Visualization of Structural Domains in a Single Crystal of Iron Pnictide EuFe₂As₂

It is known that, during the synthesis of superconducting EuRbFe₄As₄ single crystals, inclusions of its parent phase EuFe₂As₂ are encountered. The kinetics of the formation of the orthorhombic phase in EuRbFe₄As₄ crystals containing a large fraction of the EuFe₂As₂ phase is studied using polarization optical microscopy. It is shown that the orthorhombic phase grows into the tetragonal one in the form of stripes that form macrodomains with one of the possible crystallographic orientations. Then these domains are abruptly penetrated by parallel stripes of the orthorhombic phase having the second orientation, as a result of which a twin system of orthorhombic domains is formed. The process is accompanied by the appearance of macrostresses: stretching and compression waves with a characteristic period of 100–300 μm along and across the twin system, which lead to spatial modulation of the magnetic permeability. It is found that even weak magnetic fields (up to 100 Oe) significantly affect the spatial distribution of the twin structure producing an effect similar to external stresses.