Anomalous Sr distribution at La 2 CuO 4 /LaMnO 3 heterostructures

heterostructures

Introduction

The phenomena occurring at oxide heterostructure interfaces can be primarily tailored by the choice of the constituents. However, the key factor dominating the interface functionalities is the control of interface sharpness, which can be identified by the order of cationic intermixing, i.e.

elemental redistribution. Therefore, examining the interfacial structure and chemistry is vital for correlating the physical properties. For instance, in La2CuO4 (LCO) based, i.e. 214-type, high-temperature superconducting (HTSC) interfaces, the importance of dopant distribution in overdoped-undoped LCO bilayer systems [79,89] and the mechanism of cationic distribution in 2D-doped LCO systems [84] for iso-structural cuprate-cuprate interfaces have been reported recently (see chapter 4).

In these studies, aberration-corrected [104] scanning transmission electron microscopy (STEM) has provided local-probing techniques such as high-angle annular dark-field (HAADF) imaging [105], annular bright-field (ABF) imaging and electron energy-loss spectroscopy (EELS) with subatomic precision. [89]

In addition to the iso-structural systems, hetero-structural systems exhibit intriguing properties as well. One of the most important example of such heteroepitaxial systems is the interfacial contact of HTSC (Y,Ca)Ba2Cu3O7 (YBCO) and a ferromagnetic La0.67Ca0.33MnO3 (LCMO), where orbital reconstruction and charge transfer take place at the interface. [243] Recently, the antiferromagnetic coupling between Cu and Mn spins was also demonstrated in a similar heterostructure with another type of cuprate, (La,Sr)2CuO4, and considered as a “ubiquitous” behavior of cuprate/manganite interfaces. [244] The charge transfer within the layers can be tuned via controlling the compositions of the superlattices with sub-unit cell precision [245] and it is natural to connect the local lattice/chemical structure to such physical phenomena.

To understand the mechanisms of the formation of hetero-structural interfaces and their influence on the functionalities, cuprate-manganite superlattices with different cuprate layer thicknesses are fabricated by an ALL-oxide MBE system, that enables precise control (i.e. atomic layer level) of the film stoichiometry. In order to realize the cationic distribution and related local distortions at the interfaces of different phases, i.e. 113- and 214-phases, comprehensive aberration-corrected STEM techniques are applied. The Sr distributions in 113-manganite and 214-LCO phases are found to be dissimilar. In particular, Sr-O layer of SrMnO3 deposited on top of 214-cuprates strongly segregates in the growth direction, floating through the LaMnO3 layers deposited on top. The Sr-O layer (i.e. δ-doping) of LaSrCuO4 deposited on top of 113 manganites shows different elemental distribution and chemical sharpness compared to the conventional case.

99 Results & Discussion

The superlattices are grown on LSAO (001) substrates and the nominal composition of the superlattices can be written as:

21 x (2 x SrMnO

/ 6 x LaMnO

/ 1 x LaSrCuO

/ N x La

CuO

)

Here, N represents the thickness of the LCOphase (expressed in number of half unit cells – u.c.). For both of the samples, the thickness of SrMnO3 (SMO) - LaMnO3 (LMO) layer and δ-doped layer were kept constant, while the LCO layer is designed as three monolayers (MLs) for “thin” and nine MLs for “thick” layers. For clarity of the presentation, samples are defined as thin-LCO and thick-LCO, respectively. Basic sketches of the samples are presented in Figure 6.1.1.

Figure 6.1.1. Basic sketches of the (a) thick-LCO (N = 9) and (b) thin-LCO samples (N = 3). Both of the samples are composed of 21 repetitions.

To investigate the atomic arrangement of the samples, I first conducted STEM-HAADF imaging that not only demonstrates the epitaxial quality of the samples but also reveals how the adjacent layers are grown structurally. Figure 6.1.2 and Figure 6.1.3 demonstrate the high quality of the samples without extended defects, where the LSAO substrate (darker) and the epitaxial multilayers (lighter) can be distinguished via their contrast due to the difference of the mean atomic weight. The image intensity profiles averaged along the growth direction (indicated with the orange arrows) exhibit a regular contrast owing to the Sr distribution across the 214-113 and 113-214 interfaces. In particular, the decrease in the image intensity is visible (darker contrast) at the Sr-containing layers. Such obvious contrast for the thick-LCO SL structure substantiates the structurally sharp interfaces (e.g. Figure 6.1.2c).

100

Figure 6.1.2. (a) Low magnification STEM-HAADF images of the thick-LCO SL demonstrate the high-quality epitaxy. (b) STEM-HAADF image intensity profile along the orange arrow in Fig. 1a, averaged along the perpendicular direction. The image intensity indicates the periodicity of the contrast related to the Sr distribution at the interfaces. (c) High-magnification image of the area highlighted by the red rectangle in (a) confirming the structurally coherent 214-113 and 113-214 interfaces.

