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1199-F03-48

Misfit Strain Relaxation by Secondary Phase Formation in Multiferroic BiFeO3 Epitaxial Thin Films X. Wang1, Y. L. Zhu1, X. L. Ma1, C. Wang2 and H. B. Lu2 1 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

ABSTRACT Microstructures of BiFeO3 (BFO) thin films epitaxially grown on SrRuO3 (SRO) buffered SrTiO3 (STO) (001) substrates by laser molecular beam epitaxy were investigated by means of transmission electron microscopy (TEM). The results showed that the films grown under the oxygen pressures of 1Pa and 0.3Pa, respectively, contain parasitic phase embedded in the BFO phase. The parasitic phase was revealed to be poor in Bi and rich in Fe by high-angle annular dark-field (HAADF) imaging and energy dispersive X-ray spectroscopy (EDS) compositional analysis. In combination with selected area electron diffraction patterns, the parasitic phase was determined to be α-Fe2O3. By lowering oxygen pressure, the density and the size of α-Fe2O3 phase increases whereas the regularity decreases. High resolution TEM images showed that approximately periodic misfit dislocations exist at the interface between the α-Fe2O3 phase and the BFO matrix, indicating that the α-Fe2O3 particles are semi-coherently embedded in the BFO films. Less misfit dislocations were detected at the interfaces between the BFO films and the SRO/STO substrates, implying that the misfit strains in the films may be fully relaxed by the formation of α-Fe2O3 phase.

INTRODUCTION BiFeO3 (BFO) thin films have been widely investigated due to their intrinsic multiferroic properties [1-4]. It is recently shown that parasitic phases coexisting in BFO thin films influence the magnetic properties of the films [5] and also change the misfit relaxation behavior in the BFO thin films [6]. By varying deposition oxygen pressure, the main phase formed in the BFO films grown by pulsed laser deposition changes from single phase of BFO to α-Fe2O3+BFO [5] or a mixture of BFO, α-Fe2O3 and γ-Fe2O3 [6,7]. Thick-dependent microstructural evolution has also been systematically studied in Bi-Fe-O thin films [7]. In the present work, we present a detailed study of microstructural characteristics in BFO films prepared at different oxygen pressures by laser molecular-beam epitaxy. The secondary phase particles were found embedded in the as-received BFO films. We focus on the relationships between the BFO and the secondary phase. The misfit relaxation in this film system shows behavior different from that in BFO thin films previously reported. It is concluded that the misfit strains between the BFO films and the substrates are accommodated by the formation of secondary phase.

EXPERIMENTALS Single crystal SrTiO3 (STO) substrates were used to grow the films epitaxially. BFO films were prepared by laser molecular-beam epitaxy, employing a XeCl excimer laser with a