Formation of RuddlesdenâPopper Faults and Their Effect on the

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Formation of Ruddlesden–Popper faults and their effect on magnetic properties in Pr Sr CoO thin films. 0.5

0.5

3

Hongmei Jing, Sheng Cheng, Shao-Bo Mi, Lu Lu, Ming Liu, Shao-Dong Cheng, and Chunlin Jia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16341 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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ACS Applied Materials & Interfaces

Formation of Ruddlesden–Popper faults and their effect on magnetic properties in Pr0.5Sr0.5CoO3 thin films Hong-Mei Jinga,b, Sheng Chengb, Shao-Bo Mia∗, Lu Lub, Ming Liub, Shao-Dong Chenga,b, Chun-Lin Jiaa,b,c a

State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China b

c

School of Microelectronics, Xi'an Jiaotong University, Xi'an 710049, China

Ernst Ruska Center for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich, 52425 Jülich, Germany

Abstract: Epitaxial

Pr0.5Sr0.5CoO3

thin

films

have

been

grown

on

single-crystalline

(La0.289Sr0.712)(Al0.633Ta0.356)O3 (001) substrates by the pulsed laser deposition technique. The magnetic properties and microstructure of these films are investigated. It is found that Ruddlesden–Popper faults can be introduced in the films by changing the laser repetition rate. The segregation of Pr at the Ruddlesden–Popper faults is characterized by atomic-resolution chemical mapping. The formation of the Ruddlesden–Popper faults not only contributes to epitaxial

strain

relaxation,

but

also

significantly

decreases

the

ferromagnetic

long-range-order of the films, resulting in lower magnetizations than that of the fault-free films. Our results provide a strategy for tuning magnetic properties of cobalt-based perovskite films by modifying the microstructure through the film growth process. Keywords: Thin films; Electron microscopy; Microstructure; Ruddlesden–Popper faults; Magnetic property 1. Introduction Perovskite-based rare-earth cobaltates Ln1-xSrxCoO3-δ (Ln= lanthanide ion) have received significant attention due to their specific magnetic properties1-3 and ionic conductivity for potential applications in solid oxide fuel cells (SOFCs) 4-6, thermoelectric materials7, oxygen separation membranes8,9, sensors10 and catalysts11. Recently, A-site doped rare earth cobaltates Pr1-xSrxCoO3-δ have been extensively studied because of its high electrical and oxygen ion conductivity12. It has been shown that in the Pr1-xSrxCoO3 series, the half-doped

∗ Corresponding author at: State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, PR China. Email address: [email protected] 1

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Pr0.5Sr0.5CoO3 (PSCO) possesses the highest magnetization and exhibits a ferromagnetic metallic behavior below 230 K, around which phase transition occurs from a paramagnetic (PM) phase to a ferromagnetic (FM) phase13-15. In addition, the polycrystalline PSCO ceramics were found to have a second magnetic transition (TS ≈120 K), which was considered as the result of coupling between structural and magnetocrystalline anisotropy instabilities16-18. Nevertheless, this magnetostructural transition (TS) was not reported in the epitaxial Pr0.5Sr0.5CoO3 films19. In contrast to the extensive studies on the PSCO bulk materials, investigations on the microstructure and physical properties of the PSCO films were very limited. In particular, understanding of the structure-property relationship in the PSCO films is highly desired in order to control their properties for potential applications, since the physical properties of cobalt-based perovskite films, e.g. the (Y, Sr)CoO3-δ films, have been shown to depend strongly on the changes of the film growth conditions20. In

this

work,

we

have

prepared

thin

films

of

PSCO

on

the

(La0.289Sr0.712)(Al0.633Ta0.356)O3(001) (LSAT) substrates at different laser repetition rates. The magnetic transport properties of the films show evident differences depending on the laser repetition rates. The microstructure of these films has been investigated by high-resolution X-ray diffraction (XRD) and advanced electron microscopy imaging techniques. By relating the microstructure to the magnetic properties of the PSCO films prepared at laser repetition rate of 3 Hz and 5 Hz, we find that the formation of the Pr-rich Ruddlesden–Popper faults in the film prepared at 5 Hz plays a significant role in modifying the properties of the PSCO films. 2. Method Preparation of thin film A KrF excimer pulsed laser deposition (PLD) system with a wavelength of 248 nm was employed to fabricate PSCO thin films on (001) LSAT substrates using a pulse repetition rate (PRR) of 3 Hz and 5 Hz, respectively. Other parameters for film growth were the same, including the substrate temperature (900 °C), oxygen pressure (250 mTorr), target-substrate distance (10 cm), energy density (2.0 J/cm2) and laser repetition time (4500 shots). After the film growth, the as-grown PSCO films were annealed at 900 °C for 15 min at a pressure of 400 Torr in a pure oxygen atmosphere before being cooled to room temperature at a rate of 30 °C/min. Characterization of structural and electrical properties 2


