We report an observation and study of monolayer-thick structural defects in the antiferromagnetic (AFM) layered semimetal ${\mathrm{EuSn}}_2{\mathrm{As}}_2$. The high-resolution transmission electron microscopy revealed the presence of planar defects in the lattice of the studied single crystals. Using a combination of microstructural and DFT analysis, we demonstrated that a single planar defect forms a monolayer of a distinct phase ${\mathrm{EuSnAs}}_2$, which is different from the ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ phase of the bulk lattice. The smaller distance between Eu layers in the planar defect, as well as the odd number of Eu atoms in the formula of the defect $\left({\mathrm{EuSnAs}}_2\right)$ and of the defect supercell $({\mathrm{Eu}}_7{\mathrm{Sn}}_{12}{\mathrm{As}}_{14})$ promote the formation of local ferromagnetic (FM) ordering of the Eu atoms. This suggests that on average, the planar defects form a ferrimagnetic minority phase in the antiferromagnetic (AFM) host lattice. The obtained results may explain several puzzling features in the magnetic properties of various A-type AFM materials, such as the nonlinear magnetization and ferromagnetic-type hysteresis in low in-plane fields, the upturn of the magnetic susceptibility in the AFM state at temperatures approaching zero, and the emergence of the ferromagnetic-type resonance line in the electron spin resonance spectrum of the AFM state. The present finding provides new insight into understanding the mechanism of ferromagnetism in quasi-two-dimensional AFM materials: it offers a new way to create an AFM-FM interface by self-docking two different crystalline phases of a chemically pure compound. Thus this result will facilitate the design and implementation of natural AFM-FM interfaces.

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• Topological materials