Introduction

The 5d transition metal oxides have attracted much attention due to their strong spin-orbit coupling (SOC), which scales with Z4, where Z is the atomic number. Given such strong SOC, exotic phases of matter are predicted to occur, including the topological insulator, Weyl semimetal and chiral spin liquid1,2,3,4. Such materials have potential applications in quantum computing and spintronics5. The family of pyrochlore iridate compounds (A2Ir2O7) are proving to be a promising arena within which to investigate these interesting phases. In some cases, the materials are geometrically frustrated, as the magnetic species occupy a network of corner sharing tetrahedra. The spin liquid phase has been observed in Pr2Ir2O7 down to 0.3 K6 and there is evidence for the Weyl semimetal phase in Rh-doped Nd2Ir2O77 and Eu2Ir2O78. Theoretical studies have shown that the predicted topological insulator and Weyl semimetal phases can be induced by an all-in all-out spin structure of the rare earth (RE) pyrochlore iridates9,10. In addition, there has been experimental observation of metal-to-insulator transitions in some RE pyrochlore iridates (A = Y, Nd, Sm, or Eu) at the same temperature as the magnetic ordering temperature9,11,12,13.

Theoretical studies of the pyrochlore iridate thin films predict that the topological insulator and Weyl semimetal phases can be induced in the pyrochlores from epitaxial strain and finite size effects9,14 providing motivation to synthesize pyrochlore thin films. Previous research reports the synthesis of Bi2Ir2O715 and Eu2Ir2O716 thin films by pulsed laser deposition. In this paper, we report the synthesis of Nd2Ir2O7 using off-axis magnetron sputtering with ex situ post-annealing.

Synthesis

Sputter Target Synthesis

Nd2O3 and IrO2 powders were thoroughly mixed then ground for at least 2 hours in stoichiometric ratios with an excess of 5 mol% IrO2. The samples were pressed into a pellet and placed in an alumina tube, which was sealed in an evacuated silica tube to prevent the iridium from volatilizing as the IrO3 phase17 during heating before forming the ternary pyrochlore Nd2Ir2O7 phase. This would cause the powder to be neodymium rich and lead to the formation of Nd3IrO7 or Nd6Ir2O13 impurity phases (depending on the oxygen level)18,19. After sealing in a silica tube, samples were heated for 160 hours at a temperature T = 950 °C with a ramping rate of 1 °C/min. Afterwards, the samples were taken out of the silica tubes and heated at T = 1125 °C in pure O2 at atmospheric pressure in a tube furnace for several days to volatilize the excess iridium away, leaving stoichiometric pyrochlore powders with a lattice constant of 10.373 Å (see supplementary information for further analysis of the powder).

Thin Film Synthesis

The Nd2Ir2O7 powder was pressed into a 2-inch sputtering target and used for ultra-high vacuum, off-axis magnetron sputtering. Nd2Ir2O7 films were deposited on (111)-oriented 8 mol% yttria-stabilized zirconia (YSZ) substrates in 12.5 mTorr of Ar + 1% O2 atmosphere at room temperature. DC sputtering with a constant current of 60 mA was used, resulting in a deposition rate of 2 nm/minute. After the sputter deposition, the films were crystallized by annealing in atmospheric pressure N2/O2 mixture with oxygen partial pressures ranging from 30 mbar to 200 bar at 750 °C for times ranging from 5 min to 50 hr with ramping rates of 0.5–1 °C/min above 500 °C. Results showed that changing the oxygen partial pressure does not have an obvious effect on the crystal quality; however, annealing at higher temperatures (≥775 °C) results in the formation of the possible Nd3IrO7 or Nd6Ir2O13 impurity phases. A summary of the effect of annealing under different conditions is shown in Table 1. Each film was analyzed with a Bruker D8 Discover triple-axis X-ray diffractometer. Figure 1(a) shows the synchrotron XRD scan of the Nd2Ir2O7 powder converted to the Cu Kα1 wavelength for comparison with Fig. 1(b,c) showing the 2θ-ω XRD scans of a Nd2Ir2O7 film, which only exhibit the (111)-series of the pyrochlore peaks, indicating a phase-pure, (111)-oriented pyrochlore film on YSZ. The out-of-plane lattice constant was calculated to be 10.387 Å for the Nd2Ir2O7 film, which is close to the bulk value of 10.373 Å obtained from the synchrotron XRD data. The inset to Fig. 1(c) shows a rocking curve of the Nd2Ir2O7 (222) peak which reveals a narrow peak with a full-width-at-half-maximum of 0.006° and a broad baseline possibly caused by inhomogeneity in the Nd2Ir2O7 film.

