Emergence of Ferromagnetism from Planar Defects in EuSn2As2 Antiferromagnet

TL;DR

This work demonstrates that planar defects in EuSn2As2 host EuSnAs2-like layers (Eu7Sn12As14 per defect unit) that carry uncompensated in-plane ferromagnetic moments. Using TEM, EDX/EELS, and DFT+U calculations, the authors show these nanoscale ferromagnetic inclusions (~3% volume) order below $T_c \approx 18-20$ K while the bulk EuSn2As2 remains antiferromagnetic with $T_N \approx 24$ K, thereby explaining the observed low-field nonlinear magnetization and the AC susceptibility upturn. The findings recast the crystal as a metamaterial with FM nanoinclusions within an AFM matrix, offering a mechanism for FM-like features in vdW layered antiferromagnets and potential spintronic applications. The work combines microstructural characterization with first-principles modeling to link defect chemistry and magnetism in a correlated Eu-based semimetal.

Abstract

We report a study of nano-scale structural peculiarities of the antiferromagnetic layered semimetal EuSn2As2, and show that they are responsible for its puzzling magnetic properties. 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 nano-defects forms a layer 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 nano-defect promotes formation of local ferromagnetic (FM) ordering of the Eu atoms. On average, the planar defects form a weak ferromagnetic phase in the antiferromagnetic (AFM) host lattice. The obtained results explain several puzzling features in magnetic properties of A-type AFM materials: the nonlinear magnetization in low in-plane fields, ferromagnetic-type hysteresis in low field, and the upturn of the magnetic susceptibility in the AFM state at temperatures approaching zero.

Figures (8)

• Figure 1: (a,b) Crystal lattice and magnetic structure of EuSn$_2$As$_2$ in the two projections. The crystal structure of EuSn$_2$As$_2$ is rhombohedral with the space group $R\bar{3}\,m$arguilla_InChemFront_2017. The Eu spin sub-lattice is of A-type antiferromagnetic state with the $ab$ easy magnetization plane (adapted from Ref. golov_JMMM_2022). Black arrows show Eu atoms magnetization directon. (c) Bulk sample view.
• Figure 2: Temperature dependence of the AC-magnetic susceptibility, and of its inverse value (inset) for EuSn$_2$As$_2$ single crystal. Measurements were done with DC field $H=0$ and 0.2 T, applied in the easy $ab$ plane.
• Figure 3: (a) DC-magnetization $M(H)$ for EuSn$_2$As$_2$ crystal at $T=2$K for two field orientations. (b) Magnified low-field interval of the $M(H)$ curves for the 2mg-piece cutout from the same bulk crystal measured at eight temperatures for $H\|ab$ orientation. Upper left inset: $M(H)$ dependence at $T=5K$ and the definitions of the extrapolated to $H=0$ saturation magnetization $\Delta M(H=0)$ and remanence magnetization $M_{\rm rem}$. Lower right inset: temperature dependencies of $\Delta M(H=0)$ and $M_{\rm rem}(H=0)$. Vertical arrow points at $T_N$.
• Figure 4: (a) BF TEM image of EuSn$_2$As$_2$ single crystal with planar defects (one is marked with the white arrow #1). Inset: SAED obtained in $[\bar{5}410]$ zone axis. (b) FFT filtered HR HAADF STEM image of the crystal lattice. Green balls designate Eu atoms, red balls - Sn, and violet - As. The EuSn$_2$As$_2$ unit cells is marked with yellow frame, blue arrow "1" marks the EDXS analysis line and blue rectangle "2" - of the EELS analysis; the simulated image is in red frame with defect area in green frame; (c) the intensity maps obtained from HR HAADF STEM experimental image of defect area and (d) intensity map - from simulated image; (e) EELS spectra obtained from the area marked with blue rectangle 2 on panel (b); (f) DFT simulated crystal structure model Eu$_7$Sn$_{12}$As$_{14}$ of the transition defect block, including the regular layers (A, B) and defect layer itself (C); (g) a part of the HR HAADF STEM image from panel (b) showing correspondence with the simulated crystal structure; (h) triangle phase diagram (convex hull) of the Eu-Sn-As system.
• Figure 5: (a) EDXS Chemi STEM elemental maps in a region including planar defect; (b) EDXS Chemi STEM elemental line profile along blue arrow 1 on Fig. \ref{['fig:TEM']}b.