Epitaxial growth of BiFeO3 thin films by LPE and sol–gel methods

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Abstract

Epitaxial BiFeO3 thin films have been grown on the (0 0 1) SrTiO3 and LaAlO3 substrates by liquid phase epitaxy (LPE) and sol–gel methods. The films had an in-plane texture of 2° and out-plane texture of 0.6° and showed a 4-fold symmetry about the c-axis, indicating a distortion of the bulk rhombohedral cell to a tetragonal or cubic structure. The LPE films were both ferroelectric and ferromagnetic. The sol–gel films showed a small linear increase of the magnetisation with the applied magnetic field, indicating they possibly had the same antiferromagnetic behaviour as the bulk crystal. The LPE films exhibited a large leakage current, which probably originated from a relatively high level of Fe2+ in the films induced by the high growth temperature. Fe2+ might also account for the origin of the ferromagnetism in LPE films.

Introduction

In recent years there has been an increasing interest in a new class of materials, in which both electrical and magnetic ordering can coexist. Such so-called magnetoelectric materials have many potential applications including a new type of memory device based on the combination of ferromagnetism and ferroelectricity. BiFeO3 is one such material and has attracted a wide attention [1], [2], [3], [4], [5], [6], [7], [8], [9]. BiFeO3 belongs to the rhombohedral system with space group R3m and a=0.396 nm, α=89.5[6], [7]. BiFeO3 is antiferromagnetic below the Neel temperature of about 647 K [8], [9] and ferroelectric with a high Curie temperature of 1043 K [8], [9]. Most of current researches on this material were carried out using bulk polycrystalline samples [1], [2], [3]. Recently, there were reports that epitaxial thin films possessed some unique properties over the bulk crystals, including a greatly enhanced polarisation [5]. Therefore, it is of great interest to carry out more research on the thin film growth of BiFeO3. In this short paper, we report the epitaxial growth of highly textured BiFeO3 thin films on single crystal substrates of SrTiO3 (STO) and LaAlO3 (LAO) by liquid phase epitaxy (LPE) and sol–gel methods. LPE is known to be one of the best methods to grow high purity, high structure perfection thin films because in this technique, the films are grown at near thermal equilibrium conditions and unwanted impurities are excluded from films by the segregation effect of crystals grown from liquid phase. Sol–gel is a versatile and quick method for preparation of thin films. It can be carried out at lower temperatures and on a wider range of substrates, in particular on those with a large lattice mismatch.

Section snippets

Experimental procedure

LPE growth of BiFeO3 was carried out from a 85% Bi2O3+15% Fe2O3 flux contained in an alumina crucible. In order to avoid the high-temperature liquid being contaminated by the crucible, the inside surface of the crucible was coated with a thin layer of Fe2O3, which was made by painting on iron citrate gel and then sintering at 1200 °C for 24 h. The crucible, charged with well-mixed Bi2O3+Fe2O3 powder, was heated up to 830 °C to form a single liquid state [10]. After soaking for about 6 h, the

Results and discussion

An XRD θ2θ scan (2θ=2080°) is shown in Fig. 1. Only the (0 0 l) (l=1,2,3) reflections of the pseudo-cubic cell were seen for the BiFeO3 thin films grown on (0 0 1) LAO by sol–gel, indicating the films were highly c-oriented. Films grown on STO substrates showed similar diffractograms. Such well-aligned c texture could only be maintained in thin films of thickness below ∼300 nm. For the thicker films (>300 nm) some other orientations, e.g. (1 1 1), occurred as shown in the surface SEM image in Fig. 2,

Concluding remarks

Epitaxial BiFeO3 thin films have been grown on single crystal substrates of STO and LAO by LPE and sol–gel methods. The LPE films were single phase and highly oriented with an in-plane texture of 2° and out-plane texture of 0.6°. The sol–gel films grown from citrate or acetate gel solutions were also single phase and textured, but showed some porosity on the surface, possibly resulting from Bi evaporation. The LPE films grown on (0 0 1) STO showed a heteroepitaxial structure different from the

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