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Parent compounds AFe2As2 (A=Ca, Ba, Eu) of the 122 family of the iron-basedsuperconductors have been studied by 57Fe Moessbauer spectroscopy in thetemperature range 4.2 K - ~300 K. Spin density waves (SDW) have been found withsome confidence. They are either incommensurate with the lattice period or theratio of the respective periods is far away from ratio of small integers. SDWshape is very unconventional i.e. differs from the sinusoidal shape. Magneticorder starts with lowered temperature as narrow sheets of the significantelectron spin density separated by areas with very small spin density. Magneticsheets are likely to be ordered in the alternate anti-ferromagnetic fashion asthe material as whole behaves similarly to collinear anti-ferromagnet. Furtherlowering of temperature simply expands sheet thickness leading to the neartriangular SDW. Finally, sheets fill the whole available space and almostrectangular shape of SDW is reached. Substantial maximum amplitude of SDWappears at the temperature just below the magnetic onset temperature, and thismaximum amplitude increases slightly with lowering temperature. The square rootfrom the mean squared hyperfine field behaves versus temperature according tothe universality class (1, 2), i.e., with the electronic spin space havingdimensionality equal unity and the real space having dimensionality equal two.The more or less pronounced tail above transition temperature due to thedevelopment of incoherent SDW is seen.

Shape of spin density wave versus temperature in AFe2As2 (A=Ca, Ba, Eu): A M\"{o}ssbauer study