| With the rapid development of current science technology,traditional data storage methods are constantly impacted by various new technologies and materials.In the design and development of memory devices,miniaturization and multifunction are the general trend.Therefore,the research of multiferroic materials with both electric polarization and magnetic polarization as well as the mutual regulation of ferroic orders,has become an increasing need for information storage technology.This kind of materials will provide an additional degree of freedom for the design of the next generation of multi-functional electronic recording devices and greatly broaden the application scope of ferroic materials.Therefore,it has important scientific significance and application prospects.Due to the symmetry between ferromagnetism and ferroelectricity and the restriction of filling rules of electron orbits,natural single-phase multi-ferroic materials are rare and their room temperature magnetoelectric coupling is weak,which seriously restricts the application of single-phase multiferroic materials in the field of microelectronics.However,composite multiferroic materials can easily realize the coexistence of ferroelectric and ferromagnetic polarization,resulting in their obvious advantages over single-phase materials in practicality.Thus it is of great theoretical and practical value to prepare multiferroic composite films with preferred orientation or even epitaxial films,to regulate the internal stress and phase contents of the films during the growth process,to observe the morphology and micro-structures of the films with different phase dispersion,and to study the effects of the self-assembled nano-structures on the electric and magnetic properties as well as the magnetoelectric coupling property.In the aspect of experimental research,the perovskite ferroelectric materials and spinal ferromagnetic materials were used to deposit oriented grown multiferroic composite films by multiple target radio-frequency magnetron sputtering.The evolution of phase dispersion in the films was studied,and the relationship between the micro-structures and ferroelectric properties,ferromagnetic properties as well as magnetoelectric coupling property were revealed.The specific contents of this paper are summarized as follows:(1)Single phase BaTiO3 ferroelectric thin films were prepared on single crystal silicon substrates under different magnetron sputtering conditions and the effects of the deposition parameters on the micro-structures and properties were studied.The results showed that the dielectric dispersion and ferroelectric properties of the film were enhanced by introducing LNO buffer layer between Pt electrode and BTO film.The degree of preferred orientation,dielectric constant and ferroelectric polarization of the films were increased by increasing the sputtering pressure.BTO film with good dielectric frequency stability and good ferroelectric voltage resistance were obtained under appropriate deposition temperature(e.g.500?).The dielectric constant,Curie temperature and theoretical charge-discharge energy density were improved by increasing film thickness.At the same time,single-phase CoFe2O4 ferromagnetic thin films were prepared on single crystal MgO substrates and the effect of annealing process on the ferromagnetic property was studied.It was found that the ferromagnetic property of CFO thin film was changed from anisotropic into isotropic after annealing treatment.(2)BaTiO3-CoFe2O4 ferroelectric-ferromagnetic composite films were prepared on single crystal SrTiO3 substrates under different magnetron co-sputtering conditions and the effects of the deposition parameters on the micro-structures and properties were studied.The results showed that the dielectric constant and saturated magnetization of the composite films decreased when the orientation of the substrates changed in the order of(100)(110)(1 11)and the ferroelectric property also exhibited anisotropy.The degree of preferred orientation of the composite film on SRO bottom electrode was higher due to the compressive stress induced by the misfit between film and substrate while the dielectric frequency stability of the film on LSCO bottom electrode was the worst and the ferroelectric property of the film on LNO bottom electrode was the best.The increase of sputtering pressure made the films change from polycrystalline to preferred orientation growth while the dielectric constant decreased,the peak of dielectric loss moved to low frequency,and the ferroelectric polarization and piezoelectric response decreased.As deposition temperature increased,the films tended to form polycrystalline structure,and the dielectric constant decreased while the ferroelectric property became better.(3)Under the pre-optimized preparation conditions,BTO-CFO magnetoelectric composite films with different concentrations of ferroelectric and ferromagnetic phases were prepared,and the effects of different phase dispersion on the micro-structures and properties of the composite films were studied.Scanning probe microscopy analysis revealed that the composite film with a high sputtering power of CFO had a uniform dispersion of the BTO phase in the CFO matrix,forming a self-assembled nano-composite which showed a dielectric percolation effect with significantly enhanced dielectric constant and suppressed dielectric loss in a broad frequency range.In contrast,the other film with a low sputtering power of CFO showed dispersion of the CFO phase in the weakly inter-connected BTO matrix,corresponding to an enhanced low-frequency dielectric constant and an increased dielectric loss,as well as stronger frequency dependences of the two parameters.The two distinctively different percolation behaviors of dielectric properties might be explained by considering the competing effects between the space charge polarization and 'micro-capacitors' model.The leakage current and ferroelectric characteristics of the films were consistent with their dielectric properties.At the same time,the degree of phase dispersion had an influence on the ferromagnetic and magnetoelectric coupling preperties of the composite films. |
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