Studies On The Preparation And Magnetoelectric Coupling Properties Of Co/Co3O4/PZT Composite Films

Multiferroic material is a kind of material with the coexistence of ferroelectric and ferromagnetic orderings or other order parameter, and the coupling interaction between them engenders the materials with the unique magnetoelectric effect, it is widely used in the area of storage and sensory, the magnetoelectric voltage coefficient of the single-phase compounds is too small and most of them would show magnetoelectric effect only at low temperature, so it is difficult for the single-phase multiferroic compounds to be used. Comparatively, the magnetoelectric effect in the composite multiferroic materials is much higher than that of the single-phase compounds. So it is promising for the multiferroic composites to be used in the practical application.In the system of ferromagnetic/antiferromagnetic, exchange bias effect will appear when the temperature reduces below Neel temperature of the antiferromagnetic material. When the magnetic domain of ferromagnetic materials reversals with the external magnetic field, it need to overcome the pinning effect of the anti-ferromagnetic material, resulting the offset of the hysteresis loop to the opposite direction of the cooling field. In the system of CO/CO3O4, the Neel temperature is about40K, so we expect that the exchange bias effect would appear when the temperature drops below40K. Pb(Zro.52Ti0.48)03(PZT) is a kind of multiferroic material with piezoelectric effect, the purpose of this paper is to successfully make CO/CO3O4/PZT composite film, and expect to observe the magnetoelectric coupling effect. The main results are as following:Focusing on the fabrication and characterization of cobalt and cobalt oxide films, we successfully prepared cobalt and cobalt oxide film by cluster beam deposition, magnetron sputtering deposition system and pulsed laser deposition system. Then we made characterization on the film and find the cobalt clusters made by cluster beam deposition hade uniform size. The micrographs of TEM shows that the cobalt nanoclusters are5nm in size and are all cubic crystal. The film made by magnetron sputtering deposition system is very flat on the surface. We also measured the resistance of cobalt oxide at low temperature and found the resistance increased as the temperature decreased.We successfully prepared PZT film by Sol-Gel method and then made CO/CO3O4/PZT composite film, after that, we measured magnetic and electrical properties of composite film. SEM photography shows the surface of composite film is dense and smooth, the thickness of Co layer is approximately30nm, the thickness of CO3O4layer is about25nm and the thickness of PZT layer is about180nm. We know the Neel temperature of CoO is almost290K, higher than that temperature of CO3O4(40K). VSM measurements at different temperatures showed that exchange bias happened of composite film at about77K, proved the existence of CoO.We first used PPMS to measure the capacitance with varies temperature at different magnetic field, and found the capacitance mainly decreased as the temperature changed from300K to10K and there were mutations at180K and40K, we consider the mutation at180K was occurred by the phase transition of PZT from tetragonal to rhombohedral phase while the mutation at40K was caused by the transition from paramagnetic to antimagnetic. When magnetic field was applied on the composite film, the center position of the peak shifted to190K while there was no shift at40K. The shift at190K was mainly caused by the internal stress of the PZT film, indicating there is magnetoelectric coupling effect in composite film.Then we used PPMS to measure the capacitance with varies magnetic field at different temperature, found the capacitance curve would become asymmetry at low temperature due to the existence of exchange bias. When the magnetic field was positive, The magnetic capacitance of the film is about-7%and would not change with temperature. When the magnetic field was negative, The magnetic capacitance of the film is very small(no more than1%) and would became smaller as the temperature decreased. The significant inconsistency of the magnetic capacitor in the positive and negative magnetic field is mainly caused by the exchange bias effect at low temperature. When the magnetic field is positive, the antiferromagnetic layer has tiny affection to the switching of Co magnetic domain, therefor the efficiency of the magnetic-force-electric coupling is high and the magnetic capacitance increases significantly; when the magnetic field is negative, due to the strong pinning effect of the antiferromagnetic layer, the Co magnetic domain flipping needs to overcome the exchange coupling, resulting serious decline of magnetic-force-electric coupling in the composite film and the magnetic capacitance changes very little.