⇦Chinese Physics Letters, 2019, Vol. 36, No. 4, Article code 046102 Strain Induced Nanopillars and Variation of Magnetic Properties in La$_{0.825}$Sr$_{0.175}$MnO$_{3}$/LaAlO$_{3}$ Films * Xin Li (李鑫)1, Jing-Zhi Han (韩景智)1**, Xiong-Zuo Zhang (张雄祚)1, Yin-Feng Zhang (张银峰)1, Hai-Dong Tian (田海东)1, Ming-Zhu Xue (薛明珠)1, Kun Li (李昆)1, Xin Wen (闻馨)1, Wen-Yun Yang (杨文云)1, Shun-Quan Liu (刘顺荃)1, Chang-Sheng Wang (王常生)1, Hong-Lin Du (杜红林)1, Xiao-Dong Zhang (张晓东)1, Xin-An Wang (王心安)3, Ying-Chang Yang (杨应昌)1, Jin-Bo Yang (杨金波)1,2 Affiliations 1State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871 2Beijing Key Laboratory for Magnetoelectric Materials and Devices, Beijing 100871 3Ningxia Magvalley Novel Materials Technology Co., Ltd. New Energy Sub-Park, Ningdong Energy & Chemical Industry Base, Ningxia 751411 Received 7 January 2019, online 23 March 2019 *Supported by the National Key Research and Development Program of China under Grant Nos 2017YFA206303 and 2016YFB0700901, and the National Natural Science Foundation of China under Grant Nos 51731001, 51371009 and 51271004.
**Corresponding author. Email: hanjingzhi@pku.edu.cn Citation Text: Li X, Han J Z, Zhang X Z, Zhang Y F and Tian H D et al 2019 Chin. Phys. Lett. 36 046102    Abstract To investigate the process of strain relaxation and resultant variation of microstructure and magnetic properties, low-doped La$_{0.825}$Sr$_{0.175}$MnO$_{3}$ epitaxial films with different thicknesses are deposited on LaAlO$_{3}$ substrates and strain induced nanopillars are discovered inside the La$_{0.825}$Sr$_{0.175}$MnO$_{3}$ film. Perpendicular oriented nanopillars mainly exist below 30 nm and tend to disappear above 30 nm. The distribution of nanopillars not only induce the variation of lattice parameters and local structural distortion but also lead to the deviation of easy magnetization axis from the perpendicular direction. Specifically, the out-of-plane lattice parameters of the film decrease quickly with the increase of the thickness but tend to be constant when the thickness is above 30 nm. Meanwhile, the variations of magnetic properties along in-plane and out-of-plane directions would also decline at first and they then remain nearly unchanged. Our work constructs the relationship between nanopillars and magnetic properties inside films. We are able to clearly reveal the effects of inhomogeneous strain relaxation. DOI:10.1088/0256-307X/36/4/046102 PACS:61.05.-a, 61.05.cp, 74.78.Na, 75.47.Lx © 2019 Chinese Physics Society Article Text Strongly correlated oxides, due to the inter-coupling between spin, charge, orbital and lattice,[1-4] exhibit rich physical properties, such as ferromagnetism,[5] ferroelectricity,[6] and superconductivity.[7] For epitaxial films with perovskite-structure, orbital reconstruction at the interface,[8] electronic phase separation[9] induced by inhomogeneity of local strain, and the transformation from ferromagnetism to A-type (or C-type) antiferromagnetism under tensile (or compressive) strain[10] are highly sensitive to the strain induced distortion of lattice structures. Among these materials, La$_{1-x}$Sr$_{x}$MnO$_{3}$ has been widely studied due to colossal magnetoresistance effect,[11] room-temperature ferromagnetism,[12] high spin polarization[13] and potential application in spintronics device.[14,15] Recently, skyrmions-like magnetic bubbles have been reported in low-doped La$_{0.825}$Sr$_{0.175}$MnO$_{3}$ (LSMO) single crystal.[16-18] This topological spin texture can potentially be used for the next-generation magnetic storage device.[19-22] However, the epitaxial film of LSMO has not been prepared and systematically investigated. In addition, the effects of strain, applied by the substrate, on magnetic properties and intrinsic change of lattice structure are still unclear. Therefore, the investigation for the process of strain relaxation may not only reveal strain induced variation of microstructure and magnetic properties in La$_{0.825}$Sr$_{0.175}$MnO$_{3}$ epitaxial films but could also lay the foundation to investigate the relationship between strain state and topological magnetic structure in correlated oxide films. In this work, we fabricate epitaxial LSMO films on LaAlO$_{3}$ (LAO) substrates to reveal the process of strain relaxation and resultant influence on the microstructure and magnetic properties of films with different thicknesses. X-ray diffraction patterns demonstrate that out-of-plane lattice parameters of LSMO would significantly decrease due to strain relaxation when the thickness increases from 10 nm to 30 nm. The change of lattice parameters can be ignored above 30 nm. The distribution of nanopillars along the out-of-plane direction is discovered in the LSMO film according to the investigation of high resolution transmission electron microscopy (HRTEM). Compared with the surrounding regions of nanopillars, the lattice structure inside the nanopillars undergoes a higher degree of distortion due to the larger elongation along the out-of-plane direction. Meanwhile, the difference of in-plane and out-of-plane magnetic hysteresis loops of films tends to decline with the increase of the thickness from 10 nm to 30 nm. This indicates the relationship between nanopillars and magnetic anisotropy of the film. Our work suggests that the formation of nanopillars and resultant variation of magnetic anisotropy originate from inhomogeneous relaxation of the strain. Consequently, the radiation-like distribution of nanopillars could provide the chance to achieve more detailed modulation of magnetic properties in the epitaxial LSMO film.
