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

TL;DR

This study uses 57Fe Mössbauer spectroscopy to investigate the unconventional, incommensurate spin density wave shapes and their temperature evolution in parent compounds of iron-based superconductors, revealing complex magnetic ordering behaviors.

Contribution

It provides detailed insights into the shape, incommensurability, and temperature dependence of SDWs in AFe2As2 compounds, highlighting their unconventional nature and evolution with temperature.

Findings

SDWs are incommensurate or far from simple ratios

SDW shape evolves from sheets to rectangular form with cooling

Maximum SDW amplitude appears just below magnetic transition temperature

Abstract

Parent compounds AFe2As2 (A=Ca, Ba, Eu) of the 122 family of the iron-based superconductors have been studied by 57Fe Moessbauer spectroscopy in the temperature range 4.2 K - ~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 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 anti-ferromagnetic fashion as the material as whole behaves similarly to collinear anti-ferromagnet. Further lowering of temperature simply expands sheet thickness leading to the near triangular SDW. Finally, sheets fill the whole available space and almost rectangular shape of SDW is reached. 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 unity and the real space having dimensionality equal two. The more or less pronounced tail above transition temperature due to the development of incoherent SDW is seen.