The Effect of Bi-excess Surface Layers on BiFeO 3 Thin Films Prepared by Chemical Solution Deposition
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The Effect of Bi-excess Surface Layers on BiFeO3 Thin Films Prepared by Chemical Solution Deposition Yoshitaka Nakamura, Seiji Nakashima, Dan Ricinschi, and Masanori Okuyama Systems Innovation, Graduate Schools of Engineering Science, Osaka University, 1-3 Machikaneyama-Cho, Toyonaka, Osaka, 560-8531, Japan ABSTRACT We have investigated the effect of Bi-excess surface layers of stoichiometric BiFeO3 thin films prepared by chemical solution deposition. A stoichiometric BiFeO3 thin film with both the Bi-excess top and bottom surface layers shows improved crystallinity with the remanent polarization of 65 µC/cm2 at 80 K, which is larger than BiFeO3 film prepared by the same process using stoichiometric solution. These results are attributed to the reduction of the imperfect crystal at the interface between BiFeO3 film and electrode. By inserting Bi-excess layers, the saturated magnetization of all the films becomes smaller than that of the film using stoichiometric solution. Preparing stoichiometric BiFeO3 thin films with Bi-excess surface layers is an effective way to obtain good ferroelectric property. INTRODUCTION Recently, multiferroics have gathered much attention because of the coexistence of ferroelectricity, magnetism, and ferroelasticity. Furthermore, multiferroics are expected to have the interaction between these properties, i.e. ferroelectric (magnetic) property can be controlled by magnetic (electric) field. This leads new types of devices such as ferroelectric random access memory (FeRAM), sensors, and actuators. Among all multiferroic materials, BiFeO3 is one of the most promising materials with antiferromagnetic ordering below the Neel temperature (TN) of about 643 K and ferroelectric ordering below the Curie temperature (TC) of about 1103 K [1]. In bulk form, BiFeO3 shows leaky ferroelectric properties with spontaneous polarization of 3.5 µC/cm2 along the (001) direction and 6.1 µC/cm2 along the (111) direction [2]. In contrast, BiFeO3 thin film shows good ferroelectric polarization prepared by pulsed laser deposition as well as chemical solution deposition [3] [4] [5]. We have reported that BiFeO3 thin film prepared by chemical solution deposition shows improved ferroelectric property with 10% Bi-excess precursor solution, while better magnetic property is obtained with stoichiometric solution [6]. In the film prepared by stoichiometric solution, however, the crystallinity is lower than that of 10% Bi-excess solution because the chemical reaction temperature of stoichiometric solution is higher than that of 10% Bi-excess solution and Bi evaporation occurring from the surface of the film may prevent the crystallization. Recently, BiFeO3 thin film prepared by chemical solution deposition shows a noncrystalline layer between BiFeO3 thin film and electrodes [7]. Such a layer is also reported as the interfacial layer in BaTiO3 thin film and degrades the ferroelectric property [8]. This suggests that the low crystallinity area can be concentrated at the interface between the film and electrode.
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