Research On The Magneto-dielectric Effect Of Ferrite And Its Composite Materials

The variation in the dielectric permittivity of a material under an applied magnetic field,namely magnetodielectric(MD)effect,has been attracting an intense interest due to its potential and important applications in electric devices,such as magnetic sensors,magnetic memories and microwave resonators.Magnetodielectric effect has been observed in various single phase ferrite materials and magnetoelectric composites.However,considering the requirements of the practical application for magnetodielectric effect,three questions remain to be resolved.Firstly,preparing the multiferroic composites with excellent magnetoelectric properties,which contain ferroelectrics with large piezoelectric coefficient,ferromagnets with high magnetostriction coefficient,and good elastic coupling between the two phases.Secondly,the whole magnetodielectric hysteresis loops over a large range of magnetic field are required,which is quite important as a comprehensive representation of the magnetoelectric property for the composites.Thirdly,proposing a suitable theoretical model fitting the experimental data and a quantitative analysis on the basis of the specific conditions of magnetic field or electric field frequency.The coexistence of various dielectric mechanisms often occurs in the experimental measurement.Different magnetic field,electric field frequency or the material composition can all impact(or induce different)dielectric mechanisms.In order to solve those problems,the polycrystalline Mn-Zn ferrites and(1-x)(Ba0.88Ca0.12)(Ti0.88Zr0.12)O3-xCoFe2O4 serial composites were synthesized.The characters of ferromagnetic,ferroelectric and magnetoelectric coupling interaction were studied.The physical mechanism of magnetodielectric effect in the single phase ferrites and magnetostriction/piezoelectricity composites were investigated.The influences of resonance frequency and magnetostriction effect on magnetodielectric effect were researched.The experimental and theoretical researches of magnetodielectric effect in this thesis are a stepping stone for the development of magnetodielectric materials and magnetodielectric-relevant devices in the future.The research work of this thesis mainly includes three parts,as follows:Firstly,the dielectric properties and the magnetodielectric effect in polycrystalline Mn-Zn ferrites at resonant frequency were studied.The dimensional-resonance-induced abnormal dielectric spectrum was observed at f≈1 MHz.A relatively large room temperature magnetodielectric ratio of 4500%in a magnetic field of 3.5 kOe was achieved from the Mn-Zn ferrite sample with the initial permeability of 15K at resonant frequency.Theoretical analysis suggests that the combination of the Maxwell-Wanger effect and magnetoresistance effect makes contribution to the magnetodielectric effect.The large magnetodielectric effect at resonant frequency is attributed to the enhanced magnetostriction effect.Secondly,the hysteresis behavior of the magnetodielectric effect for Mn-Zn ferrites was investigated.Noticeable magnetodielectric effect was observed in the Mn-Zn ferrite samples at relative low frequency(f≤100 kHz).Both the magnetodielectric and magnetoresistance curves showed similar magnetic field dependence and shape anisotropy.The hysteresis behavior of the magnetodielectric effect depended on the measuring frequency and Zn doping of the ferrite samples.Theoretical analysis suggests that the hysteresis behavior of the magnetodielectric effect results from the combination of magnetoresistance and magnetostriction effects.Thirdly,the magnetodielectric effect of polycrystalline(1-x)(Ba0.88Ca0.12)(Ti0.88Zr0.12)O3-xCoFe2O4(x=0.10,0.20,0.30,0.40)ceramic was studied.The room temperature magnetic and ferroelectric behaviors of the synthesized composites were measured.For the composite with x = 0.40,a room temperature magnetodielectric ratio of 5.37%was achieved under a magnetic field of 1.5 T atf= 1 kHz.The measured "butterfly hysteresis" magnetodielectric curves exhibit an obvious dielectric anomaly.Theoretical analysis suggests that the observed magnetodielectric effect is attributed to the magnetoresistance effect and magnetoelectric coupling.The butterfly-like hysteresis of magnetodielectric radios was understood from the ferroelasticity of the ferroelectric phase(Ba0.88Ca0.12)(Ti0.88Zr0.12)O3.