Table 1 Summary of results of annealing amorphous Nd2Ir2O7 films.
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Figure 1
figure 1

(a) θ/2θ XRD scan of Nd2Ir2O7 powders (taken at Argonne National Lab Advanced Photon Source using an X-ray wavelength of 0.413191 Å), which was converted to the Cu Kα1 wavelength of 1.5405 Å for comparison. (b) 2θ-ω XRD scan of a 200-nm thick Nd2Ir2O7 film grown on YSZ (111). The sample was post-growth annealed at 750 °C in pure O2 at atmospheric pressure with a ramp rate of 0.5 °C/min. (c) High resolution XRD scan of the same film around the YSZ (111) and Nd2Ir2O7 (222) peaks. The inset is the rocking curve of the Nd2Ir2O7 (222) peak, which consists of a narrow peak with full-width-at-half-maximum of 0.006° and a broad baseline.

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Microscopy Analysis and Discussion

In order to study the early stages of the growth mechanism of Nd2Ir2O7, an 800 nm Nd2Ir2O7 film was deposited, followed by a 5-minute post-growth annealing at 750 °C (with the same ramp rate specified above). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) of the pyrochlore film was performed using an FEI probe-corrected Titan3 80–300 S/TEM at 300 kV with a detector collection range of 55–375 mrad. Figure 2(a) shows a low magnification HAADF-STEM image of a cross-section of the annealed film. The film was predominantly amorphous with crystallites nucleated at the YSZ interface. Close examination reveals {111} and {001} faceted crystallites [Fig. 2(b)] surrounded by regions exhibiting dark contrast [Fig. 2(c)] in the HAADF image.

To fully understand the nature of the faceted crystals and Nd2Ir2O7/YSZ interface, elemental analysis was performed by energy-dispersive X-ray (EDX) spectroscopy using the FEI Super-X quad-silicon drift detector system. An EDX profile across the crystalline/amorphous Nd2Ir2O7 interface [Fig. 2(d)] was quantified using experimental Cliff-Lorimer k-factors obtained from the Nd2Ir2O7 crystal assuming perfect 2:2:7 stoichiometry. This demonstrates that the as-deposited (amorphous) material was stoichiometric, which implies that the nanocrystals formed during the post-anneal. Note that spectra were collected at specimen orientations far from major crystallographic zone axes to minimize probe-channeling effects that could differ significantly between the amorphous and crystalline materials.

Films that had been post-annealed for significantly longer times (approximately 12 hours) were observed to be completely crystalline. Higher magnification HAADF-STEM images of the Nd2Ir2O7/YSZ interface in these films [Fig. 3(a)] reveals an epitaxial relationship: <111>Nd2Ir2O7 ‖ <111>YSZ and <110>Nd2Ir2O7 ‖ <110>YSZ. In HAADF-STEM or “Z-contrast” imaging, the intensity of an atomic column is approximately proportional to Z1.8. As shown in Fig. 3(b,c), this contrast in the images of the Nd2Ir2O7 film is characteristic of a high degree of pyrochlore ordering. The most intense atomic columns contain only Ir atoms (Z = 77), while the least intense contain only Nd (Z = 60) and the columns of intermediate contrast are alternating Nd and Ir atoms with 1:1 ratio.

Figure 3
figure 3

(a) STEM image of a Nd2Ir2O7 film grown on YSZ (111) looking down the <110> axis. The Nd2Ir2O7 film is ordered in pyrochlore structure and has an epitaxial relationship with the YSZ substrate. (b) High resolution STEM image of the Nd2Ir2O7 film away from the interface shows the pyrochlore ordering of the Nd and Ir sublattices as schematically shown in (c).

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In some regions of the Nd2Ir2O7/YSZ interface, a thin “cloudy” layer was observed in the HAADF-STEM images, as shown in Fig. 4(a). This observation might be mis-interpreted as an amorphous layer at the interface between the YSZ and the Nd2Ir2O7, but this would be inconsistent with the epitaxial relationship described above. In order to demonstrate that this epitaxial relationship remains despite the presence of the “cloudy” layer, the image in Fig. 4(a) was Fourier filtered to enhance the contrast from the crystalline materials. In the Fourier filtered image, the epitaxial relationship is clear [Fig. 4(b)], although the filtering process does reduce the Z-contrast and introduces mathematical artifacts around the edges of the image. HAADF-STEM images acquired from thinner TEM foils, Fig. 5(a), revealed nanocrystals, approximately 1-2 nm in diameter in the interfacial region. The nanocrystals exhibit a cubic crystal structure, with spatial frequencies in the fast Fourier Transform (FFT) that match the lattice parameter of metallic Ir [Fig. 5(b)] and are distinctly different from those of the Nd2Ir2O7 film [Fig. 5(c)]. When the dark contrast observed at the triple junction of the YSZ substrate, amorphous Nd2Ir2O7 and crystalline faceted Nd2Ir2O7 noted in Fig. 2(b), is examined at higher magnification [Fig. 5(d)], it is observed that the Ir nanoparticles are also present [Fig. 5(e)].

Figure 4
figure 4

(a) STEM image revealing a “cloudy” interface in a region of the sample. (b) Fourier filtering of (a) reveals an epitaxial relationship at the interface despite the “cloudy” layer.

