Parent compounds AFe2As2 (A = Ca, Ba, Eu) of the 122 family of the iron-based superconductors have been studied by Fe57 M\"ossbauer spectroscopy in the temperature range 4.2\char21{}$\sim${}300 K. Spin density waves (SDW) have been found with some confidence. They are either incommensurate with the lattice period or the ratio of the respective periods is far away from the ratio of small integers. SDW shape is very unconventional (i.e., differs from the sinusoidal shape). Magnetic order starts with lowered temperature as narrow sheets of the significant electron spin density separated by areas with very small spin density. Magnetic sheets are likely to be ordered in the alternate antiferromagnetic fashion as the material as a whole behaves similarly to the collinear antiferromagnet. A further lowering of temperature simply expands sheet thickness leading to the near triangular SDW. Finally, sheets fill the whole available space and the almost rectangular shape of the SDW is reached. The substantial maximum amplitude of SDW appears at the temperature just below the magnetic onset temperature, and this maximum amplitude increases slightly with lowering temperature. The square root from the mean squared hyperfine field behaves versus temperature according to the universality class (1,2) (i.e., with the electronic spin space having dimensionality equal to unity and the real space having dimensionality equal to 2). The more or less pronounced tail above transition temperature due to the development of incoherent SDW is seen.

In this work the synthesis of hercynite from Fe₂O₃ and Al₂O₃ powders was carried out by arc-melting method under the protective argon atmosphere. The obtained material was characterized with the use of powder X-ray diffractometry (XRD) and Mössbauer spectroscopy (MS). A Mössbauer effect in hercynite obtained by the arc-melting method indicated the cations distribution in the spinel structure among the tetrahedral and octahedral interstices. The presence of Fe²⁺ ions was detected in both tetrahedral and octahedral sites while Fe³⁺ ions occupied only the octahedral interstices. The approximate formula of the obtained iron-aluminate spinel was as follows (Fe²⁺ _0.77Al³⁺ _0.23) (Fe³⁺ _0.07Fe²⁺ _0.05Al_0.88)₂O₄.

The Fe–Al end-member spinel hercynite was prepared by the arc plasma synthesis (APS) technique and subjected to annealing at 1273 K for 1, 24 and 36 h in air atmosphere. The X-ray diffraction (XRD) analyses conducted before and after the thermal treatment revealed an evident alteration of the original hercynite. About half of the original spinel is preserved after the longest annealing experiment (36 h), the reminder of the Fe–Al spinel underwent decomposition into iron and aluminum oxide phases, among which non-stoichiometric magnetite and corundum were the most important. Mössbauer spectroscopy revealed that the initial hercynite was a normal spinel with a slight inversion degree of 11%, which was increased to 34% after the 36-h annealing process. An overlapping of reflections for spinel-like phases γFe₂ O₃ and γAl₂ O₃ with Fe–Al spinel impeded their detection by XRD. Mössbauer spectroscopy (MS) confirmed the presence of magnetite in the annealed samples. The SEM-EPMA observations of the oxidized surfaces of hercynite showed well-developed, scalenohedral crystals with different Fe/O ratios, and well defined compositions corresponding to Al-bearing Fe₂ O₃ as measured by WDS spectroscopy. The results of XRD, MS and SEM-EPMA permit to establish a general oxidation mechanism of hercynite decomposition.