Magnetic structure of EuZn$_2$Sb$_2$ single-crystal thin-film

TL;DR

The paper investigates how Eu magnetism in EuZn2Sb2 tunes its electronic topology, predicting distinct topological phases for AFM vs FM states and mapping the magnetic order in thin-film crystals. Using DFT with GGA+U and SOC, the authors show AFM yields Dirac TCI or Dirac DSM and FM yields Weyl nodes. Resonant X-ray elastic scattering reveals a coexistence of bulk AFM and surface FM, consistent with surface Weyl states and lower-layer Dirac/TCI behavior. The findings highlight the critical role of surface oxidation in stabilizing surface FM and suggest EuZn2Sb2 as a platform for layer-selective topological phases.

Abstract

Magnetic topological materials are a class of compounds which can host massless electrons controlled by the magnetic order. One such compound is EuZn$_2$Sb$_2$, which has recently garnered interest due to its strong interplay between the Eu magnetism and charge carriers. However the topology of the electronic band structure, which depends on the ground state magnetic configuration of the europium sublattice, has not been determined. Based on our \textit{ab-initio} calculations, we find that an in-plane and out-of-plane \textit{A}-type antiferromagnetic (AFM) order generates a topological crystalline insulator and Dirac semimetal respectively, whereas a ferromagnetic (FM) order stabilizes a Weyl semimetal. Our resonant x-ray elastic scattering measurements of single-crystal thin film EuZn$_2$Sb$_2$ reveal both a sharp magnetic peak at $\textit{\textbf{Q}}$=$(0,0,\frac{1}{2})$ and broad $\textit{\textbf{Q}}$=$(0,0,1)$ below $T_{\mathrm{N}}=12.9$\,K, which is associated with an \textit{A}-type AFM and FM order, respectively. Our measurements indicate that the FM and AFM layers are spatially separated along the crystal $c$ axis, with the former limited to the top three atomic layers. We propose that EuZn$_2$Sb$_2$ behaves as a Weyl semimetal in the surface FM layers, and as a topological crystalline insulator in the lower AFM layers.

Figures (4)

• Figure 1: a Trigonal crystal structure of EuZn$_2$Sb$_2$, showing $P\bar{3}m1$ space group. b The corresponding bulk Brillouin zone of EuZn$_2$Sb$_2$. Red lines indicate the irreducible Brillouin zone with high symmetry points labelled. c Schematic diagram of REXS set-up, showing the incident and reflected x-ray (green arrow), and Bragg reflection peak of the structure (black), FM order (blue), and AFM order (red).
• Figure 2: (a--d) Possible magnetic structures of EuZn$_2$Sb$_2$ with the Eu$^{2+}$ magnetic moments represented by black arrows and (e--h) corresponding electronic band structure along $\Gamma - \mathrm{A}$ high symmetry line, where the dashed lines at 0 eV denote the Fermi energy ($E_{\mathrm{F}}$). Magnetic structure and corresponding band structure of a, e in-plane AFM with doubly-degenerate bands and gapped Dirac; b, f in-plane FM with Weyl nodes; c,g out-of-plane AFM with doubly-degenerate bands and Dirac point; d, h out-of-plane FM with Weyl nodes.
• Figure 3: a REXS intensity in arbitrary units (arb. units) as a function of $(0, 0, l)$ at various temperatures, showing the emergence of peaks attributed to AFM order at $l = 0.5$ (red) and FM order at $l = 0.95$ (blue) from $12$ K onwards. b REXS intensity as a function of temperature along the $l = 0.5$ and $l = 0.95$ line respectively, clearly showing the emergence of AFM order at $T_\mathrm{N} = 12.9$ K and FM order at $T_\mathrm{C} \approx 13$ K.
• Figure 4: Laue oscillations of AFM peak at $(0, 0, \frac{1}{2})$ and structural peak at $(0, 0, 1)$, both measured at $3.5$ K. Inset diagrams depict the thickness measured by the respective oscillations. a REXS intensity of $(0, 0, \frac{1}{2})$ AFM peak measured at the slightly off-resonance condition at $E_i = 1.128$ keV, using $\pi$-polarized x-rays. This measures thickness of AFM layer. b Non-resonant x-ray scattering intensity of $(0, 0, 1)$ structural peak measured at $E_i = 1.121$ keV, using $\sigma$-polarized x-rays. This measures total number of layers.