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The crystallinity of the PSCO films was analyzed by high-resolution XRD using PANalyticalX’Pert MRD. For (scanning) transmission electron microscopy ((S)TEM) investigations, plan-view specimens were prepared by traditional ion milling method21, and cross-sectional specimens were prepared by focused ion beam (FIB) method in a dual-beam scanning electron microscope (SEM) (FEI Helios600i FIB/SEM)22. Bright-field TEM images and selected area electron diffraction (SAED) patterns were recorded on a JEOL 2100 microscope. Atomic-resolution high-angle annular dark-field (HAADF) and annular bright-field (ABF) images were obtained on a JEOL ARM 200F microscope with a probe aberration corrector, operated at 200 kV. In STEM mode, a probe size of 0.1 nm and convergence semi-angle of 22 mrad were used for HAADF and ABF imaging experiments. The HAADF and ABF detector covered angular range is 90–176 and 11–22 mrad, respectively. The energy-dispersive X-ray spectroscopy (EDS) was performed on a JEOL ARM 200F microscope equipped with Oxford X-MaxN 100TLE spectrometer. The magnetic transport properties of the PSCO films were determined using a physical property measurement system (PPMS, Quantum Design). The applied magnetic field is 500 Oe in a field-cooled (FC) condition. 3. Results and discussion 3.1 Microstructure of PSCO films PSCO has an orthorhombically distorted perovskite-type structure at room temperature23, which can be treated as a pseudo-cubic perovskite structure with a lattice parameter of 0.3799 nm in facilitating discussion, as a schematic drawing of the PSCO unit cell in Figure 1a. The Pr/Sr atoms sit at the 12-coordinated A sites in a disordered manner, while the Co atoms occupy the 6-coordinated B sites. Single-crystalline (001) LSAT (a=0.3868 nm)24 was selected as the substrate for epitaxial growth of PSCO thin films due to the relatively small lattice mismatch (~1.6%). The crystallinity of the PSCO films prepared at different laser repetition rates was characterized by high-resolution XRD. Figure 1b shows typical XRD

θ/2θ scan profiles of the PSCO films on the LSAT (001) substrates prepared at a laser repetition rate of 3 Hz and 5 Hz. To simplify the following description, these two types of films are referred as 3 Hz films and 5 Hz films, respectively. Apparently, in the XRD spectra only (00l) reflections of the films and the substrates appear, indicating that the films are free from secondary phases. The splitting of (00l) peaks can be detected resulting from the difference in the out-of-plane lattice parameters between the films and the substrates. Based on the XRD results, the out-of-plane lattice parameter of the 3 Hz and the 5 Hz films is 3



