Self-assembling behavior and interface structure in vertically aligned nanocomposite (Pr0.5Ba0.5MnO3)1-x:(CeO2)x films on (001) (La,Sr)(Al,Ta)O3 substrates

Heteroepitaxial oxide-based nanocomposite films possessing a variety of functional properties have attracted tremendous research interest. Here, self-assembled vertically aligned nanocomposite (Pr0.5Ba0.5MnO3)1-x:(CeO2)x (x = 0.2 and 0.5) films have been successfully grown on single-crystalline (001) (La,Sr)(Al,Ta)O3 substrates by the pulsed laser deposition technique. Self-assembling behavior of the nanocomposite films and atomic-scale interface structure between Pr0.5Ba0.5MnO3 matrix and CeO2 nanopillars have been investigated by advanced electron microscopy techniques. Two different orientation relationships, (001)[100]Pr0.5Ba0.5MnO3//(001)[1-10]CeO2 and (001)[100]Pr0.5Ba0.5MnO3//(110)[1-10]CeO2, form between Pr0.5Ba0.5MnO3 and CeO2 in the (Pr0.5Ba0.5MnO3)0.8:(CeO2)0.2 film along the film growth direction, which is essentially different from vertically aligned nanocomposite (Pr0.5Ba0.5MnO3)0.5:(CeO2)0.5 films having only (001)[100]Pr0.5Ba0.5MnO3//(001)[1-10]CeO2 orientation relationship. Both coherent and semi-coherent Pr0.5Ba0.5MnO3/CeO2 interface appear in the films. In contrast to semi-coherent interface with regular distribution of interfacial dislocations, interface reconstruction occurs at the coherent Pr0.5Ba0.5MnO3/CeO2 interface. Our findings indicate that epitaxial strain imposed by the concentration of CeO2 in the nanocomposite films affects the self-assembling behavior of the vertically aligned nanocomposite (Pr0.5Ba0.5MnO3)1-x:(CeO2)x films.