cpl-36-4-046102-fig1.png
Fig. 1. (a) X-ray diffraction patterns of LSMO films, where arrows in the graph indicate (002) diffraction peaks of LSMO with different thicknesses. (b) Out-of-plane lattice parameters and related fitting curve, obtained from XRD data of (a).
LSMO films with different thicknesses were fabricated through pulsed laser deposition (PLD) to investigate the process of strain relaxation. Figure 1(a) shows x-ray diffraction patterns of LSMO/LAO films with the thicknesses of 10, 20, 30, 40 and 60 nm. Only (002) diffraction peaks of substrates and films can be observed, which indicates that high-quality epitaxial films were formed. Meanwhile, (002) diffraction peaks of all LSMO films locate on the left side of LAO (002) diffraction peak, suggesting that the films are subjected to tensile strain along out-of-plane direction. When the thickness of the film increases from 10 nm to 30 nm, the LSMO (002) diffraction peak gradually moves towards the (002) peak of the substrate, and the largest variation of 2$\theta$ is close to 1$^{\circ}$ corresponding to the movement of peaks. However, the movement of the (002) diffraction peak is minor and variation of 2$\theta$ can be ignored above 30 nm. In addition, the diffraction peak is wider in the 10 nm film, which may be due to inhomogeneous distribution of lattice parameters induced by stronger strain. Figure 1(b) shows that the out-of-plane lattice parameters of the films, obtained from the XRD data in Fig. 1(a), decrease obviously when the thickness increases from 10 nm to 30 nm and remain nearly the same for the thickness 30–60 nm. The fitting curve indicates that out-of-plane lattice parameters decay exponentially with the increase of the thickness. These results demonstrate that the relaxation of strain mainly occurs below 30 nm, and the strain will gradually relax until it has no obvious influence on the lattice parameters with the increase of the thickness.
cpl-36-4-046102-fig2.png
Fig. 2. Surface morphology of 10, 20, 30 and 60 nm LSMO films obtained from AFM. The scan area is 500 nm$\times $500 nm: (a) 10 nm LSMO/LAO, (b) 20 nm LSMO/LAO, (c) 30 nm LSMO/LAO, and (d) 60 nm LSMO/LAO.
