Superstructural ordering of Fe³⁺ and Mo⁵⁺ cations in Sr₂FeMoO_6-δ

The SFMO sample was synthesized from a stoichiometric mixture: (a) MoO₃, Fe₂O₃ and SrCO₃ simple oxides, and (b) SrFeO_3-х and SrMoO₄ precursors.

The use of partially reduced precursors promotes the increase of the rate of SFMO growth in the sample No. 2. The temperature of synthesis lowers as well.  Superstructural ordering of Fe³⁺ and Mo⁵⁺ cations is created. Besides, according to the XRD analysis data, additional (101) and (103) reflexes were determined.

Magnetization of SFMO samples synthesized from SCO, FO, MO simple oxides

A temperature dependence of magnetization of the sample No.1, preliminary cooled down from 300 to 4.2 К without magnetic field (zero-field cooling, ZFC) and in magnetic field (field cooling, FC) with a subsequent heating of the sample at B = 0.04T have been measured. From the analysis of these results one can see that a sharp increase of magnetization on 2.37% takes place in the region 4.2 – 11 К. It is caused by the presence of magnetic regions with low coercive force and appearance of the superparamagnetic effect. On the base of calculations of temperature difference ΔТ = 241(Т_В) - 180(Т_max) = 61К one can confirm the large magnetic inhomogeneity of the samples.

Magnetization of SFMO samples synthesized from SFO and SMO simple oxides

A sharp increase of magnetization on 8.23%, caused by the presence of magnetic regions with low coercive force and appearance of the superparamagnetic effect takes place in the range 4.2 – 11К. On the base of calculations of temperature difference ΔТ=241(Т_В) – 180(Тmax) = 61 К one can confirm the smaller magnetic inhomogeneity of the sample No.2 in comparison with the sample No.1.

Optimization of the modes of SFMO synthesis from precursors

A stage-like change of the speed of  SFMO crystallization (β=dα/dt) with fixed heating rates was specified. Increasing the heating rate one can achieve a considerable increase of amplitude values β_max =f(Т) with their shift to a larger temperatures region. At the same time values β_max=f(α) are practically constant.

Comparative analysis of superstructural ordering of Fe³⁺ and Mo⁵⁺ cations in SFMO

On the base of optimization of SFMO synthesis conditions we have succeeded in increase the degree of superstructural ordering. This is confirmed by the XRD spectra of reflexes of SFMO synthesized from: MoO₃, Fe₂O₃ and SrCO₃ mixture (a), SrFeO_3-х and SrMoO₄ precursors (b), and SrFeO_3-х, SrMoO₄ precursors (c), with staging annealing at  vheat = 7deg/min up to Т = 1050 К (exposure 5 h) and heating up to 1350 К (exposure 20 h) (3).

Temperature dependence of magnetization of samples synthesized from a stoichiometric mixture of MoO₃, Fe₂O₃ and SrCO₃ (1), SrFeO_3-х and SrMoO₄ precursors (2) and SrFeO_3-х, SrMoO_4-y, precursors with staging annealing at vheat = 7 deg/min up to Т = 1050 К  (exposure 5 h), and heating up to 1350 К (exposure 20 h) (3).

Influence of annealing modes on SFMO films microstructure

SFMO thin films sputtered in the Ar gaseous medium at the rate 7 - 9 nm/min, remnant pressure 10^-3 Pa and substrate temperature 923 К, are not single-phase and has a weak adhesion to substrate surface (а). The substrate temperature increase to 1223 К at oxygen partial pressure 10^-8 Pa during 1 h leads to a formation of coarse-grain and inhomogeneous structure (b).

SFMO thin films sputtered at the rate 8 nm/min with substrate temperature  923 К in the Ar medium with an additional annealing in evacuated quartz ampules in the presence of a getter at 1173 К and 10^-8 Pa during 1 h are characterized with tetragonal symmetry of the unit cell (I4/m) and reveal the superstructural ordering on Fe/Mo cations.

SFMO thin films with superstructural ordering of Fe/Mo cations are ferrimagnetics with Curie temperature ~ 368 К. The films have ΔТ=0 and Т_B=29.8 К, which indicates the presence of low-coercive grains with Нс→0 and realization of superparamagnetic state.