Rapid preparations of Bi1-xLaxFeO3± δ thin films and their ferroelectric properties

Abstract

Multiferroic materials exhibiting the features of ferroelectricity, ferromagnetism and even ferroelasticity simultaneously have attracted much attention because of their vast potential applications in multifunctional devices as well as their interesting physical connotations. BiFeO3 (BFO) is the multiferroic material most studied because it has only single phase of multiferroic oxide with giant remanent polarization above room temperature. Although BFO has many excellent advantages, the large leakage current is a chief obstacle for its practical application in some devices. As is well known, the leakage current of BFO is due to the valence transformation from Fe3+ to Fe2+ which results in the oxygen vacancy defect and low ferroelectric properties. Some experiments have confirmed that substituting some cations at A site (Bi) or B site (Fe) can improve the multiferroic property of BFO. In addition, we can reduce the leakage current by increasing the oxygen pressure to compensate for the vacancy defect during annealing. In the present work, we employ the sol-gel method which has been widely used in industries to prepare lanthanum doped BFO thin films (La =0, 5%, 10%, 15%, 20% and 25%) (BLFO) and Bi0.75La0.25FeO3± δ thin films separately in air and pure oxygen annealing atmosphere. And we are to achieve the optimal ferroelectric properties of BFO thin films. The traditional trial-and-error method which is used to check the value of a certain parameter one by one always takes rather long time. The high throughput methodology can screen the parameters simultaneously, which greatly reduces the optimizing time. Employing the high throughput methodology, we successfully realize a faster optimizing process to achieve the strongest ferroelectric property in La-doping BFO thin film. We analyze the structures and the ferroelectric properties of the samples grown in different conditions, such as the annealing temperature, the concentration of La-doping and the annealing atmosphere, etc. Results are as follows. 1) The optimal annealing temperature for achieving a single phase thin film is around 560℃. X-ray diffraction (XRD) patterns show that all the samples, including La-doping thin films with different concentrations, are of perfect single phase. Bi0.75La0.25FeO3± δ thin films are prepared separately in air and pure oxygen annealing atmosphere. 2) We calculate the lattice constants for all the doping samples of BLFO. With the increase of La-doping concentration, both a and b values reach the largest lattice constants of a=b=5.59~Å at La=15%. 3) Among all the doping samples, the sample with a La-doping concentration of 15% has the largest polarization 26.7 μC/cm2, which is consistent with its largest lattice constants. 4) The degrees of crystallinity and the ferroelectric properties of Bi0.75La0.25FeO3±δ thin films annealed in pure oxygen atmosphere are much better than those in air. The high throughput method is successfully used in the present work, and it plays an important role in exploring new materials in high-efficiency, speediness and objectivity. Therefore, it can be extended to many other materials for optimizing the grow conditions.