To examine the effect of strain relaxation on the surface morphology, atomic force microscopy (AFM) was used to investigate the difference of surface morphology with the increase of the thickness. As shown in Fig. 2, all of the films are nearly atomic-level flat, and the surface undulation is less than 0.6 nm according to contrast of AFM images. The surface of the 10 nm film is the flattest (see Fig. 1(a)). Cluster-like morphology is obvious in the 20 nm film (see Fig. 2(b)), becomes ambiguous in the 30 nm film (see Fig. 2(c)), and then gradually disappears in the 60 nm film (see Fig. 2(d)). The differences of surface flatness of different-thickness films imply the effect of strain relaxation, and inhomogeneous strain inside the film may contribute to the formation of this cluster-like morphology on the surface. Together with the significant decline of lattice constant $c$ and obvious change of morphology on the surface, there may be a distortion of the lattice structure during the process of strain relaxation. A detailed investigation for cross section of LSMO/LAO films is carried out by HRTEM, and the 60 nm LSMO film can be divided into two areas according to the variation of lattice structure. Nanopillars are found to be mainly distributed below 30 nm along the out-of-plane direction of the film, and the average width of pillar is about 6 nm (see Fig. 3(a)). Above 30 nm, the nanopillars nearly disappear. Figure 3(b) shows an enlarged area of Fig. 3(a), in which the nanopillars exhibit a radiation-like distribution along the perpendicular direction. There is no pillar-shape structure near the interface, which indicates the coherent growth of films on substrates at the early stage of deposition. The misfit dislocations may form above 10 nm due to the inhomogeneity of strain relaxation. In addition, regions with higher strain may be energetically more favorable for the growth of nanopillar. Therefore, the growth behavior of LSMO epitaxial films with nanopillars on LAO substrates may follow the Stranski–Krastanov growth mode,[23] in which the film would firstly grow coherently on the substrate and then exhibit island-like growth with the increase of the thickness. Figure 3(c) shows selected area electron diffraction (SAED) patterns of Fig. 3(a). The white square indicates the positions of LAO reference spots. The diffraction patterns of LSMO contains weakly split spots with radial distribution. This indicates the superposition of two quasi-square patterns with slight different lattice constants, which should be attributed to the existence of nanopillars. The phase of nanopillars may be orthohombic phase.[24] In Fig. 3(d), to fully reveal the difference between nanopillars and surrounding regions, the distribution of distorted lattice of nanopillars and surrounding regions (green parts) can be further reconstructed on the original HRTEM image according to additional reciprocal lattice getting through FFT. The green parts belong to nanopillars and the purple parts represent the surrounding regions where lattice distortion could be ignored. The radiation-like distribution of nanopillars is clearly identified again and nanopillars gradually disperse from the bottom with the increase of the thickness.
cpl-36-4-046102-fig3.png
Fig. 3. (a) Cross-section HRTEM image of LSMO/LAO with the thickness of 60 nm, which exhibits nanopillars extending from the bottom and disappearing gradually with the increase of the thickness. (b) Enlarged area of nanopillar structure of (a). (c) SAED patterns of cross-section sample in (a). (d) Separated nanopillar structure in HRTEM according to FFT, the green part represents the distribution of nanopillars, and purple part indicates the surrounding regions.
cpl-36-4-046102-fig4.png
Fig. 4. (a) HRTEM image of local structure inside the nanopillar. (b) Line profile of energy dispersive spectrometer (EDS) from the interface to the surface of the film. (c) HRTEM image of the nanopillar and surrounding region. (d) Strain distribution of (c), obtained through geometrical phase analysis.
Further investigations of the nanopillar's microstructure indicate that the lattice inside the nanopillar undergoes higher degree of elongation along the $\langle001\rangle$ direction. Since the elongation of the out-of-plane lattice constant is usually accompanied by the compression of the in-plane lattice constant,[10] and the in-plane lattice constant (0.367 nm) is smaller than the out-of-plane lattice constant (0.422 nm) (see Fig. 4(a)), thus the lattice is subjected to in-plane compressive strain inside the nanopillar. Moreover, in the surrounding region of the pillar, the out-of-plane lattice constant (0.394 nm) is smaller than the central area of nanopillar, which suggests that the compressive strength is higher inside the nanopillar. Figure 4(b) is the line profile of Mn and Sr elements from the substrate to the surface of the film through energy dispersive spectrometer (EDS), and no obvious inhomogeneous distribution of chemical elements can be recognized according to Fig. 4(b). Figure 4(c) shows another area containing single nanopillar and surrounding regions, the outline of a pillar is marked in the image. According to the distribution of strain tensor $e_{xy}$ obtained from geometrical phase analysis (see Fig. 4(d)), the distribution of strain tensor $e_{xy}$ is nearly random in surrounding regions compared with the pillar, thus the formation of nanopillars indeed comes from inhomogeneous distribution of strain relaxation, where the in-plane compressive strain is concentrated inside the area with nanopillars. Out-of-plane elongation and in-plane compression of the lattice would lead to the tetragonal change. Higher degree of tetragonal change inside nanopillars suggests strong Jahn–Teller distortion, and the degenerate 3$d$ orbital of Mn3$+$ is split in association with uniaxial distortion of MnO$_{6}$ octahedron,[25] thus the electron would tend to occupy 3$z^{2}-r^{2}$ orbital under this condition and may further lead to orbital polarization inside the pillars. Moreover, since the perovskite oxides consist of networks of corner-sharing octahedra, the in-plane compressive strain would also lead to additional rotation modes of MnO$_{6}$ octahedron or polar off-centering in addition to the Jahn–Teller elongation. Under compressive strain, the MnO$_{6}$ octahedra are rotated in phase around $[100]_{\rm c}$ direction and out of phase around $[010]_{\rm c}$ direction.[26] Therefore, the rotation angles inside and outside nanopillars should be different. However, the detailed rotation patterns and possible displacement of ions still cannot be identified through HRTEM investigations.
