Nanoscale crystal imperfection-induced characterization changes of manganite nanolayers with various crystallographic textures
© Liang et al.; licensee Springer. 2013
Received: 9 May 2013
Accepted: 6 July 2013
Published: 6 August 2013
(La,Sr)MnO3 (LSMO) nanolayers with various crystallographic textures were grown on the sapphire substrate with and without In2O3 epitaxial buffering. The LSMO nanolayer with In2O3 epitaxial buffering has a (110) preferred orientation. However, the nanolayer without buffering shows a highly (100)-oriented texture. Detailed microstructure analyses show that the LSMO nanolayer with In2O3 epitaxial buffering has a high degree of nanoscale disordered regions (such as subgrain boundaries and incoherent heterointerfaces) in the film. These structural inhomogeneities caused a low degree of ferromagnetic ordering in LSMO with In2O3 epitaxial buffering, which leads to a lower saturation magnetization value and Curie temperature, and higher coercivity and resistivity.
KeywordsCrystallographic texture Manganite Epitaxy Magnetic properties Buffering
Because of their versatile physical properties, various transition metal oxides, specifically perovskite-based manganites, have attracted considerable scientific and technological attention [1–3]. There is potential for the application of La1 - xSr x MnO3 (LSMO) in the magnetic storage device and spin-sensitive device field, or it can be used as an important hole-doping material to construct microelectronic devices [2, 4, 5]. To realize nanodevice applications with high efficiency, it is imperative that LSMO thin films be fabricated on a nanometric scale.
High-quality epitaxial manganite films with specific orientations are essential for the next-generation of microelectronic and magnetic devices. However, single-crystalline perovskite oxide substrates are expensive, and a large diameter substrate is currently technologically unavailable. These factors hinder the practical application of epitaxial LSMO films in the electronic industry [4, 6]. Two factors might cause lattice stress in nanoscale manganite thin films. An ultra-thin LSMO epilayer grown on the lattice-mismatched perovskite oxide substrate usually induces built-in stresses in the film, which greatly affect its physical properties [4, 7–9]. Moreover, a large thermal expansion coefficient (TEC) difference between the film and substrate also significantly affects the lattice stress in nanoscale manganite thin films. In comparison to randomly oriented thin films, the highly crystallographic textured film usually exhibits superior crystal quality. If the TEC value of a substrate and film is similar, then highly textured ultra-thin polycrystalline LSMO films would not suffer from the lattice distortion that was caused by a lattice mismatch on the single crystalline substrates. This might be promising for practical applications in devices. The sapphire substrate and LSMO have similar TEC sizes . Sapphire substrates can be fabricated with a large diameter and relatively low cost in comparison to perovskite oxide substrates. Such fabrication could attain the practical mass production of a device. Moreover, to form functional heterostructure microelectronic devices, sapphire substrates can be used to integrate LSMO nanofilms with other high-quality optoelectronic thin films [11, 12]. During this project, two different crystallographic textured LSMO thin films with a nanoscale thickness were grown using In2O3 epitaxial underlayering. These films did not suffer lattice stress. These results enable an analysis of the correlation between nanoscale crystal imperfections and manganite nanofilm physical properties.
LSMO nanolayers (the Sr content is approximately 39%) with thickness of approximately 60 nm were grown on the c-axis-oriented sapphire substrates with and without 40-nm-thick In2O3 (222) epitaxial buffering. The deposition of the In2O3 epitaxy layers and LSMO nanolayers was performed using a radiofrequency magnetron-sputtering system. During the deposition, the substrate temperature for the thin-film growth of the In2O3 epitaxy and LSMO nanolayer was kept at 600°C and 750°C, respectively. Moreover, the gas pressure of deposition was fixed at 10 mTorr with an Ar/O2 ratio of 3:1. The as-synthesized samples are further annealed in air ambient at 950°C for 30 min.
The crystal structure of the samples was investigated by X-ray diffraction (XRD) with Cu Kα radiation. The detailed microstructure of the as-synthesized samples was characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The composition analysis was performed using energy dispersive X-ray spectrometer (EDS) attached to the TEM. The surface morphology of the LSMO nanolayers was investigated by atomic force microscopy (AFM) with an area size of 2 μm × 2 μm. The surface current images of the LSMO nanolayers were also observed using conductive atomic force microscopy (CAFM) with PtIr tips. A superconducting quantum interference device magnetometer was used to measure the magnetic properties of the samples.
Results and discussion
In summary, 60-nm-thick LSMO nanolayers were grown on sapphire substrates with and without In2O3 (222) epitaxial buffering. The LSMO experienced improved (110) preferred crystal growth via In2O3 (222) epitaxial buffering. Comparatively, the surface grain size is more homogeneous for the LSMO nanolayer grown on the sapphire substrate. The rugged surface of the In2O3 epitaxial underlayer further incurred rougher surface morphology of the LSMO nanofilm. The columnar crystallite feature of the In2O3 epitaxial underlayer caused a relatively smaller lateral domain size of the manganite ultra-thin layer on it. Moreover, In2O3 epitaxial buffering resulted in rugged heterointerfaces between the LSMO nanolayer and In2O3 epitaxy. These factors contributed to a higher content of subgrain boundaries and incoherent interfaces on a nanometric scale in the LSMO nanofilm via In2O3 epitaxial buffering. These disordered regions caused disordered spins to exist in the LSMO nanolayer. Therefore, lower saturation magnetization value and Curie temperature, and higher coercivity and resistivity are found in the highly (110)-textured LSMO nanolayer.
YCL is a professor of the Institute of Materials Engineering at National Taiwan Ocean University (Taiwan). HZ received his Masters degree in Materials Engineering at National Taiwan Ocean University (Taiwan) in 2013. WKL is a graduate student of the Institute of Materials Engineering at National Taiwan Ocean University (Taiwan).
This work is supported by the National Science Council of Taiwan (grant nos.: NSC102-2221-E-019-006-MY3 and NSC100-2628-E-019-003-MY2) and National Taiwan Ocean University (grant no.: NTOU-RD-AA-2012-104012).
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