Coherent epitaxy of trilayer nickelate (Nd0.8Sr0.2)4Ni3O10 films by high-pressure magnetron sputtering

Rare-earth (R) nickelates (such as perovskite RNiO3, trilayer R4Ni3O10, and infinite layer RNiO2) have attracted tremendous interest very recently. However, unlike widely studied RNiO3 and RNiO2 films, the synthesis of trilayer nickelate R4Ni3O10 films is rarely reported. Here, single-crystalline (Nd0.8Sr0.2)4Ni3O10 epitaxial films were coherently grown on SrTiO3 substrates by high-pressure magnetron sputtering. The crystal and electronic structures of (Nd0.8Sr0.2)4Ni3O10 films were characterized by high-resolution X-ray diffraction and X-ray photoemission spectroscopy, respectively. The electrical transport measurements reveal a metal-insulator transition near 82 K and negative magnetoresistance in (Nd0.8Sr0.2)4Ni3O10 films. Our work provides a novel route to synthesize high-quality trilayer nickelate R4Ni3O10 films.

Here, high-quality NSNO 4310 epitaxial films were synthesized on SrTiO 3 (STO) substrates by high-pressure magnetron

MATERIALS AND METHODS
2-inch Nd 0.8 Sr 0.2 NiO 3 polycrystalline targets were synthesized by a standard solid-state reaction method with initial reactants consist of Nd 2 O (Aladdin, 99.99%), SrCO 3 (Aladdin, 99.99%), and NiO(Aladdin, 99.99%).The mixture was calcined at 1350 • C for 8 h and reground for 6 h.Then the powders were pressed into a 2-inch target with 30 MPa and annealed at 1300 • C for 2 h.The high-quality NSNO 4310 thin films (∼ 20 nm) were synthesized by high-pressure radio frequency (RF) magnetron sputtering (home-made) with a 2-inch target and the O 2 (purity of 99.999%) reactive gas [43,44].Before growth, the base vacuum pressure was ∼ 3 ×10 −8 torr.During growth, O 2 pressure was kept at 0.02 Torr with a gas flow of 1.8 sccm, and the substrate temperature was held at 550 • C. The power of the RF generator was kept at 60 W. To ensure the uniformity of the films, the heating stage was rotating at a speed of 5 rams/min during growth.After growth, the films were cooled down to room temperature at 25 • C per minute in the 0.02 Torr O 2 atmosphere.A topochemical reduction process transforms NSNO 4310 films into NSNO 438 films.The NSNO 438 films and reducing agent CaH 2 were sealed in an evacuated quartz tube and heated at 280 • C for 6 hours.
The crystal structure of NSNO 4310 films was characterized by the High-resolution X-ray diffractometer (Bruker D8 Discovery) with the Cu K α source (λ = 1.5405Å).The 2θ-ω scans and asymmetrical reciprocal space mappings were performed to reveal the coherent growth and lattice parameters of films.The electronic structure of the films was probed by PHI 5000 Versa Probe x-ray photoelectron spectroscopy (XPS) at an acceptance angle of 45

RESULTS AND DISCUSSIONS
Figure 1 shows the crystal structures of NSNO 113 and NSNO 4310 .NSNO 113 has a symmetry of orthorhombic structure with the lattice parameters a = 5.39 Å, b = 5.38 Å,c = 7.61 Å (the pseudocubic lattice parameter is ∼ 3.81 Å).It is noted that the replacement of Nd by Sr atoms via doping has little impact on the symmetry of crystal structure and the lattice parameters [45].
In contrast, the NSNO 4310 has a monoclinic symmetry (space group P2 1 /a) with the lattice parameters a = 5.365 Å, b = 5.455 Å, c = 27.418Å, and β = 90.31• at room temperature [34,46].First, we performed HRXRD to characterize the crystal structure of NSNO 4310 films.The wide-range 2θ-ω scans in Figure 2 (A) show that the (220) diffraction peak of NSNO 4310 is along with the (00L) diffraction peaks of STO substrates without any secondary phases, whereas the (110) peak disappears due to the symmetry [46], indicating that NSNO 4310 film is highly textured on STO substrates.The amplified view near (002) diffraction of STO substrates (see Figure 2 (B)) shows a film peak ∼ 47.46 • , corresponding to a (110) layer spacing (∼ 3.828 Å) of NSNO 4310 films, which agree well with the results in the literature [14,46].Reducing NSNO 4310 films by CaH 2 can generate (Nd 0.8 Sr 0.2 ) 4 Ni 3 O 8 (NSNO 438 ) with diffraction angle at ∼ 46.47 • , which is too close to the STO (002) peak near 46.47 • to be seen [14,47].Hence, no significant feature is observed in  From RSM, it is estimated that the (110) layer spacing is ∼ 3.828 Å, agreeing very well with the value extracted from the (220) diffraction peak of NSNO 4310 films.Thus, the RSM data further confirms the high-quality of epitaxial NSNO 4310 films.
To further characterize the chemical composition of the NSNO 4310 film, the XPS characterization was carried out.Figure 3 shows a wide-energy core-level XPS spectrum from 0 to 1300 eV.As seen, besides adsorbed carbon on the NSNO 4310 film surface, no detectable impurity signal is observed.Figure 4 (A) shows the temperature-dependent resistivity of NSNO 4310 films from 2.5 K (1.88 mΩ • cm) to 300 K (1.59 mΩ • cm).There is a metal-insulator transition at ∼ 82 K.It is noted that the resistivity remains small at the range of 2.5 -300 K, indicating a metallic behavior.Magnetoresistance (MR = ∆ρ/ρ 0 × 100%) at 2.5 K was also measured (see Figure 4 (B)).The negative magnetoresistance of NSNO 4310 films has also been observed in the bulk [48], resulting from the weak localization at

2 ) 4
Ni 3 O 10 (NSNO 4310 ) films as a representative material to investigate the synthesis and the properties of R 4 Ni 3 O 10 .The trilayer nickelate R 4 Ni 3 O 10 belongs to the n = 3 memeber of the Ruddlesden-Popper (RP) series of R n+1 Ni n O 3n+1 , which shows a rich phase diagram of novel quantum states

Figure 3 .
Figure 3. XPS spectra of NSNO4310 films on STO single-crystalline substrates at room temperature.

Figure 2 (
Figure 2 (A) for NSNO 438 films.Next, to characterize the in-plane lattice parameters of NSNO 4310 films, we measured the reciprocal space mapping (RSM).

Figure 2 (
Figure 2 (C) shows the RSM recorded around asymmetric STO (103) diffraction.The reciprocal lattice vectors Q x and Q z low temperature and being different from the electrical behavior of La 4 Ni 3 O 10 [49] and Pr 4 Ni 3 O 10 [33].
In this work, high-quality (Nd 0.8 Sr 0.2 ) 4 Ni 3 O 10 films have been synthesized on SrTiO 3 by high-pressure magnetron sputtering.The crystal and electronic structures of (Nd 0.8 Sr 0.2 ) 4 Ni 3 O 10 films are characterized by XRD and XPS.The electrical transport measurements reveal a metal-insulator transition and negative magnetoresistance.Our work provides a novel route to synthesize high-quality trilayer nickelate R 4 Ni 3 O 10 films.