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Figure 5
figure 5

(a) High magnification STEM image of nanoparticles at the Nd2Ir2O7/YSZ interface. (b) FFT of the nanoparticle indicated by red arrow in (a) clearly indicates a cubic structure, with spatial frequencies matching the lattice parameter of metallic Ir, which is distinct from (c) the FFT of the pyrochlore Nd2Ir2O7. (d) HAADF STEM image of the bottom-left corner of the island marked by the green box in Fig. 2(b) A saturated image of (d) shows metallic Ir nanoparticles filling the void space at the island growth front. Scale bars on all images are 5 nm.

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The accumulation of Ir nanocrystals, specifically at the triple junction, can help explain the appearance of Ir metal in some regions of the interface but not others due to projection considerations in STEM imaging. It is reasonable to believe that the dark triple junction seen in Fig. 5(d) completely surrounds the base of the crystalline Nd2Ir2O7 island [Fig. 6(a)]. Consequently, some areas of the TEM specimen near the interface may include partially the dark disordered region and partially crystalline Nd2Ir2O7 along the electron beam path, resulting in a “cloudy” interface, as shown in Fig. 6(a–c).

Figure 6
figure 6

(a) Schematic of the crystallite shown in Fig. 2(b) assuming the crystallite forms facets primarily along {111} and {001} planes. Features at the Nd2Ir2O7/YSZ/amorphous triple junction represent Ir nanocrystals. (b) Schematic of the sample after being sliced into a <110> TEM foil with Ir nanocrystals visible only at the edges. (c) STEM image of crystallite with a schematic of the pyrochlore structure overlaid on the crystallite and the miller indices of the facets labeled. This confirms the crystal facets primarily on {111} planes, but also on {001} planes.

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It is clear that the nucleation and growth of crystalline Nd2Ir2O7 films is influenced by multiple factors. During the annealing process, it is common in some systems for nucleation sites to form at all edges of the film and not exclusively at the interface, resulting in the formation of a polycrystalline thin film, such as the case for YSZ grown on sapphire20. In other systems, nucleation sites at the interface grow epitaxially accompanied by the formation of a random polycrystalline film far from the interface, such as the case for YBa2Cu3O7 films grown on SrTiO3 with ex situ annealing21. YSZ is well lattice matched to Nd2Ir2O7 and its cubic fluorite structure is a variant of the pyrochlore structure favoring the formation of nucleation sites exclusively at the interface with an epitaxial orientation. This potentially enables the entire film to be epitaxial as the nucleation sites grow by consuming the surrounding materials until the film is fully crystallized and the epitaxial orientation is well preserved. The curved surfaces that are apparent at the triple junctions [Figs 2(b) and 5(e)] are consistent with wetting phenomena suggesting liquid- or glassy-like behavior at the amorphous-crystalline interface at elevated temperatures. This is worth considering further as the nominal annealing temperature is well below the melting temperature of Nd2Ir2O7. We note that the crystallization of the pyrochlore phase is an exothermic reaction with an enthalpy of formation of 88.7 kJ/mol19. According to the Neumann–Kopp rule22 the heat capacity of Nd2Ir2O7 is

Previous research23 has determined the heat capacity of IrO2 to be

and Nd2O3 has been determined24 to be

Eqs. (1, 2, 3) predict that the energy released from the crystallization could raise the temperature in the vicinity of the interface from 750 °C to about 1030 °C. This is above the decomposition temperature of IrO2 of 1020 °C19, thus causing the formation of metallic Ir nanoparticles near the crystalline/amorphous interface. The high oxygen ion conductivity of YSZ, which peaks at 8 mol% Y2O325, may enhance the formation of Ir nanoparticles by creating an oxygen deficient interfacial layer. It may be possible to prevent such an interfacial layer by using a buffer material to block the oxygen flow to the substrate. In addition, it may be possible to prevent IrO2 decomposition by annealing at lower temperatures; however, crystallizing Nd2Ir2O7 at temperatures below 750 °C is challenging.

Conclusions

In summary, Nd2Ir2O7 powders were synthesized in an evacuated silica tube and their structural properties were analyzed using laboratory as well as synchrotron X-ray diffraction. We report growth of Nd2Ir2O7 epitaxial films using off-axis magnetron sputtering followed by post-annealing. XRD results demonstrate phase-pure epitaxial films grown on YSZ (111). HAADF-STEM images reveal clear pyrochlore ordering. Additionally, Nd2Ir2O7 grows by the formation of epitaxial nucleation sites exclusively at the YSZ interface that slowly absorb the surrounding amorphous material forming a (111)-oriented crystalline film.

Additional Information

How to cite this article: Gallagher, J. C. et al. Epitaxial growth of iridate pyrochlore Nd2Ir2O7 films. Sci. Rep. 6, 22282; doi: 10.1038/srep22282 (2016).