Journal of Alloys and Compounds 509 (2011) 7742–7748 Contents lists available at ScienceDirect Journal of Alloys and Compounds j our na l ho me p ag e: www.elsevier.com/locate/jallcom Bismuth ferrite bilayered thin films of different constituent layer thicknesses Jiagang Wu a,b,∗ , John Wang b , Dingquan Xiao a , Jianguo Zhu a a Department of Materials Science, Sichuan University, Chengdu 610064, PR China b Department of Materials Science and Engineering, National University of Singapore, Singapore 117574, Singapore a r t i c l e i n f o Article history: Received 25 March 2011 Received in revised form 27 April 2011 Accepted 29 April 2011 Available online 10 May 2011 Keywords: Bismuth ferrite Bilayer structure Multiferroic behavior Fatigue behavior a b s t r a c t Bilayered thin films consisting of (Bi 0.90 La 0.10 )(Fe 0.85 Zn 0.15 )O 3 and (Bi 0.90 La 0.10 )(Fe 0.90 Zn 0.10 )O 3 layers have been fabricated by radio frequency sputtering. Both multiferroic layers are well retained in these bilayers. Their leakage current, multiferroic properties, and fatigue behavior are largely dependent on the thick- nesses of (Bi 0.90 La 0.10 )(Fe 0.85 Zn 0.15 )O 3 . With an increase of the thickness in the (Bi 0.90 La 0.10 )(Fe 0.85 Zn 0.15 )O 3 layer, the leakage current density of bilayers is degraded due to different grain growth modes and an increase in oxygen vacancies, the dielectric constant (ε r ) becomes larger due to the introduction of (Bi 0.90 La 0.10 )(Fe 0.85 Zn 0.15 )O 3 with a high ε r value, and their magnetic properties are deteriorated with increasing the thickness ratios of (Bi 0.90 La 0.10 )(Fe 0.85 Zn 0.15 )O 3 with a weaker magnetization. All bilayers exhibit a good ferroelectric behavior regardless of varying thicknesses of the (Bi 0.90 La 0.10 )(Fe 0.85 Zn 0.15 )O 3 layer, while their coercive field decreases with increasing the thickness of the (Bi 0.90 La 0.10 )(Fe 0.85 Zn 0.15 )O 3 layer. An anomalous enhancement in switchable polarization is demonstrated by these bilayers, owing to the involvement of space charges accumulated at the interfaces between two constituent layers. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Multilayered ferroelectric thin films differing in their structures or/and compositions exhibit novel magnetics [1,2], an improved polarization [3–5], a high dielectric constant [6,7], a new structural phase [8], and other new functional properties [9,10] owing to the epitaxial strain and the interface coupling and interactions among these constituent interfacial layers. Multiferroic materials have recently attracted much attention [11–24]. Among those, multiferroic BiFeO 3 (BFO) material with a rhombohedrally distorted perovskite structure has recently been given to considerable attention due to its giant remanent polariza- tion, a high Curie temperature, and the existence of ferroelectric and ferromagnetic properties at and above room temperature [13–26], and the material shows promise for practical applications in high density ferroelectric random access memories, spintron- ics, and sensors and actuators, and so on [11–26]. In contrast, a very high leakage current density at room temperature seriously degrades the electrical behavior of BFO material, and thus hin- ders its practical applications [11–26]. Although some methods have been used to decrease the leakage current density of BFO thin films, their electrical behavior is still not ideal [2,5,27,28]. Among these promising methods, the bilayered structure consisting of ∗ Corresponding author at: Department of Materials Science, Sichuan University, Chengdu 610064, PR China. Tel.: +86 28 85412202; fax: +86 28 85412202. E-mail addresses: wujiagang0208@163.com, msewujg@scu.edu.cn (J. Wu). BFO and other ferroelectrics may be a more promising method for decreasing the leakage current density and improving the elec- trical properties of BFO, owing to the advantage of the bilayered structure. In the past, the bilayered thin films consisting of BFO and other ferroelectric layer have been reported [2,5,28], such as Bi 1/2 Na 1/2 TiO 3 , Pb(Zr, Ti)O 3 . Some interesting results have been demonstrated by such a bilayered structure, such as the reduction in leakage current density, the improvement in phase purity, and the enhancement in fatigue behavior [2,5,26], but their polarization value is much lower as compared with the intrinsic polarization one of BFO thin films [2,5,28]. In the present work, the bilayered thin films con- sisting of (Bi 0.90 La 0.10 )(Fe 0.85 Zn 0.15 )O 3 (BLFZO15) and (Bi 0.90 La 0.10 )(Fe 0.90 Zn 0.10 )O 3 (BLFZO10) layers were grown in situ on Pt/TiO 2 /SiO 2 /Si(1 0 0) substrates without any buffer layers by radio frequency (rf) sputtering, where the thickness of the BLFZO10 layer is fixed, and the thickness of the BLFZO15 layer is changed from 20 nm to 180 nm. The thicknesses of the BLFZO15 layer seri- ously affect the leakage current density, dielectric properties, and multiferroic and fatigue behavior of BLFZO15/BLFZO10 bilayered thin films, and the underlying physics mechanisms were clearly addressed. 2. Experimental procedure The two-inch BLFZO10 and BLFZO15 ceramic targets with an excess of 10% Bi have been synthesized via a solid state reaction of constituent oxides, namely Bi2O3 (99.9%), Fe2O3 (99.9%), La2O3 (99.99%), and ZnO (99.9%). The powder mix- tures of appropriate compositions of these oxides were calcined at ∼700 ◦ C in 0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2011.04.140