cpl-36-4-046102-fig5.png
Fig. 5. (a)–(c) The magnetic hysteresis loops of LSMO/LAO films with the thicknesses of 10, 20 and 30 nm along the in-plane (IP) and out-of-plane (OOP) directions. (d) The dependence of coercivity on thickness.
Further magnetic measurements indicate the difference of magnetism along the in-plane and out-of-plane directions. Figures 5(a)–5(c) represent the dependence of magnetic hysteresis loops of the film on the thickness. With the increasing thickness, the difference of magnetic hysteresis loops along two directions tends to decrease. This decrease indicates that the easy axis of magnetization would gradually deviate from the original direction due to the variation of perpendicular anisotropy inside films. The changes of coercivity along two directions exhibit different trends below 30 nm (see Fig. 5(d)). The out-of-plane coercivity decreases drastically, but the in-plane coercivity increases gradually from 10 nm to 30 nm. Then, the difference of coercivity between two directions tends to be constant with a further increase of the thickness. Generally speaking, the coercivity mechanism is related to not only intrinsic magnetocrystalline anisotropy but also to the extrinsic microstructure of the material. In the 10 nm film, since the film is coherently grown on the substrate and the direction of easy magnetization is along the vertical direction, the coercive force of the 10 nm film is the largest in the vertical direction, whereas the coercive force in the in-plane direction is the smallest. As the thickness of the film increases, due to the appearance of the radial nanopillars, the direction of easy magnetization of the film will gradually deviate from the vertical direction, thus the coercive force in the vertical direction of the film gradually decreases, while the coercive force in the parallel direction gradually increases. When the film thickness exceeds 30 nm, the changes of the easy magnetization direction are minor due to the disappearance of the nanopillars. Therefore, the change of coercivity is very small in either the vertical direction or the parallel direction. Thus, both the out-of-plane decrease and in-plane increase of coercivity should be mainly ascribed to the deviation of easy-axis from the original perpendicular direction. This deviation is caused by radial distribution of nanopillars. These results indicate that the process of strain relaxation can be categorized into two stages in LSMO/LAO films. When the thickness of the film is below 30 nm, the strain relaxation could not only induce higher-degree decline of out-of-plane lattice parameters (about 2% from 10 nm to 30 nm) but may also lead to the formation of perpendicular oriented nanopillars due to the inhomogeneity of strain relaxation. With a further increase of thickness, the distortion of lattice structure tends to be negligible and nanopillars gradually disappear due to the gradual relaxation of strain. The variations of magnetic properties could also be identified according to magnetic hysteresis loops along in-plane and out-of-plane directions. In general, the easy magnetization axis of the LSMO film is supposed to be along the out-of-plane direction with larger cell parameters.[27] Therefore, it stays at the out-of-plane direction when the film is grown on LAO substrates, which could cause the elongation of lattice constant $c$ due to in-plane compressive strain. However, magnetic anisotropy would further undergo variation due to the complexity of lattice distortion at each stage during the process of strain relaxation. Specifically, the differences of hysteresis loops along in-plane and out-of-plane directions tend to decline with the increase of the thickness at the first stage of strain relaxation below 30 nm, and the coercivity along two directions exhibit a contrary trend. This indicates the deviation of easy magnetization axis from the perpendicular direction. The consistency between the distribution of nanopillars and resultant magnetic response below and above 30 nm indicates the two-stage change of strain relaxation in films. In LSMO/LAO films, the change of microstructure and magnetic properties with thickness is correlated with the formation of strain induced nanopillars. Nanopillars are mainly distributed below 30 nm due to the incomplete relaxation of strain. In addition, there is a higher degree of distortion inside nanopillars compared with surrounding regions. The variation of magnetic anisotropy exhibits its dependent relationship with the distribution of nanopillars. When the film is above 30 nm, the difference of magnetic properties tends to decrease with the increase of the thickness. This is accompanied with the disappearance of nanopillars due to gradual relaxation of strain. The formation and distribution of nanopillars could also deepen our current understanding of modulating magnetic anisotropy through the process of strain relaxation. 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