Epitaxial stabilization of magnetic GdAuSb/LaAuSb superlattices

Abstract

We report the epitaxial stabilization of GdAuSb films and GdAuSb/LaAuSb superlattices via molecular beam epitaxy on (0001)-oriented Al$_{2}$O$_{3}$ substrates. GdAuSb crystallize in the Au-Au dimerized YPtAs structure type (space group $P6_{3}/mmc$), the same structure as the Dirac semimetal LaAuSb. Angle-resolved photoemission spectroscopy (ARPES) measurements show similar near $E_F$ bandstructures for GdAuSb and LaAuSb, plus a rigid band shift for GdAuSb towards more hole-like behavior and core-like Gd $4f$ states $\sim 9$~eV below the Fermi energy. LaAuSb/GdAuSb superlattices exhibit sharp superlattice fringes by X-ray diffraction and atomically-precise interfaces by scanning transmission electron microscopy. Superlattices display two transitions in temperature-dependent resistvity, compared to a single Néel temperature for thick GdAuSb films. Superlattices of $Ln$AuSb materials ($Ln=$ rare earth) with atomically abrupt interfaces offer a new epitaxial platform for control of magnetic and topological order via tunable intralayer exchange and reduced dimensionality.

Figures (4)

• Figure 1: (a) Survey $\omega$-2$\theta$ x-ray diffraction scan comparing the new GdAuSb compound to the previously reported LaAuSb alloy. 000l reflections are labeled, substrate reflections are labeled with asterisks (*). (b) Zoom-in on (0002) reflection for GdAuSb and LaAuSb films to highlight the pronounced Keissig fringes. (c) Rocking curve about the GdAuSb (0002) and Al$_{2}$O$_{3}$ (0006) reflections displaying the low mosaicity exhibited in the film. (d) Schematic crystal structures of the 19-valence LnAuSb compounds crystallizing in the YPtAs structure.
• Figure 2: Electronic structure of thick GdAuSb and LaAuSb films.(a) Angle-integrated photoemission spectroscopy of the valence bands for GdAuSb and LaAuSb. The Gd 4f states are on a split scale at 0.1x the original scale to fit the states into the same plot. (b) Angle-resolved photoemission spectroscopy (ARPES) for LaAuSb (left) and GdAuSb (right) taken along $\overline{\Gamma}-\overline{M}$ directions, as indicated by the red line in (c). The overlaid traces are simulated band structure calculated using the WIEN2K DFT package wien2k. The colors refer to specific $k_z$ values ranging from 0 ($\Gamma$, red) to 0.385 Å$^{-1}$ ($A$, blue) in equal steps of 0.064 Å$^{-1}$(c) Fermi surface for LaAuSb and GdAuSb with schematic projected surface Brillouin Zone with high symmetry points $\overline{\Gamma}, \overline{M},$ and $\overline{K}$ labeled. The red line represents the cut taken for the dispersion presented in b. The 3-Dimensional Brillouin Zone is also presented with the high symmetry points of the hexagonal system labeled. The color plots are DFT calculations for the Fermi surface showing the minimal dispersion along the $\Gamma$-A direction.
• Figure 3: (a) Exemplary long-range $\theta$-$2\theta$ out-of-plane XRD scan of [(LaAuSb)$_{8}$/(GdAuSb)$_{8}$] superlattice structure grown on a sapphire (0001) substrate. (b) Cross-sectional STEM image taken along a $[10\overline{1}0]$ direction on the $[0001]$ zone axis of the same [(LaAuSb)$_{8}$/(GdAuSb)$_{8}$] superlattice structure in (a). The dotted yellow circles highlight stacking fault defects and the sharp horizontal white lines we believe to be due to Gd-rich off-stoichiometries. The dark and light stripes are the LaAuSb and GdAuSb layers, respectively. The red arrows denote a line defect that traces through the frame of the image. (c) Zoom in on region defined in (b). The LaAuSb and GdAuSb layer thicknesses are presented and the inset shows a zoom-in on the GdAuSb crystal structure with an overlaid ball-and-stick model to signify the atoms. In the schematic, green atoms are Gd, purple atoms are Sb, and gold atoms are Au.
• Figure 4: a Longitudinal resistivity versus temperature for GdAuSb, LaAuSb, and n=2, m=6 superlattice structure. The resistivity is normalized to the value at 273 K for all three samples to remove effects from interface scattering in the superlattice structure. We observe one transition at 17.85 K in both the pure GdAuSb and superlattice samples, and in the latter case a second transition is observed at 6.13 K. b Presents the derivative of the sheet resistance with respect to temperature for both the superlattice (blue circles) and the pure bulk-like GdAuSb film (red triangles). The transition temperatures were extracted by fitting the two peaks as shown by the black lines where the dashed lines are the individual peaks. We note an asymmetry in the peak structure in $T_\textup{N1}$ that appears in both samples is not present in the lower temperature transition, thus we attribute this to the thick GdAuSb buffer layer and not an effect caused by the superlattice. The schematic of the superlattice presents the different layer thicknesses and how changing LaAuSb spacer layer thickness affects the interlayer exchange coupling between GdAuSb layers. c Magnetization measurements of pure GdAuSb thin films in a zero-field cooled (black) and field cooled (red) configuration. Field cooling is conducted at 0.4 T, and the data was taken during warm-up. A clear antiferromagnetic transition is observed at 18 K, which agrees well with the transition $T_\textup{N1}$ observed in the longitudinal resistivity measurement.