| The polarization can be controlled by magnetic field or the magnetic moment can be controlled by electric field through the magnetoelectric coupling (ME) effect in multiferroic materials, which have attracted wide attention. Due to the requirements of the outer-shell electronic structure for ferromagnetism and ferroelectricity are mutually exclusive, single-phase multiferroic materials are very rare. Only BiFeO3 has both above room temperature ferroelectric (FE) (Tc-830℃) and antiferromagnetic (AFM) (TN-370℃) orderings, which can meet the requirements of room temperature applications for devices. Thus, BiFeO3 has become the focus of research.Due to antiferromagnetic nature which limits its macroscopic magnetization, the most plausible application of BiFeO3 is to be used in spintronics devices as antiferromagnetic pinning layer. Thus, the ferroelectric properties of BiFeO3 films and exchange bias with ferromagnetic layer have attracted considerable research interests. However, due to the mixed valence state of Fe induced by O deficiency and Bi evaporation, BiFeO3 always suffers from high leakage current. Thus the good insulation and ferroelectric properties of samples must be ensured. Furthermore, the exchange bias should at least be maintained, to realize the magnetoelectric manipulation.The main results are as following:1. We have explored the growth conditions of BiFeO3 films by pulsed laser deposition (PLD). Through optimizing the growth conditions, we found the best preparation conditions of BiFeO3 films:substrate temperature:750℃; oxygen pressure:2 Pa; the laser pulse energy: 360 mJ; repetition rate:10 Hz; the distance between target and substrate:5 cm.2. The BixFeO3 and Bi1-yLiyFeO3 films were prepared by PLD on (001) SrTiO3 substrate. The structure, ferroelectric, and magnetic properties were studied systematically. The properties of magnetoelectricity and reproducibility are good. The conclusions are:(1) Except for the samples with x≥1.1, the others are pure phase. When the concentration of Bi exceeds 1.1, the impurity phase of Bi2O3 emerges in the samples, which can also be observed in the SEM as islands.(2) The shapes of ferroelectric hysteresis loop of the samples with 0.9≤Bi≤1.2 are normal and the others are irregular. There is little influence of Bi concentration on leakage current.(3) The sample of 3% Li doping possesses the best ferroelectric properties, and the leakage current is also the lowest. That is due to the neutralization of p-type doping by Li replacing Bi on the intrinsic n-type behavior in the samples.(4) Though the fatigue can be improved through 3% Li doping, it still needs further optimization.(5) The exchange bias is not significantly affected by the Li and Bi doping. So the samples with improved ferroelectric properties can be used in the experiments of magnetoelectric manipulation.3. Solid solutions of 80% BiFeO3 and 20% Na0.5Bi0.5TiO3 (BFO-NBT),80% Bi0.95La0.05FeO3 and 20% NBT (BLFO-NBT) were prepared by conventional sol-gel method. Clear ferromagnetic hysteresis loops were observed for both samples. With poling field applied on both samples, the magnetization of BFO-NBT was strongly suppressed (from 0.726 emu/g to 0.282 emu/g at 60 kOe), while that of BLFO-NBT was nearly unchanged. The reorientation of magnetization in poled BFO-NBT has been excluded. The strongly suppressed magnetization in poled BFO-NBT is attributed to the redistribution of oxygen vacancies under electric field, leading to the decrease of defect configurations. The mobility of oxygen vacancies was weakened by 5% La substitution, hindering the redistribution of oxygen vacancies under electric field. |
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