Figure 6.1.3. (a) Low magnification STEM-HAADF images of the thin-LCO SL demonstrate the high-quality epitaxy. (b) STEM-HAADF image intensity profile along the orange arrow in Fig. 1a, averaged along the perpendicular direction. The image intensity indicates the periodicity of the contrast related to the Sr distribution at the interfaces. (c) High-magnification image of the area highlighted by the red rectangle in (a) confirming the structurally coherent 214-113 and 113-214 interfaces.

101

After revealing the structurally coherent interfaces, atomically-resolved STEM-EELS investigations are performed in order to evaluate the elemental intermixing (i.e. the chemical sharpness) at the interfaces. Figure 6.1.4a-d show atomic resolution maps of Sr (blue), La (green), Mn (red), Cu (cyan) and the overlay (RGB) of these elements as obtained via STEM-EELS, respectively.

The La and Sr substitution in the manganite phase (Figure 6.1.4a,b) and the eight-monolayer-thick Mn distribution (Figure 6.1.4c) are revealed. Here, the substitution of La³⁺ with Sr²⁺ (divalent alkaline-earth metal), which provides potential charge carriers for electrical conductivity [246], results in a mixed valance of Mn³⁺ and Mn⁴⁺, where the ions exchange the carriers via O-2p orbitals as explained by the double-exchange mechanism. [72,247,8] Moreover, as a consequence of such atomic intermixing emerging at the SMO-LMO interface, the expected electronic structure is a mixture of t2g

3eg

1 and t2g 3eg

0 configurations. EELS results clearly indicate that the elemental intermixing is mostly confined to 2-3 UC (Figure 6.1.4). That is in a good agreement with the polarized neutron reflectometry (PNR) measurements demonstrating the magnetic profile of the individual layers as employed for identical systems. [245]

Figure 6.1.4. The atomically resolved elemental maps obtained via EELS spectroscopy, are displayed in panels a), b), c) and d) for Sr (blue), La (green), Mn (red), and Cu (cyan) respectively.

e) displays the overlay of the three elements. f) shows the Sr distribution profiles across both interfaces. The linear distortion in the experimental spectrum image is corrected via STEM SI Warp software [129] for the Sr profile plot in (f). Please note that, for clarity, the nominal SrMnO₃ and Sr δ-doped La2CuO₄ layers are shaded light-blue.

102

Furthermore, in the 113-phase (i.e. SMO-LMO contact) the increased Sr concentration in growth direction is noteworthy, in which the position of the Sr-O layers are majorly deviated from the position they were nominally deposited (Figure 6.1.4). This is an example of Sr segregation towards the surface triggered by the locally induced in-plane strain via La and Sr substitution. In particular, the larger Sr atoms [96] – compared to La – are re-arranged in the structure, in order to relieve the strain and the system is relaxed via the mild modification (i.e. segregation) instead of any defect formation, as high-epitaxial quality is evidenced via STEM-HAADF imaging (Figure 6.1.2, 6.1.3).

This behavior is similar to the Ba segregation in epitaxial La2CuO4-based systems. [89] The difference in the present case is the sudden stop of Sr distribution after 4 MLs. The importance of the interface sharpness (smoothness) for SMO-LMO superlattices, which directly affects the enhancement (reduction) of the ferromagnetic moment at the interfaces, have been reported.

[176,248] But our observation is slightly different from that of the SMO-LMO interfaces reported before. [249]. Such difference could be due to the compressive strain induced by the LSAO substrate and LCO interlayers, which maximizes the effect of the local strain. These findings suggest that the interface sharpness can be further tuned via heteroepitaxial contacts, which influences the magnetic profiles.

Simple segregation is not enough to explain the obvious presence of La-rich (or Sr-deficient) A–O column detected on the first nominal SMO layer and the sudden termination of the SrO segregation. In addition to the HAADF image contrast (Figure 6.1.3), STEM-EELS analyses clearly evidence the La-rich first layer and the Sr segregation in growth direction as well as the A- and B-type interface formation [244] at two interfaces (Figure 6.1.4). Here, one can suggest that the local charge of the layers (polar and unipolar layers) plays a role in this case.

The negatively charged CuO

layers attract the positively charged LaO layers and the SrO layers are pushed upwards.

[139] The abrupt termination of the SrO segregation takes place after the charge is compensated by the LaO layers. Therefore, one can speculate that in addition to the elastic strain energy induced by the size difference of La³⁺ and Sr²⁺, the local charge difference triggered by La³⁺ and Sr²⁺ substitution plays role in this picture as well. Nevertheless, these interfaces need further examination for detailed evaluation of the mechanism lays behind.

On the other hand, the elemental intermixing on the next interface (i.e. 113-214 interface;

manganite-cuprate) shows an interesting contrast at the SMO-LMO interface. The Sr distribution in the system clearly demonstrates the dissimilarity of Sr profiles in two different phases (Figure 6.1.4).

