| Aging behaviors are frequently observed in many kinds of materials, indicating the gradual changes of certain properties of materials with time elapsing. For ferroelectric materials, the aging behaviors mainly include the phenomena of double hysteresis loops and asymmetry of coercive field.As a lead-free multiferroic material, BiFeO3 (BFO) has been investigated extensively for the potential application in multifunctional devices. It has been demonstrated that the BFO films with desired ferroelectric properties can be obtained in N2 or an atmosphere with low oxygen pressure at annealing temperatures ranging from 500℃to 670℃, which are much lower than the Curie temperature of BFO (~850℃). Therefore, the film should be subjected to aging during the deposition process or the annealing treatment, that is, the domains will be gradually stabilized by the defect complexes formed between the oxygen vacancies and low-valence ions such as Fe2+.Aiming to demonstrate this hypothesis, various BFO thin films doped with low-valent ions at A and B sites were fabricated using a metal organic decomposition method combined with sequential layer annealing, and their individual properties and comparative study were performed.The BiFe0.95Zn0.05O3 (BFZO) thin film was fabricated on indium tin oxide(ITO)/glass substrate, and double hysteresis loops and asymmetric coercive fields which were triggered by aging were studied. Double ferroelectric hysteresis loops were observed, indicating that the film is partially aged. This aging phenomenon can be attributed to the formation of defect complexes between oxygen vacancies and acceptor defects in the film. The nonreversible transition from double ferroelectric hysteresis loop to the single one can occur during the course of increasing the applied electric field, which should be due to the breaking of defect complexes as well as the switching of the 71°/109°domains. The double ferroelectric hysteresis loops in low electric field region should be due to the backswitching of 180°domains. Additionally, an asymmetry of the coercive field can be found to be strongly dependent on the magnitude and frequency of the electric field, and the observed asymmetric coercivity should be due to that the local fields produced by defect complexes which can provide a preferential driving force for domain backswitching in BFZO film with a preferential polarization.To compare the effect of aging behaviors on multiferroic properties of A- and B-site-doped BFO, Bi0.95Ca0.05FeO3 (BCFO) and BiFe0.95Ni0.05O3 (BFNO) thin films with complete (001) orientation were epitaxially grown on (001)-oriented LaNiO3 oxide electrodes respectively, and their aging behaviors were studied. With the increasing of applied voltage, the evolutions of hysteresis loops under different applied voltages were observed, so that aging behaviors exist in both BCFO and BFNO films. However, the voltage of de-aging for BCFO is lower than that for BFNO, suggesting that the aging degree of BCFO is lower than that of BFNO. Based on crystal structure and defect chemistry, the aging model that the distinction of aging degree between BCFO and BFNO stems from the different lattice sites which defects occupied as well as the different symmetry of local fields triggered by defects was proposed. Additionally, the leakage current of BCFO was of almost three-order higher than that of BFNO, indicating that there are fewer or weaker defect complexes in BCFO, as well as doping at B site can more effectively suppress the presence of Fe2+. In term of ferromagnetic properties, the remanent magnetization of BCFO is apparently lower than that of BFNO, implying that the local fields produced by defect complexes also affect the magnetic properties of both BCFO and BFNO.A series of Bi1-xCaxFeO3 (BCFO) thin films with no other phase were prepared on ITO/glass substrates. After detecting the BCFO films of different Ca doping concentration by an x-ray diffractometer, the instable ferroelectric phase region of BCFO films was proved to be enlarged. As a result, it remains to be ferroelectric phase at the point of x=0.2. And it is found that the points at x=0.125 and x=0.15 show a large deviation from classical Vegard's law behavior. We explain the reason for different orientations of BCFO films of different Ca doping ratio through analyzing the Ca-substitution dependence of the surface energy. The Au/BCFO/ITO/glass FE capacitors of different Ca doping ratio were prepared. After de-aged by repeatly applying high votage to polarize the films, the ferroelectric properties at voltages ranging from 8V to 50V were measured and well P-E hysteresis loops were obtained, so that this de-aging method was proved to be appropriate. The leakage current of Ca-doped BCFO films decreased, and the orientation of main polarization axis of BCFO does not change. It is found that the remanent polarization value varies with the (202)-oriented peak indensity, and the BCFO film with Ca doping concentration of 0.05 shows the best ferroelectric property, with the remanent polarization and coercive field of 48μC/cm2 and 545kV/cm, respectively.
|
PDF Full Text Request