Stabilization of a honeycomb lattice of IrO$_6$ octahedra in superlattices with ilmenite-type MnTiO$_3$

In the quest for quantum spin liquids, thin films are expected to open the way for the control of intricate magnetic interactions in actual materials by exploiting epitaxial strain and two-dimensionality. However, materials compatible with conventional thin-film growth methods have largely remained undeveloped. As a promising candidate towards the materialization of quantum spin liquids in thin films, we here present a robust ilmenite-type oxide with a honeycomb lattice of edge-sharing IrO$_6$ octahedra artificially stabilized by superlattice formation with an ilmenite-type antiferromagnetic oxide MnTiO$_3$. The stabilized sub-unit-cell-thick Mn-Ir-O layer is isostructural to MnTiO$_3$, having the atomic arrangement corresponding to ilmenite-type MnTiO$_3$ not discovered yet. By spin Hall magnetoresistance measurements, we found that antiferromagnetic ordering in the ilmenite Mn sublattice is suppressed by modified magnetic interactions in the MnO$_6$ planes via the IrO$_6$ planes. These findings lay the foundation for the creation of two-dimensional Kitaev candidate materials, accelerating the discovery of exotic physics and applications specific to quantum spin liquids.


Introduction
Recent progress in the physics of quantum spin liquids has intensified search for new candidate materials 1 . In the materials design, an exactly solvable S = 1/2 spin model on a twodimensional honeycomb lattice has given a rigorous theoretical framework, which was firstly proposed by Kitaev 2 and later reformalized with realistic materials by Jackeli and Khaliullin 3 .
One of the major challenges in this research field is to incorporate those materials into thin films, especially atomically thin monolayer forms. Thin-film techniques are not only essential for eventual practical applications but also effective for providing additional degrees of freedom for controlling magnetic interactions in actual materials. For example, epitaxial 4 strain at the interface and dimensionality control [16][17][18] can offer new options to address the issue that the expected Kitaev-type interactions are often broken by the antiferromagnetic ordering due to structural distortion and interlayer coupling [6][7][8][9][10][11][12][13]19,20 . However, it is quite difficult to prepare high-quality films of -RuCl3 (refs. 6,7 ) and iridates with H and Li (refs. [10][11][12][13][14] ) that have so far gained attention as the bulk candidate materials. This is because their volatile and/or diffusible nature must entail extremely careful control of stoichiometry and suppression of inter-diffusion for reproducing the bulk properties. Considering these problems in known Kitaev  and MgIrO3, which possess a honeycomb lattice of edge-sharing IrO6 octahedra in ab plane 19 (see Fig. 1(a) for the ilmenite-type structure). The authors discussed an XY-like magnetic anisotropy and a tilting magnetic structure that is possibly related to  .
In this study, starting from ilmenite-type MnTiO3 for which the single-crystalline film growth on Al2O3(0001) has been established 21,22 , we attempted to stabilize Ir at the B-site of ilmenitetype MnBO3, as shown in Fig. 1(a). By taking advantage of epitaxial strain in superlattices with ilmenite-type MnTiO3, which shares the A-site MnO6 plane, we materialized the honeycomb lattice of edge-sharing IrO6 octahedra in the ilmenite lattice. This ultrathin-film form of iridate will serve as an intriguing two-dimensional platform for pursuing the physics of Kitaev 5 materials.

Results
Structural characterization by x-ray diffraction (XRD). The films were grown on Al2O3(0001) substrates by pulsed-laser deposition using a KrF excimer laser (see Methods).
Our early attempt to grow ilmenite-type MnIrO3 directly on Al2O3(0001) was not successful, resulting in phase-separated films of Mn3O4 and IrO2 (see Fig. S1 in Supporting Information for the XRD pattern). We then took a superlattice approach 16-18 based on ilmenite-type MnTiO3 thin films 21,22 . In the superlattice structure, the common A-site MnO6 plane is expected to promote the ilmenite-type stacking of MnO6 and IrO6 planes along the c-axis direction (the left of Fig. 1(a)). A schematic structure of a typical superlattice is given in Fig. 1(b), where a