In particular, the 2D-doping of Sr in the La2CuO4 phase exhibits an asymmetric profile, as reported in previous studies. [55,84] However, our results demonstrate different redistribution lengths in both substrate and growth directions. The distribution in the growth and substrate directions are measured around 1.3 nm (~2 uc) and ~0.65 nm (~ 1 uc), respectively, which gives rise to a sharper interface similar to that of Sr and Ca-doped M-I bilayer structures reported recently. [79] Such difference can be ascribed to the epitaxial strain from LaMnO3, since the growth conditions (~600-650 °C, 3x10^-5 Torr) are the same as for the previous 2D-doping case [84], and therefore, cannot induce different growth kinetics and one can refer to an indirect influence. Nominally, when the compressive strain is induced via the LSAO substrate, 2D-doping in related La2CuO4-based systems demonstrate superconducting transition temperatures over 35K [55]. However, in this case, the superconducting transition temperatures are around 25 K instead [245].

103

The octahedral distortions at oxide heterostructure interfaces are crucially affected by local elemental distribution [89] and have a direct link to the magnetic [153] and electronic [16,79]

properties. Therefore, I further conducted STEM-ABF and STEM-HAADF imaging (Figure 6.1.5) that are appropriate methods to quantify the local distortions when combined. Basal (in-plane) and apical (out-of-plane) O–O and La–La interatomic distortions are measured on ABF (Figure 6.1.5a) and HAADF (Figure 6.1.5c) images, respectively. Measured distances versus the number of Cu-O (or Mn-O) layers, which are visible in this particular projection, are displayed in Figure 6.1.5b,d.

Figure 6.1.5. a) STEM-ABF image displaying all atomic column positions including O-column positions. b) O–O interatomic distances along basal (in-plane) and apical (out-of-plane) directions exhibit the variations across the interfaces. c) The STEM-HAADF image displays the coherent interfaces and the weaker contrast in the Sr-rich region (1-2 MLs). d) La–La interatomic distances along basal (in-plane) and apical (out-of-plane) directions exhibit the variations across the interfaces. The blue and orange arrows on the images indicate the nominal 214-113 and 113-214 interfaces. The apical and basal distances are indicated with straight and dashed lines, respectively.

The error bars give the 95% confidence interval (corresponding to 2 times the standard error) of the average of 14 unit cells of the perovskite lattice along the basal (in-plane) direction.

First of all, the basal (in-plane) interatomic distances are about the same along the layers and are measured around 3.90 Å for both of the O–O and La–La basal (in-plane) distances (Figures 6.1.5b,d). This is an indication of the compressive in-plane epitaxial strain induced by the substrate.

Regarding the O–O interatomic distance, the larger distances measured in cuprate layers (~ 4.30 ± 0.05 Å) compared to manganite layers, which saturates around 3.90Å, show an abrupt decrease after the 4^th layer. Interesting findings stand out when both Sr-containing interfaces are taken into consideration and compared. In particular, the abrupt increase of O–O distance at the last unit cell of the manganite layer (i.e. 12^th layer) is noteworthy. Such increase may be related to the space-charge region formed next to the nominal interface. [55] Moreover, the gradual increase of O–O interatomic (apical) distances starting from 13^th layer is an example of an anti-Jahn–Teller distortion as observed in homogenously-doped La2CuO4 systems. [33] This gradual increase instead of an abrupt change is

104

due to the longer Sr tail in growth direction. The measurements also showed that, instead of a sharp transition, the apical O–O distances shows a gradual increase just after the 113-214 interface, which suggests that the 214-113 interface is sharper than the 113-214 interface.

Another point to consider is the local effect of substituting La³⁺ cations with Sr²⁺ cations, for which the measurement of La-La apical interatomic distances provides direct information (Figure 6.1.5d). In particular, while in the first LCO layer the La–La distances are constant (~3.61 Å), in the last Sr-containing unit cell of SMO-LMO contact (i.e. 7^th layer), the distance is measured as 3.82±0.04 Å. This is followed by the LMO layer with a La–La distance measured as 3.93±0.04 Å which demonstrates that the presence of a larger (i.e. Sr²⁺) cation in the system suppresses the La–La interatomic distances. Besides, on the 113-214 interface, (again, similar to O–O distances) a smooth transition of the interatomic distances is observed, and this can be explained via the Sr redistribution – with a decay length of 1.3 nm – in growth direction after the nominal interface.

Conclusion

In conclusion, in order to examine the interface formation and the structural and chemical sharpness at the interfaces, ALL-oxide MBE grown lanthanum cuprate-lanthanum manganite heterostructures are probed utilizing comprehensive STEM techniques. The investigations demonstrate the differences of Sr profiles together with the interrelated local lattice deformations that affect the interface sharpness. The observations not only substantiate the tunability of Sr redistribution at oxide heterostructures interfaces via employing different phases but also indicate that sharper interfaces can be obtained via 113-214 contacts. It is further suggested that hetero-epitaxial contacts can be used as a tool to tailor the sharpness of the interfaces, which may lead to tune the physical properties.

105

7. Nickelate-based oxide