Results and Discussion
PBMO zone axis, which shows the existence of two types of OR between CeO 2 and PBMO in the nanocomposite films. It is known that under the HAADF imaging conditions, the atomic columns appear dots in a dark background, and the intensity (I) of bright dots is roughly proportional to the square of the atomic number (Z) of the atom column 24 . The CeO 2 nanopillars have a bright contrast in the PBMO matrix. It is found that CeO 2 /PBMO interface can be either semi-coherent or coherent along the film-growth direction, as shown by yellow dashed lines and by red dashed lines, respectively. Interfacial dislocations are visible at the semi-coherent interface, as demonstrated by a horizontal yellow arrow. The atom-scale structure of the semi-coherent PBMO/CeO 2 interface has been investigated by EDS element mapping 28 . Fig. 2b is a typical high-resolution HAADF-STEM image of the PBMO/CeO 2 interface. The corresponding EDS maps of Mn, Ba, Ce and Pr are shown in Figs. 2c−f, respectively. In the PBMO matrix, Pr and Ba cations site at the same atomic columns, indicating that A-site disordered PBMO is obtained. According to the EDS measurements, no elemental segregation at the PBMO/CeO 2 interface. In the CeO 2 nanopillar, Pr and Ce site at the same atomic columns, implying that Pr 3+ ions dope into CeO 2 and partially replace Ce 4+ ions. The substitution of Pr 3+ in Ce 4+ can result in the formation of (Ce,Pr)O 2-δ , and oxygen vacancies generated in the (Ce,Pr)O 2-δ In other words, interface reconstruction occurs at the PBMO/CeO 2 interface, resulting in the formation of a single unit-cell thickness of A-site ordered PBMO structure. It is worth mentioning that the distortion of MnO 6 octahedra is different between A-site ordered and disordered PBMO 34 . In addition, the A-site ordered PBMO occurs a ferromagnetic-paramagnetic transition at about 320 K, while A-site disordered PBMO has T C ≈ 140 K 25 .
It is worth noting that in our work the VAN (PBMO) 1-x :(CeO 2 ) x (x = 0.2 and 0.5) films coherently grow on the LSAT substrates. For the CeO 2 nanopillars embedded in the PBMO matrix with the OR-I, with the change of the molar ratio (x) of CeO 2 to PBMO, the strain of the VAN (PBMO) 1-x :(CeO 2 ) x films can be estimated by  www.nature.com/scientificreports www.nature.com/scientificreports/ is in agreement with the experimental observations that no OR-II occurs between CeO 2 and PBMO in the VAN (PBMO) 0.5 :(CeO 2 ) 0.5 film.
Compare with A-site disordered PBMO, A-site ordered PBMO has a relative low Ms and high magnetoresistance at low temperatures 25 . Nevertheless, considering a very small volume fraction (~20%) of the A-site ordered PBMO in the (PBMO) 0.8 :(CeO 2 ) 0.2 film, the magnetic properties (e.g., M s ) of the (PBMO) 1-x :(CeO 2 ) x films on the LSAT substrates would be mainly affected by the epitaxial strain imposed by the CeO 2 nanopillars within the films 23 . In other words, the volume fraction of CeO 2 and the crystallographic OR between CeO 2 and PBMO in the VAN films change the strain state and the magnetic properties of the PBMO film 22,23 . In addition, it was found that the electrical resistivity of the VAN (PBMO) 0.5 :(CeO 2 ) 0.5 film is over 4 times larger than that of the pure PBMO film in our previous work 22 . It is believed that the vertical semi-coherent phase boundary can increase the difficulty of charge carriers transport, and result in an increase of resistivity of the film system 9,36 . The appearance of A-site ordered PBMO at the coherent PBMO/CeO 2 interface could lead to a decrease of electrical  www.nature.com/scientificreports www.nature.com/scientificreports/ resistivity since A-site ordered PBMO has two orders lower electrical resistivity than A-site disordered PBMO 25 . But, the A-site ordered PBMO in the (PBMO) 0.8 :(CeO 2 ) 0.2 film has a very small volume fraction, which could not strongly affect the resistivity of the VAN (PBMO) 0.8 :(CeO 2 ) 0.2 film 23 . Importantly, our work demonstrates that the epitaxial strain can lead to the formation of A-site ordered PBMO at the heterointerface, which provides a strategy to fabricate A-site ordered PBMO thin films on the substrates (e.g., CeO 2 /YSZ buffered Si substrates 37,38 ) by using strain engineering in the heterosystems.
In summary, the VAN (PBMO) 1-x :(CeO 2 ) x films prepared on (001)-oriented LSAT substrates have been systematically studied by advanced electron microscopy. While the VAN  and PBMO. In addition, interface reconstruction occurs at the coherent PBMO/CeO 2 interface, resulting in the formation of a single unit-cell-thick layer of A-site ordered PBMO at the interface. Our results demonstrate that self-assembling behavior of the nanocomposite (PBMO) 1-x :(CeO 2 ) x films can be modulated by epitaxial strain.

Material and Methods
Thin film preparation. The composite targets of (PBMO) 1-x :(CeO 2 ) x (x = 0.2 and 0.5) were sintered by a standard ceramic sintering method. The (PBMO) 1-x :(CeO 2 ) x films were fabricated on (001) LSAT single-crystalline substrates by a KrF (wavelength λ = 248 nm) excimer pulsed laser deposition system with laser energy density of 2.0 J cm −2 at 3 Hz. During the film deposition, oxygen pressure is 250 mTorr and substrate temperature is 800 °C.
Thin film characterization. Cross-sectional transmission and scanning transmission electron microscopy (TEM/STEM) specimens were prepared by focused ion beam (FIB) milling (FEI Helios NanoLab 600i) 39 . Diffraction contrast imaging, selected-area electron diffraction (SAED), high-angle annular dark-field (HAADF) and annular bright-field (ABF) imaging, energy dispersive X-ray spectroscopy (EDS) mapping and electron energy-loss spectroscopy (EELS) mapping were carried out on a probe aberration-corrected JEOL JEM-ARM200F equipped with an Oxford X-Max N 100TLE spectrometer and a Gatan Enfina spectrometer, operated at 200 kV. In STEM mode, a probe size of 0.1 nm at semi-convergence angle of 22 mrad was used. HAADF and ABF detectors covered angular ranges of 90-176 and 11-22 mrad, respectively. EELS collection angle was 72 mrad and energy resolution was 1.2 eV at the dispersion of 0.3 eV/pixel.

Data availability
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