Emergence of Ferromagnetism from Planar Defects in EuSn2As2 Antiferromagnet
We report an observation and study of monolayer thick structural defects in the antiferromagnetic (AFM) layered semimetal EuSn2As2. 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 EuSnAs2, that is different from the EuSn2As2 phase of the bulk lattice. The smaller distance between Eu layers in the planar defect as well as odd number of Eu atoms in the formula of the defect (EuSnAs2) and of the defect supercell (Eu7Sn12As14) promote 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 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.