Investigation Of The Dielectric Properties 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 magnetoelectric coupling between them. The dielectric anomaly around the magnetic-phase-transition-temperature observed in experiments is indicative of the magnetoelectric coupling in magnetic relaxor ferroelectric materials. Furthermore, the application of the external magnetic field induces the dielectric behaviors which are different to the dielectric behaviors without external magnetic field. The dielectric behaviors which are induced by the external magnetic field named as magnetocapacitance (MC) effect. The magnetocapacitance effect in magnetic relaxor ferroelectric materials are more obvious than in perovskite manganites.In technologic area, the ability to couple with either the magnetic or the electric polarization offers an extra freedom in the design of convention transducers, actuators, and storage devices. A number of device applications have been suggested for ferroelectromagnets, including multiple state memory elements, ferromagnetic resonance devices controlled by electric field, and variable transducers with either magnetically modulated piezoelectricity or electrically modulated piezomagnetism. The investigation of the magnetocapacitance effect has important fundamental values and extensive technological applications.In our thesis, we have done the following work:1. The investigation of the magnetoelectric coupling in magnetic relaxor ferroelectric materials.As far as the investigation of the magnetoelectric coupling in conventional ferroelectromagnets is concerned, much theoretical work has been produced. However, it has never been considered for the magnetoelectric coupling in magnetic relaxor ferroelectric materials. Based on SRBRF model and Heisenberg model, we propose an advisable magnetoelectric coupling term of . We find that the spin-pair-correlation via the magnetoelectric coupling plays an important role on the relaxor ferroelectric subsystem, while the correlation of the dimensionless order parameter does not influence the properties of magnetic subsystem without the external electric field. At last, we offer the value of spin-pair-correlation and its fluctuation in magnetic relaxor ferroelectric materials.2. The investigation of static dielectric behavior in magnetic relaxor ferroelectric materials.The occurrence of spontaneous magnetic order results in the anomaly of static dielectric behavior around the magnetic-phase-transition temperature in magnetic relaxor ferroelectric materials. We think the spontaneous magnetic order induces the occurrence of the spin-pair-correlation which would modify the random interaction among polar clusters via the magnetoelectric coupling. Then the spherical glass order parameter and the static dielectric constant would be amended.3. The investigation of dynamic properties in magnetic relaxor ferroelectric materials.CdCr₂S₄, a representative member of magnetic relaxor ferroelectric materials which take place phase transition between paramagnetic and ferromagnetic phase at its magnetic-phase-transition temperature (84.5k), shows the behavior of conventional relaxor ferroelectric materials in the higher temperature range. Based on the superparaelectric model, one polarization process which is associated with the thermally activated flips of the polar clusters is considered. Therefore, the reorientable polar clusters have the main contribution for the dielectric behavior. It is indicated that the dielectric anomaly below magnetic-phase-transition temperature is attributed to the reason that the activation energy against polar clusters reorientation are amended by the spin-pair correlation via the magnetoelectric coupling. Moreover, we consider the influence of the external magnetic field in dielectric constant. The obtained real part of dielectric constants and the magnetocapacitance are in good agreement with the experimental results which confirm the correctness of our idea.