The Nonlinear Dielectric Response And Impurity Effect In Magnetic Relaxor Ferroelectrics

Magnetic relaxor ferroelectric materials (MRF) are compounds in which the relaxor ferroelectricity and ferromagnetic (antiferromagnetic) order coexist simultaneously in certain temperature range. The coexistence of the two order parameters may result in the inherent magnetoelectric (ME) effect. Experimentally the dielectric anomaly around the magnetic-phase-transition-temperature is observed due to the inherent magnetoelectric coupling. After further investigation, we have found that the magnetoelectric coupling can also result in the anomaly of the third-order nonlinear dielectric susceptibility, which provides a great deal of important information for us to better understand the relaxor ferroelectricity. By doping, the dielectric and magnetic property of this kind of material exhibit novel and valuable features. Colossal magnetocapacitance phenomena have already been reported experimentally in doped magnetic relaxor ferroelectrics. Besides, theoretical researches have shown that doping can enhance the magnetocapacitance of magnetic relaxor ferroelectrics.Magnetic relaxor ferroelectric materials have great potential application in making multi-layer capacitors, storage devices and photoelectric memory devices because of its relatively big magnetocapacitance. Therefore, the investigation of the dielectric response and doping effect in magnetic relaxor ferroelectrics is meaningful, which has become the leading issues of material science and condense physics.In the thesis, the research work on magnetic relaxor ferroelectric materials has been performed as follows:1. The investigation on third-order static nonlinear dielectric response in magnetic relaxor ferroelectric materials. The static dielectric property of magnetic relaxor ferroelectric materials has been studied a lot theoretically. However, little attention has been paid to the third-order nonlinear dielectric response. For magnetic relaxor ferroelectric materials, the third-order static nonlinear dielectric susceptibilityχ₃ is as important as the linear dielectric susceptibility. Another key physical quantity is the scaled nonlinear susceptibility a₃ , which is capable of discriminating between the static behavior of normal ferroelectrics and relaxors. What's more, the freezing temperature of materials can be inferred according to the position of the scaled nonlinear susceptibility's peak. Based on the SRBRF model and Heisenberg model, considering magnetoelectric effect, we find thatχ₃ also shows the dielectric anomaly around the magnetic phase transition temperature, which is similar to the behavior of the linear dielectric susceptibility. Different from the electric field, the external magnetic field will not induce the relaxor-to-ferroelectric transition, but the peaks of bothχ₃and a₃ move to the high-temperature direction with the increase of magnetic phase transition temperature and the external magnetic field. This phenomena indicates magnetism has a significant influence on the freezing temperature by means of ME coupling.2. The investigation of doping effect on spin-pair correlation and magnetocapacitance in magnetic relaxor ferroelectric materials. In Cd_1-xFe_x Cr₂ S₄, combining SRBRF model with the site dilution model based on the Heisenberg model, the Fe²⁺ ions doping effect on spin-pair correlation and magnetocapacitance has been explored. An advisable magnetoelectric coupling term of is proposed. By applying the replica trick and mean field approximation, under different external magnetic field, we get the temperature dependence of spin-pair correlation << s_i|→-s_j|→>>and its fluctuationδ<< s_i|→-s_j|→>> including both the compositional average and the thermoaverage. Investigation shows that both the magnetic phase transition temperature Tc andδ<< s_i|→-s_j|→>> increase due to Fe²⁺ ions doping. Furthermore, the effect is more obvious with the increase of Fe²⁺ content. The dielectric anomaly in Cd_1-x Fe_x Cr₂ S₄ around the magnetic phase transition temperature is also observed because of ME coupling. Moreover, compared with the pure case, the magnetocapacitance is dramatically enhanced due to the increase of the fluctuation of spin-pair correlation. Our theoretical results well agree with the experimental ones.