Study On Magnetoelectric Effects Of Bismuth Ferrite

The multiferroic materials refer to those possess two or more basic ferroic orders at the same time.It can realize the coexistence and coupling between ferroelectricity?or anti-ferroelectricity?and ferromagnetism?or anti-ferromagnetism and ferrimagnetism?.As the rare single phase multiferroic material at room temperature,the ferroelectric Curie temperature and antiferromagnetic Néel temperature of BiFeO3?BFO?are 1103 K and 643K respectively,which are much higher than room temperature.It makes BFO one of the most researched systems in the multiferroic field.However,BFO cannot exhibit the macroscopic magnetic moment due to the antiferromagnetic nature,hindered the direct applications in magnetoelectric coupling devices.Typically,coupling between ferroelectricity and ferromagnetism was achieved by the exchange bias effect near the ferromagnetic-antiferromagnetic?FM-AFM?interfaces.The realization of magnetoelectric coupling requires that BFO thin films possess excellent ferroelectric properties and strong saturation polarization.However,the polarization will decrease gradually after repeatedly polarization reversal,i.e.the so called ferroelectric fatigue effect.The research on fatigue properties of BFO is mainly based on non-magnetic nobel metal electrodes,such as Au and Pt.The researches on FM metallic electrodes are very limited,while FM electrodes are necessary for BFO magnetoelectric coupling devices.As the same time,La2/3Sr1/3MnO3?LSMO?is one of the most widely used material for magnetic coupling layer due to its excellent half metallic properties and 100%spin polarization.However,the upper temperature of exchange bias?i.e.blocking temperature?in BFO-LSMO bilayers is very low?¹00 K?,which greatly hindered the commercial applications of this system.The research for the system with higher blocking temperature is also very urgent.In this paper,we focus on the ferroelectric and magnetic properties of BFO-based heterostructure,we carried out the following aspects of studies.1.BFO epitaxial films of high quality were successfully grown on SrTiO3?STO?substrates by pulsed laser deposition.We optimized the growth parameters and found the best growth conditions:substrate temperature of 750?,oxygen partial pressure of 2 Pa,laser energy of 2.5 J/cm²,frequency of 10 Hz,target-substrate distance of 5 cm2.The ferroelectric fatigue properties of BFO thin films with ferromagnetic metallic electrodes were studied and compared with traditional Pt electrodes.We found its abnormal fatigue behavior,and attributed this phenomenon to the competitive relationship between the pinning effects to charged ferroelectric domain walls of oxygen vacancies and the migrations of oxygen vacancies in electric field.Meanwhile,the different properties and work function between Pt and NiFe electrodes will also produce significant influences on its ferroelectric properties.3.The exchange interactions at the interface of BFO and Sr2FeMoO6?SFMO?were studied.An obvious exchange bias effect was observed after cooling under field of 5000 Oe,and the bias field was about 55 Oe.The existence of spin glasses at the interface is excluded by the cooling field dependent magnetization measurements,thus the exchange bias is determined to be FM-AFM type.The blocking temperature of BFO-SFMO interface is about 160 K,which is much higher than that of BFO-LSMO system.There are two blocking temperatures TB1 and TB2.The TB1 at about 160 K corresponds to the exchange interactions of Fe³⁺-O²-Fe³⁺,while the T_B2 at about 110 K corresponds to the exchange interaction of Fe³⁺-O²-Mo⁵⁺.