Orientation-dependent stabilization of $\mathrm{Mg}{\mathrm{Cr}}_2{\mathrm{O}}_4$ spinel thin films

$A{B}_2{\mathrm{O}}_4$ normal spinels with a magnetic $B$ site can host a variety of magnetic and orbital frustrations leading to spin-liquid phases and field-induced phase transitions. Here, we report the epitaxial growth of (111)-oriented $\mathrm{Mg}{\mathrm{Cr}}_2{\mathrm{O}}_4$ thin films. By characterizing the structural and electronic properties of films grown along the (001) and (111) directions, the influence of growth orientation has been studied. Despite distinctly different growth modes observed during deposition, the comprehensive characterization reveals no measurable disorder in the cation distribution nor multivalency issue for Cr ions in either orientation. Contrary to a naive expectation, the (111) stabilized films exhibit a smoother surface and a higher degree of crystallinity than (001)-oriented films. The preference in growth orientation is explained within the framework of heteroepitaxial stabilization in connection to a significantly lower (111) surface energy. These findings open broad opportunities in the fabrication of two-dimensional kagome-triangular heterostructures with emergent magnetic behavior inaccessible in bulk crystals.

Figure 1

(a) Schematic showing Cr atoms in MCO when viewed along (111) planes. (b) RHEED pattern from the (001) surface of MAO before growth. (c) RHEED pattern of MCO(001) grown on MAO(001). (d) RHEED image of the AlO (0001) substrate. (e) RHEED pattern of MCO(111) grown on MAO(111). (f) RHEED pattern of MCO(111) grown on AlO(0001). The inset curve is a vertical line profile. All samples are grown at $550{}^{\circ }\mathrm{C}$ and 2 mTorr of pure oxygen.

Figure 2

(a) Full-range XRD scans of MCO(001) film grown on MAO, MCO(111) film grown on AlO (orange), and MCO(111) film grown on MAO under the same deposition condition. (b) An atomic force microscopy image of MCO(111) grown on the AlO substrate. The averaged rms roughness in the $1\mu \text{m}\times 1\mu \text{m}$ area is 171 pm. (c) RSM of MCO (531) film peak measured on 80-nm MCO (111) film. (d) XRR of the MCO(111) film, whose XRD is shown in (a), along with the fitting result. From the fitting, the film thickness is estimated to be around 23 nm. (e) Zoomed-in XRD scan around (222) peak measured on films grown under various conditions. The inset is the $\omega$ scan of MCO (111) ($550{}^{\circ }\mathrm{C}$, 2 mTorr) around the (222) film peak; the estimated FWHM is $0.{01}^{\circ }$.

Figure 3

(a) XAS of MCO (111) measured at room temperature in comparison to XAS of ${\mathrm{Cr}}_2{\mathrm{O}}_3$ and CrO. ${\mathrm{Cr}}_2{\mathrm{O}}_3$ and CrO XAS were adapted from Ref. [56]. (b) Experimental full-range XPS result for MCO (111) and MCO(001). (c) Zoomed-in Cr $2p_{3/2}$ XPS with fitting. The five main peaks are the fine structure of ${\mathrm{Cr}}^{3+}$ as a result of multipeak splitting, which is standard for chromium (III) oxides. The other small peak is ${\mathrm{Cr}}^0$ due to the surface charging effect. The ${\chi }^2$ of this fit is 0.399. (d) The integrated intensity of the five peaks shown in (c) in percentage, observed in other closely related systems [57, 58] compared to the fitting result of MCO(111) and MCO(100).