Preparation Of CoFe₂O₄and BiFeO₃Thin Films By Ultrasonic Spray Pyrolysis And Their Properties

Electronic system has been oriented toward integration, intelligentization, lightweightand versatility, and multiferroic magnetoelectric materials emerge under this background. The2-2type laminated structure magnetoelectric composite film, which can effectively solve theproblem of leakage guide and improve the magnetoelectric effect, has attracted muchattention. Developing a new simple chemical method to prepare the2-2type magnetoelectricfilm is of great significance. Spray pyrolysis is a very promising thin film fabrication methodwith simple process and low cost. In this paper, the dual-source ultrasonic atomization spraypyrolysis film deposition system was firstly designed and improved. The CoFe2O4films andBiFeO3films were prepared using this facility. The present work has laid a good foundationfor the future preparation and investigation of2-2type magnetoelectric films. The maincontents and results of this work are as follows:The dual-source ultrasonic atomization spray pyrolysis film deposition system wasdesigned by the author and the manufacturers. A series of improvements have also been madeon the initial facility according to our experience on the film preparation. The characteristicsof this system are as follows. Firstly, there are two independent atomizing chambers, whichare convenient for the preparation of doped films and multi-layer structures. Secondly,vertically up feeding mode was adopted in this system, and the gravity is used to filter largedroplets, so the size distribution of droplet is very narrow. Thirdly, substrate rotating devicewas added to the system to make the substrate rotate under a certain speed, thus more uniformthin films could be obtained. What’s more, two extra carrier gas import interfaces weredesigned for the convenience of carrier gas selection according to different needs.The magnetic films of CoFe2O4were prepared by ultrasonic spray pyrolysis. Theinvestigations were focused on the influences of substrate temperature on film structure andproperties, the forming mechanism of the film, and the improvement of film properties byannealing treatment. The study showed that the film began to form spinal phase when thesubstrate temperature was350℃. The crystallinity increased with the increase of substrate temperature, but it would form impurity phases such as Fe2O3if the substrate temperature wastoo high. The growth mode of cobalt ferrite film on Si substrate was island mode. The surfaceof the films was covered with large particales formed by polycrystalline aggregate, and withthe increase of the substrate temperature, the film roughness reduced. The thickness of thefilm firstly increased and then decreased with raise of substrate temperature, and it reachedthe maximum value of15.7μm at350℃. High substrate temperature can obviously enhancethe coercivity, which reached3400Oe at500℃, but it cannot improve the saturationmagnetization greatly. Annealing treatment can change the microstructure and improve theproperties of as-deposited CoFe2O4film. For cobalt ferrite films deposited at300℃, the bigparticals fell off from the surface after annealing. The saturation magnetization was up to43.5emu/cm3after800℃annealing, which was almost10times higher than that ofas-deposited film, and the coercivity reached the maximum value of1120Oe after700℃annealing. But the property improvement for the film deposited at high temperature byannealing treatment was limited, and some soft magnetic or antiferromagnetic secondaryphase would form after annealing.The preparation process of BiFeO3films and their structures and magnetic properties byultrasonic spray pyrolysis were also investigated in this work. The most difficult point in thepreparation of BiFeO3film was the inhibition of Bi3+hydrolysis in the precursor solution. Thesubstrate temperature had a great influence on the phase constitution of the film. The pureBiFeO3phase can be obtained only at about350℃, and Bi2Fe4O9phase would appear athigher substrate temperatures. In general, BiFeO3film prepared by spray pyrolysis also grewon Si substrate in island mode, which had a bad continuty. Annealing treatment was carriedout for the film deposited at300℃. The BiFeO3and a small amount Bi2Fe4O9phases werefound after500℃annealing, which can be considered as the best annealing temperature. Inorder to compensate the volatilization loss of Bi3+under high temperature, appropriate amountof extra Bi3+(less than5%) can be added in the precursor solution to suppress the formationof Bi2Fe4O9or Bi2O3phase. BiFeO3film prepared by ultrasonic spray pyrolysis only showedweak magnetism at room temperature. The coercive force was very small. The Neil temperature of BiFeO3phase can be deduced from the Magnetization vs. Temperature curve,which is about670K.The current research shows that the dual-source ultrasonic atomization spray pyrolysisfilm deposition system has its own characteristics compared with the similar systems at homeand abroad. The system can be used for the preparation of different types of oxide films.Magnetic CoFe2O4and ferroelectric BiFeO3films were successfully obtained using thisfacility and based on optimal processes. Although the structures of the films need to be furtherimproved, the work in this paper laid a good foundation for further research of high qualitycomplex functional oxide films and composite films.