 197Nanosized single-phase powders of the ferrimagnetic double perovskite Sr 2 FeMoO 6-d (SFMO) with various degrees of the superstructural ordering of the Fe 3þ and Mo 5þ cations have been synthesized by the citrate-gel method at pH ¼ 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 2þ increases and that of Fe 3þ 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 M€ ossbauer spectroscopy. The charge disproportionation for the powders obtained at pH ¼ 6 and 9 facilitates the segregation of a-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. 57Fe M€ ossbauer spectra taken at 20 K of Sr 2 FeMoO 6-d prepared at pH ¼ 6.

The Role of the Fe/Mo Cations Ordering Degree and Oxygen Non‐Stoichiometry on the Formation of the Crystalline and Magnetic Structure of Sr2FeMoO6−δ

“…The monotonous growth of Т C with increasing P correlates also with the diminution of the electrical resistivity of the investigated samples and, respectively, with a rise of the spin‐polarized charge carrier concentration. This fact indicates that the spin‐polarized charge carriers at the Fermi level have a dominating role on the exchange interactions, which corroborates the conclusions drawn by other authors …”

Valence State of Iron and Molybdenum Cations under Conditions of Anionic Deficiency in Sr2FeMoO6–δ

“…An increase in the annealing time up to 90 h in a stream of 5%H 2 /Ar at T = 1420 K facilitates the transfer of part of molybdenum cations to another valence state: Mo 6+ + x e —1 → Mo 5+ + Mo 4+ + Mo x + (Figure ). This indicates a redistribution of the electron density stimulated by the formation of oxygen vacancy associates …”

“…This circumstance indicates the presence of regions with ordered Fe/Mo cations that occupy a larger volume in A —3 than in A —1 or A —2. According to the Mössbauer spectroscopy data, the B hf and δ IS values for the S1 sextet (and to a lesser extent for the S2 and S3 sextets) are shifted from the values characteristic of Fe 3+ and correspond to the state with a mixed valence of iron cations Fe 2+/3+ (between Fe 3+ and Fe 2+ ). Thus, the isomeric shift of the S1 sextet for samples A —1, A —2, and A —3 is δ IS ≈ 0.7 mm s −1 , which is a value intermediate between those for Fe 3+ ( δ IS ≈ 0.5 mm s −1 ) and Fe 2+ ( δ IS ≈ 1.2 mm s −1 ), as previously found for Sr 2 FeWO 6 .…”

“…Thus, a number of authors, using X‐ray photoelectron spectroscopy (XPS), found that a mixed‐valence state of the Fe and Mo cations is observed in SFMO, depending on the oxygen concentration and superstructural ordering degree of those cations. In this situation, the coexistence of both chain fragments, 30% (Fe 3+ —Mo 5+ ) and 70% (Fe 2+ —Mo 6+ ), is possible …”

“…The Mössbauer spectroscopy technique using the 57 Fe isotope allows us to obtain qualitative and quantitative information on the phase composition and specific properties of the electronic structure (density) and magnetic ordering of each phase, being based on a complex analysis of hyperfine parameters of local configurations of iron ions …”