calculated to be 0.3768 nm and 0.3776 nm, respectively. To further figure out the microstructural details of PSCO films grown at different laser repetition rates and understand the possible relations between the microstructure and the measured properties, TEM investigations have been performed on cross-sectional TEM specimens. Figure 1c and Figure 1d show a low-magnification bright-field (BF) TEM image of the 3 Hz and the 5 Hz PSCO films on LSAT substrates, respectively, viewed along the [100] LSAT zone axis. The contrast difference for the film and the substrate allows to locate the film-substrate interface, as indicated by a horizontal white arrow. Under the used film-growth conditions, the total thickness of the 3 Hz and the 5 Hz PSCO film is about 120 nm and 160 nm, respectively. In Figure 1c, the 3 Hz PSCO film shows a homogeneous contrast, indicating that the films are free of planar defects. In contrast, in Figure 1d the dark line contrast appears within the 5 Hz PSCO films, indicating the existence of a large number of planar defects. We also note a distinct layer of the 5 Hz PSCO film directly above the interface, which is free of planar defects and thus makes a visible boundary with the PSCO film part containing planar defects, as indicated by a horizontal red arrow. In most cases, the planar defects start from the boundary and penetrate the whole films. 3.2 Identification of Ruddlesden–Popper faults in the films The distribution and atomic structure of the planar defects in the 5 Hz PSCO films have been investigated in detail based on both plan-view and cross-section specimens. Figure 2a shows a plan-view low-magnification HAADF image of the 5 Hz PSCO films viewed along the [001]P (the subscript P denotes a pseudo-cubic perovskite structure) zone axis of PSCO, which provides an overview of the distribution of planar defects in the film matrix. We note that the intensity of the planar defects is evidently higher than that of the perfect films. It is known that under the HADDF imaging conditions, the atomic column intensity is approximately proportional to Z2, where Z is the atomic number averaged in the atomic columns25. Therefore, the planar defects can be considered as either Pr-rich (Sr-deficient) or contain high density of Pr/Sr in the atomic columns compared to the perfect area in Figure 2a. In addition, it is found that each planar defect forms a closed polygon in the film and the edges of the polygon are parallel to the planes of PSCO. A magnified HAADF image is displayed in the inset in Figure 2a. It can be seen that the relative displacement of the adjacent (Pr,Sr)O layers separated by the planar defects in PSCO is a/2[100]P and a/2[010]P (a is the lattice parameter of the pseudo-cubic unit cell), as indicated by red lines. Therefore, the planar fault have an in-plane displacement of a/2[110]P. 4


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Figure 2b displays a low-magnification cross-sectional HAADF image of the PSCO/LSAT interface, viewed along the [100] LSAT zone axis. The PSCO films coherently grow on the LSAT substrates. Above the substrates, there is a fault-free PSCO layer with a thickness of about 8 nm. In addition, it can be seen that the planar defects form above the fault-free PSCO layer, and nearby no dislocations exist. A high-resolution HAADF image of the planar defects is displayed in Figure 2c. We noted that the continuity of perovskite-type structure is interrupted by the planar fault indicated by red arrows. In the cross-sectional view, the relative displacement between two adjacent (Pr,Sr)O planes separated by the planar defects is a/2[001]P in the out-of-plane direction denoted by horizontal white double-head arrows. Combining the observed in-plane displacement (Figure 2a) the planar faults have a displacement vector of a/2P. It is necessary to mention that the displacements a/2P observed in Figure 2a and 2c are the projected components of the displacement vector (a/2P) of the planar defects along the viewing direction. Considering that the planar defects contain two (Pr,Sr)O layers and the circumference of each individual polygon formed by planar defects can be calculated in the plan-view images, e.g. Figure 2a, the ratio of the area of the planar defects to the whole area of the PSCO films is estimated to be about 4.3% in the 5 Hz PSCO films. The chemical information of the planar defects is further characterized by advanced imaging techniques combined with atomic-resolved EDS mapping. Figure 3a shows a high-resolution HAADF image of the 5 Hz PSCO films containing the planar defects indicated by a vertical red arrow, viewed along the [100]P PSCO zone axis. Apparently, the intensity of atomic planes in the planar defects is higher than that of (Pr,Sr)O layers in the perfect region. Figure 3b is an atomic-resolution ABF image taken from the area containing planar defects in Figure 3a. The planar defects are indicated by a pair of vertical red arrows. It can be seen that under ABF imaging conditions oxygen columns can be imaged in the planar defects, as denoted by horizontal blue arrows in Figure 3b. In addition, by atomic-resolved EDS mapping, the distribution of Co, Sr and Pr in the planar defects has been determined, as shown in Figure 3c-3e, respectively. In the perfect region, Pr and Sr atoms site at the same atomic columns, indicating that A-site disordered PSCO is obtained. In comparison, the atomic plane in the planar defects is obviously Pr-rich and Sr-deficient, as indicated by a pair of vertical arrows in Figure 3c-3e. Based on the observations of the planar defects, a kind of antiphase boundary connected with a crystallographic displacement vector of a/2 is obtained in the 5 Hz PSCO films, which is known as the Ruddlesden–Popper 5



