Formation And Properties Of BTO-NZFO Nanocomposite Multiferroic Thin Film Synthesized In Situ By RF Sputtering

Nowadays, miniaturization and multifunctionality have been the inevitable trends for the development of electronic devices. Multiferroic materials combining impressive dielectric and magnetic performances and magnetoelectric coupling, are thus attracting more and more attention. Because of the absence and low properties at room temperature of single-phase multiferroic materials, multiferroic composites become the optimal candidate materials for next generation device. Among all kinds of multiferroic composites, the composite thin film with 1-3 structure is the hotpot of research due to the largest theoretical magnetoelectric coupling. But some shortages, such as occurrence of conducting percolation, mutual restrain between different ferroic parameters and complex preparation process, restrict the further study and application of multiferroic composite thin film with 1-3 structure. To resolve these problems, BaTiO3(BTO)-Ni0.5Zn0.5Fe2O4(NZFO) composite thin film prepared by RF magneton sputtering was investigated in this work. Some achievements were obtained.Firstly, the BTO-NZFO composite thin film with low loss was fabricated successfully. In this composite thin film, the grain size of NZFO was controlled to about 20 nm by choosing the suitable sintering temperature and thickness of thin film. High fraction of the grain boundary with deformation lattice of the nanosized ferrite particles restrains the charge hopping which contributes importantly to the conductivity of the BTO-NZFO composite, making the dielectric loss of the nanocomposites (0.009) near the transition threshold to be more than one orders of magnitude lower than those of traditional composites.Secondly, the BTO-NZFO composite thin film possessing simultaneously low dielectric loss and high permeability was fabricated successfully. In this thin film, the communication between magnetic particles cannot be blocked up by one layer BTO by controlling the size of non-magnetic BTO grains to be smaller than 20 nm and using the characteristic of magnetic communication which can across non-magnetic layer with nanoscale. It makes the magnetic threshold of the nanocomposite shift downwards from the topological one, and the magnetization is free of the control from demagnetizing field before the ferrite conducting path is formed. The permeability of 0.5BTO-0.5NZFO nanocomposite improves 33% compared with that of traditional composite.Thirdly, the BTO-NZFO composite thin film with high magnetoelectric coupling was fabricated successfully. The topological structure of the nanocomposite thin film is modelled mathematically by the sites network. The anisotropy of the transition threshold of low-dimensional composite along normal and radial direction was revealed theoretically and experimentally. Using this characteristic, the similarity between percolation path and nano pillar and the inducing effect of (111) silicon substrate on NZFO in composite, the composite thin film with 1-3 like structure and (100) oriented ferrite was fabricated. The magnetoelectric coupling of this multiferroic composite thin film is high to 9.2%.In addition, to further know essence of multiferroic composite material, the mechanism of the formation and properties, and the relation between them were investigated deeply, some physical models were established and conclusions were obtained.(1) The craft-microstructure relation in BTO-NZFO composite thin film was constructed by investigating and revealing the formation mechanism of nanocrystalline solid solution and the control of substrate on the gradient activation energies of constituent phases. Knowing this relation, the optimal preparation conditions, including sputtering crafts, sintering temperature and thickness of thin film, for specific microstructure were determined. The effective control on microstructure of composite thin film was achieved.(2) The universal whole circuit model was constructed by analyzing the scope of application of existing physical model for the electric behavior of composite near the transition threshold of topological structure, and via finding and avoiding the reasons for their limitations. In this whole circuit model, the control of dielectric property of constituent phases on the dielectric behavior of multiferroic composite near transition threshold of topological structure was revealled.(3) The controls of the transition of topological structure on the magnetic exchange interaction and equivalent demagnetizing field were interpreted based on magnetization mechanism of multiferroic nanocrystalline. These control relations result in the change of the coercivity and permeability of nanocrystalline composite near the transition threshold.(4) Based on the polarization and magnetization mechanism of NZFO, the azimuthal control of magnetic coupling and charge hopping on magnetic and dielectric properties of nanocomposite was revealed. The smaller and larger angle deviations of resultant A-O-B superexchange coupling in final and initial state from external magnetic field in NZFO lattice of BTO-NZFO composite with (100) oriented ferrite, contribute the higher saturation magnetization and permeability than that of the thin film with randomly distributed NZFO, respectively. The larger angle deviation of resultant direction Fe2+-Fe3+ hopping pair in final and initial state from external electric field in NZFO lattice of BTO-NZFO composite with (100) oriented ferrite, contribute the lower dc conductivity and dielectric loss than that of the thin film with randomly distributed NZFO, respectively. The intrinsic dielectric and magnetic properties can be promoted to optimal values by controlling the arrangement of magnetic coupling and charge hopping, or the orientation of NZFO phase in composite thin film.As a result, the main difficulties for multiferroic composite were well solved in this work. An effective and efficient way to fabricate multiferroic composite thin film with excellent dielectric and magnetic properties and outstanding magnetoelectric coupling was found.