Microstructural characterization by scanning transmission electron microscopy (STEM).
Our concept to form honeycomb-lattice IrO6 planes was further evidenced by cross-sectional  Fig. 3(a). 8 Owing to the Z-contrast nature (Z: atomic number) 23 , the bright layers must contain much Ir that has the largest Z among the constituent elements (Mn, Ti, Ir, and O). Also, there are tiny bright island-like regions possibly because of segregated Ir-rich impurities. A close inspection on the single unit of the superlattice (Fig. 3(b)) reveals that a Mn−Ir−O layer between MnTiO3 layers contains a couple of bright atomic planes along the [0001] direction. Using the atomically resolved HAADF image of Fig. 3(c) (the area marked by yellow dashed lines in Fig. 3(b)), we compared each atomic site with a model structure ( Fig. 3(d)). It was reported for MnTiO3 that the MnO6 (TiO6) plane has a larger (smaller) atomic displacement along the [0001] direction between intra-plane Mn (Ti) ions 24 . By measuring the atomic displacement in the image, we identified that Ir atoms are located at the B-site, which is occupied with Ti atoms in MnTiO3, as schematically shown in Fig. 3(d) (also see the left of Fig. 3(c)). In addition, the bright Ircontaining planes are regularly sandwiched between MnO6 planes, with having similar dumbbell-like characteristic atomic arrangement. On the basis of these results, we conclude that a honeycomb lattice of edge-sharing IrO6 octahedra crystallizes in the Mn−Ir−O layer with ilmenite-type atomic ordering. Note here that the Mn−Ir−O layer composed of two pairs of IrO6 and MnO6 planes corresponds to the 2/3 unit cell (u.c.) of the ilmenite lattice ( Fig. 1(a)). We also conducted electron energy loss spectroscopy (EELS) for Mn L, Ti L, and O K edges in the MnTiO3 buffer and Mn−Ir−O layer regions (Fig. S3). While the Mn L-edge spectra agree well between the two regions (i.e., Mn 2+ ), partial reduction of Ti 4+ to Ti 3+ occurs around the 9 Mn−Ir−O layer 25,26 . Associated with this reduction, the existence of oxygen deficiency is suggested from the O K-edge spectra. We infer that Mn 2+ and Ir 4+ are stable under the oxygen pressure of 10 mTorr used for the deposition, whereas Ti 4+ is slightly reduced to Ti 3+ . Charge transfer between B-site TiO6 and IrO6 planes via A-site MnO6 planes may be relevant to this point, though it is not clear at this stage.

Investigation of surface magnetic order by spin Hall magnetoresistance measurements.
Judging from these structural analyses, a honeycomb lattice of edge-sharing IrO6 octahedra should partly form a two-dimensional network in the superlattice. While some iridates are known to exhibit metallic conduction [27][28][29][30][31] 22,33,34 ). In fact, R for both the thick-MnTiO3 monolayer ( Fig. 4(a)) and the trilayer C (purple in Fig. 4(c)) exhibits the sudden increase around T = 60 K upon heating, around which bulk MnTiO3 undergoes the antiferromagnetic transition 36 . These results indicate that antiferromagnetic order develops in MnTiO3/Al2O3(0001) structure. In stark contrast, R for the bilayer (Fig. 4(b)), and the trilayer A and B (green and light blue in Fig. 4(c)) exhibits the monotonous increase with increasing T without clear anomalies. Although A-site spinful MnO6 planes are common to all samples, 11 magnetic ordering behavior is thus completely different, which we believe reflects magnetic interactions in Mn−Ir−O, i.e., the unique stacking of honeycomb-lattice IrO6 and MnO6 planes.

Discussion and Conclusions
Comparing the SMR results, we discuss responses. Another support to the negligible role of surface roughness is that the 2-u.c.-thick thin-MnTiO3 monolayer, despite its island-like film morphology with increased surface roughness, exhibits the bulk-like TN as in thicker and much flatter samples (≥ 3 u.c.) 22 . Secondly, the total film thicknesses of the heterostructures (including MnTiO3 and Mn−Ir−O layers) are much thicker than the 2 u.c. so that a sufficiently large volume of the Mn sublattice is ensured for the whole heterostructure. This also helps minimize the possible influence of domain disconnection, which generally becomes pronounced in ultrathin films. In stark contrast, the systematic recovery of antiferromagnetic SMR responses with an increase of the thickness of MnTiO3 top layer, from the trilayer A (2/3 u.c.), B (4/3 u.c.), to C (3 u.c.), signals the intrinsic origin that is responsible for spin interactions in a characteristic length. Notably, the striking difference between the antiferromagnetic 2-u.c.-thick thin-MnTiO3 monolayer 22   films were characterized by XRD using Cu K  radiation and EDX, respectively.

SMR measurements.
A Pt film with a thickness of approximately 2 nm was deposited on the film surface by radio-frequency magnetron sputtering at 150 °C. The heterostructured film was then patterned into an L-shaped multi-terminal device structure using photolithography and Arion milling, followed by electron-beam evaporation of Au/Ti electrodes. Resistance was measured by the four-probe method using a semiconductor parameter analyzer (Agilent 4155C) and nano-volt meters (Keithley 2182A) in a physical property measurement system (Quantum Design Inc.) equipped with a one-axis sample rotator. Details for the measurement scheme and analysis were reported in ref. 22 .

Electron microscopy.
To obtain an electron-transparent thin specimen, the grown thin film with the substrate was mechanically polished, and Ar-ion beam milling was performed at 0.5 kV in the final stage. For the atomic and electronic structure analyses, an aberration corrected STEM system (JEOL ARM300CF) was used, equipped with a DELTA corrector, a cold field emission gun, and an EELS spectrometer (Quantum, Gatan Inc.), operated at 300 kV. The probe forming aperture was 30 mrad and the collection semi-angle for HAADF was 85-200 mrad.