faults (RP-faults)26-29. Since no misfit dislocations could be found in the 3 Hz PSCO films on LSAT, the tensile strain of the films imposed by the substrate is fully accommodated by the elastic deformation of the lattice. In comparison, the formation of RP-faults in the 5 Hz PSCO films contributes to the lattice strain relaxation. Figure 4a shows an atomic-resolution HAADF image of the 5 Hz PSCO films containing typical RP-faults indicated by a pair of vertical red arrows, viewed along the [100]P PSCO zone axis. The CoO layers adjacent to the RP-faults are indicated by vertical white arrows. Along the in-plane [010] direction, the interlayer spacing between the PrO layer in the RP-faults and its neighboring CoO layer is about 0.164 nm, which is smaller than one-half unit cell of pseudo-cubic lattice (about 0.193 nm). However, the interlayer spacing measured between the two PrO layers of the RP-faults is about 0.286 nm. The interlayer spacing between the two CoO layers indicated by vertical white arrows is 0.613 nm along the in-plane direction, which is larger than one and a half unit cell of pseudo-cubic lattice (about 0.579 nm). Therefore, the formation of RP-faults leads to local lattice expansion, which could contribute to release the lattice mismatch strain in the PSCO/LSAT heterostructures. A schematic model of PSCO films containing a RP-fault is illustrated in Figure 4b, viewed along the [100]P PSCO zone axis. It should be noted that in most cases the normal of the RP-faults is perpendicular to the film growth direction, which contributes to strain relaxation more efficiently. The above results show that RP-faults form in the PSCO films prepared at the pulse repetition rate of 5 Hz. It has been reported that the microstructure and physical properties of the films can be tuned by controlling pulse repetition rate during the film growth30-33. For instance, the ZnO films grown at 5 Hz have excellent ultraviolet (UV) emission and high-quality crystallinity compared with the films prepared at 10 Hz30. Kinetic Monte Carlo (KMC) simulations indicated that at higher pulse repetition rates the nuclei will not be given more time to ripen, which results in higher density of islands in the films34. In principle, the formation of RP-faults in the PSCO films could be an intrinsic consequence of the nucleation process and growth kinetics of the films. While the PSCO films prepared at 3 Hz, the nucleus ripening time could be prolonged sufficiently. As a result, no RP-faults appear in the films during the film growth. In contrast, smaller size and higher density of islands could be formed while the PSCO films prepared at 5 Hz. The coalescence of the PSCO nuclei results in the formation of RP-faults at the crystalline-nuclei boundaries. It is necessary to mention that the coalescence of the islands could also lead to the formation of planar faults in other 6


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film systems, e.g. anti-phase boundaries (APBs) in Fe3O435,36 and Li0.5Fe2.5O4 films37. 3.3 Magnetic transport properties of the films Figure 5 shows the temperature dependence of the field-cooled (FC) (under 500 Oe) magnetization measurements for the 3 Hz and the 5 Hz PSCO films. Both PSCO films exhibit spontaneous magnetizations (M) below ∼205 K, which indicates that the films undergo a phase transition from paramagnetic (PM) to ferromagnetic (FM) around 205 K. In addition, upon cooling the magnetic moment for both films has an abrupt jump around 185 K, exhibiting well-defined long-range ferromagnetic ordering in both films. Below 150 K the FC magnetic moment curve for both PSCO films shows a monotonic increase with decreasing temperature. In comparison, the 5 Hz PSCO films have relatively low magnetizations (M). Extrapolating the two curves to 0 K leads to M3HZ≈180.8 emu/cc and M5HZ≈100.7 emu/cc. The ratio of M3HZ/M5HZ is approximately 180%. It is noted that apart from the PM-FM phase transition, the second low-temperature magnetostructural transition (Ts =120 K) observed for bulk ceramics is not present on cooling, which is consistent with the Ref. 17. The ferromagnetic phase transition of our PSCO films is around 205 K, which is lower than that of the bulk materials (~230 K)38,39. The intrinsic strain in the epitaxial films imposed by the substrate may affect the Curie temperature (Tc) of ferromagnetic transition1,40. In addition, the epitaxial strain in both PSCO films may suppress the second low-temperature transition (TS), which is correlated to an orthorhombic-to-monoclinic symmetry change in PSCO19. Additionally, magnetic phenomena can be seen in the magnetic hysteresis (M–H) curves measurements. The inset in Figure 5 shows the magnetic field dependence of the magnetic moment at 20 K. The 3 Hz PSCO films show a relatively narrow magnetic hysteresis loop with a small coercive field (Hc) of 3.69 kOe in comparison with the 5 Hz PSCO film with a coercive field of 4.42 kOe. Saturation magnetization (Ms) of the films can be obtained by the extrapolation of high magnetic field (H

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