Hydrothermal epitaxy and resultant properties of EuTiO3 films on SrTiO3(001) substrate
© Lv et al.; licensee Springer. 2014
Received: 14 April 2014
Accepted: 19 May 2014
Published: 29 May 2014
We report a novel epitaxial growth of EuTiO3 films on SrTiO3(001) substrate by hydrothermal method. The morphological, structural, chemical, and magnetic properties of these epitaxial EuTiO3 films were examined by scanning electron microscopy, transmission electron microscopy, high-resolution X-ray diffractometry, X-ray photoelectron spectroscopy, and superconducting quantum interference device magnetometry, respectively. As-grown EuTiO3 films with a perovskite structure were found to show an out-of-plane lattice shrinkage and room-temperature ferromagnetism, possibly resulting from an existence of Eu3+. Postannealing at 1,000°C could reduce the amount of Eu3+, relax the out-of-plane lattice shrinkage, and impact the magnetic properties of the films.
81.10.Aj; 81.15.-z; 61.05.-a
KeywordsEuTiO3 films Hydrothermal growth Epitaxy Multiferroics
Interest in multiferroics has been recently revived, since coexistence and interactions of ferroelectric, ferromagnetic, and ferroelastic orderings in multiferroics[1–6] could be applied potentially to a range of novel multifunctional devices[6, 7]. As one of the special multiferroic materials, EuTiO3 was found that in the bulk exhibits a G-type antiferromagnetic ordering below 5.3 K[8, 9], and its epitaxial films transform into ferromagnetic under large enough lattice strain[10–13].
A variety of techniques are available to grow fine epitaxial perovskite films, such as pulsed laser deposition, molecular beam epitaxy, radio-frequency magnetron sputtering, and metal-organic chemical vapor deposition. These methods share a common feature that high growth temperatures (>500°C) and costly equipments are usually necessary. In contrast, an attractive alternative technique for preparing epitaxial perovskite films is hydrothermal epitaxy[16–20], which allows direct deposition crystalline films using mild aqueous solutions at temperatures as low as 150°C[16, 18] and avoids the research dependence on the costly aforementioned epitaxial growth equipments. In consideration of the merits of the hydrothermal epitaxy, however, nothing is currently known about the hydrothermal growth of epitaxial EuTiO3 films and their properties.
In this paper, we report the hydrothermal epitaxy of EuTiO3 films on SrTiO3(001) substrate at 150°C and the properties of the films. We find that the as-grown epitaxial EuTiO3 films show an out-of-plane lattice shrinkage and room-temperature ferromagnetism. Postannealing at 1,000°C evidences that this lattice shrinkage relates to the instabilities of Eu oxidation state in the films.
The heteroepitaxial EuTiO3 films investigated were grown on SrTiO3(001) substrate by hydrothermal method. Prior to growth, a solution of KOH (10 M, 15 mL) was added into a suspension which was composed of TiO2 (0.2 g), Eu(NO3)3 · x H2O (1.0 g) and H2O (50 mL) with a subsequent constant stirring for 30 min. The resulting solution was then introduced into a 100-mL Teflon-lined stainless autoclave with a fill factor of 65%, where the SrTiO3(001) substrate was fixed inside. The autoclave was shifted to a preheated oven holding at 150°C. After 24 h of growth, the sample was removed from the autoclave, cleaned by deionized water, and then dried ready in the air for the subsequent measurements. The phase structure of the films was assessed by high-resolution X-ray diffractometry (HRXRD; Bede D1, Durham, UK). HRXRD longitudinal ω- 2θ scans were recorded with an analyzer composed of Ge channel-cut crystals, while a pole figure was taken in skew geometry and with open detector. To assess the morphology and microstructure of the films, the samples were cleaved into smaller pieces for investigation by scanning electron microscopy (SEM; Hitachi S-4800, Chiyoda-ku, Tokyo, Japan) and transmission electron microscopy (TEM; TecnaiTMG2F30, FEI, Hillsboro, OR, USA), the latter through the standard mechanical thinning and ion-milling processes. The elemental composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS; Kratos AXIS UltraDLD, Manchester, UK). The absence of water or hydroxyl in the films was evidenced by Fourier transform infrared spectroscopy (FTIR; Nexus870, Nicolet, Madison, WI, USA). The magnetic properties of the as-grown and annealed samples were measured in a superconducting quantum interference device magnetometry (SQUID). All magnetization data presented here are corrected for the diamagnetic background of the substrate. Postannealing of the as-grown sample was carried out in an Ar ambient for 10 h at 1,000°C.
Results and discussion
To summarize and conclude, using a hydrothermal method, EuTiO3 films with high crystalline quality were successfully grown on SrTiO3(001) substrate at a temperature of 150°C. The films show highly oriented and regularly shaped morphologies with graded spacial distribution, which reflects a sequential growth process of the films. Using this growth technique, EuTiO3 films grown on SrTiO3 substrate exhibit an out-of-plane lattice shrinkage, which could be relaxed by postannealing. Valence instabilities of Eu were found in the sample and result in the EuTiO3 films being ferromagnetic at room temperature, which provides an opportunity to study further their properties and potential applications.
Fourier transform infrared spectroscopy
high-resolution X-ray diffractometry
selected area electron diffraction
scanning electron microscopy
superconducting quantum interference device magnetometry
transmission electron microscopy
X-ray photoelectron spectroscopy.
We thank Tielong Shen and Ji Wang from the Institute of Modern Physics, Chinese Academy of Sciences for their technical help on TEM measurements. This work was supported by the National Basic Research Program of China (Grant No. 2012CB933101), National Natural Science Foundation of China (Grant Nos. 11274147, 51371093, and 11034004), PCSIRT (Grant No. IRT1251), and the Fundamental Research Funds for the Central Universities (Grant No. lzujbky-2013-ct01 and lzujbky-2014-174).
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