| Multiferroic magnetoelectric (ME) materials not only exhibit ferroelectricity and ferromagnetism simultaneously, but also the magnetoelectric coupling effects, which is defined as induced electric polarization under applied external magnetic field or induced magnetization by the external electric field. Recently, multiferroic magnetoelectric materials have stimulated increasing interest due to the extensive and potential applications in sensors, transducers, filters, phase shifters, actuators and memories, etc.The (1-x)Ba0.8Sr0.2TO3-xCoFe2O4(x=0.1,0.2,0.3,0.4) and (1-x)Ba0.6Sr0.4TiO3-xCoFe2O4(x=0.15,0.3,0.45) multiferroic composite ceramics were fabricated by conventional solid state reaction method, respectively. X-ray diffraction and scanning electron microscope (SEM) show the ceramic composites are composed of the tetragonal perovskite Ba0.8Sr0.2TiO3phase (or cubic perovskite Ba0.6Sr0.4TiO3phase) and cubic spinel CoFe2O4phase, without the other intermediate and secondary phase. The influences of different ferrite concentration on the dielectric, ferromagnetic and ferroelectric properties of the ceramic composites were investigated. The variations of the dielectric constant and dielectric loss with frequency at the low frequency for the composites demonstrate dielectric dispersion. In the aspect of the variations of dielectric properties with temperature, the dielectric constant of each composite increases with the increase in temperature up to a peak and then declines at the higher temperature. The dielectric constant decreases with increasing the measured frequencies and the ferroelectric Curie temperature (Tc) shifts to higher temperature as the frequencies increase. These show that the composites behave as relaxor ferroelectrics. Measured by magnetic and ferroelectric tests, the composites display ferromagnetic and ferroelectric properties simultaneously. The saturated magnetization (Ms) values of the composites almost linearly increase with the increase in ferrite content. The magnetic coercivity (Hc) values increase with increasing ferrite content, which indicates the motion and rotation of magnetic domain wall of the composites become harder and harder with increasing ferrite content. In addition, the saturated polarization (2PS) values of the composites decrease with increasing ferrite content, while the remnant polarization (2Pr) values increase with increasing ferrite content. The enhanced ferroelectricity of the composites may be attributed to the space charge effect in the composites. The electric coercivity (2EC) values increase with increasing ferrite content, showing that the motion and rotation of electric domain wall of the ferroelectric regions become harder and harder as ferrite content increases.Theoretically, in order to explain the origin of ferroelectricity for the ferroelectric matrix—BST perovskite structure in the multiferroic particulate composite ceramics from microstructure, first principle was used to calculate the energy of the tetragonal Ba0.8Sr0.2TiO3and cubic Bao.6Sro.4Ti03crystalline cell, in which the lattice constants of tetragonal Ba0.8Sr0.2TiO3and cubic Bao.6Sro.4Ti03are calculated from the X-ray diffraction data of the ceramic samples fabricated in this experiment. The results demonstrate that the displacement of Ti ion away from the center of oxygen octahedron is3pm when the energy of the tetragonal Ba0.8Sr0.2TiO lattice is minimum, while Ti ion is located in the center of the oxygen octahedron when the energy of the cubic Bao.6Sro.4TiO3lattice is minimum. The separation of positive and negative charges in the crystalline structure results in spontaneous polarization. In terms of the calculation of first principle, no spontaneous polarization exists in Ba0.6Sr0.4TiO3crystal at room temperature, exhibiting no ferroelectricity. However, measured by ferroelectric tests, it shows obvious ferroelectricity at room temperature. The ferroelectricity of the Ba0.6Sr0.4TiO3at room temperature propably originates from its compositional fluctuation.In order to incorporate into the microelectronic integration technics, the CoFe2O4/(Ba0.8Sr0.2TiO film as bottom layer, CoFe2O4film as top layer) and CoFe2O4(CoFe2O4film as bottom layer, Ba0.8Sr0.2TiO3film as top layer)2-2style layered heterostructure magnetoelectric composite films were grown on Pt/TiO2/SiO/Si (100) substrate by rf-magnetron sputtering, respectively. X-ray diffraction and scanning electron microscope (SEM) show that the composite films consist of tetragonal perovskite Bao.8Sro.2Ti03and cubic spinel CoFe2O4phases, without the other intermediate and secondary phase. Qualitatively, the conduction mechanism of the heterostructure composite films follow by Schottky thermionic emission at high electric field. The heterostructure composite films diplay distinct ferroelectricity and ferromagnetism simultaneously, which are comparable with the respective pure BST and CFO film. Moreover, an obvious magnetoelectric coupling effect was observed in the heterostructure films, with a maximum magnetoelectric voltage coefficient of5.0mV/cm-Oe in the CoFe2O4/Ba0.8Sr0.2Ti03heterostructure film, which is about six times larger than that of the Ba0.8Sr0.2TiO3-CoFe204particulate composite ceramics in the previous report. The mechanism for the magnetoelectric voltage coefficient of CoFe2O4/Bao.8Sro.2Ti03heterostructure composite film larger than the Bao.8Sr0.2Ti03-CoFe2O4particulate ceramic composites was discussed. Additionally, it is found that the maximum magnetoelectric voltage coefficient of the Ba0.8Sr0.2Ti03/CoFe2O4composite film is9.4mV/cmOe, which is larger than the one of CoFe2O4/Ba0.8Sr0.2TiO3heterostructure composite film. The mechanism for the magnetoelectric voltage coefficient of Ba0.8Sr0.2TiO3/CoFe2O4heterostructure composite film larger than the CoFe2O4/Bao.8Sro.2TiO3composite film was discussed. |
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