Physical Properties Of Ion-doped M-type Strontium Ferrite

The single-phase multiferroic materials are new multi-functional materials,in which ferroelectric sequence and ferromagnetic sequence coexist in same crystal lattice.The magneto-electric coupling of this type of material realizes a mutual regulation that an external electric field affects its magnetic order or an external magnetic field affects its ferroelectric order.The discovery of this phenomenon provides the possibility to realize new multifunctional materials.Hexagonal ferrites have been widely studied and applied for their special physical properties,such as high saturation magnetization,high coercivity and high Curie temperature.At the same time,the magnetoelectric coupling produced by the special magnetic structure of the hexagonal ferrite is more exciting for researchers.In this thesis,the M type hexagonal ferrite as the main research object is doped with different ions to study its influence of the structure and physical properties.At the same time,a simple physical analysis and discussion of the magnetoelectric coupling phenomenon was proposed.The main research results are as follows:(1)SrFe12-xInxO19(x=0,1.8,2.4 and 3.0)ceramic samples were successfully synthesized by solid state reaction method.Structural analysis shows that indium doping can affect the crystal structure and morphology of the material.The dielectric and magnetic properties of the material of the ceramic samples were significantly affected by the indium doping.The systematic study of the influence of the magnetic field on the dielectric properties found that the magnetic field has a certain modulation effect on the dielectric constant.The positive and negative magnetodielectric transition temperatures are different for the samples with different doping amounts.The transition temperature of x=0 is between 50 K and 75 K,while the transition temperature of x=2.4?3.0 is between 25 and 50 K.The magnetodielectric effect of samples with x=0 and x=2.4 was studied at room temperature.The results show that the sample with x=0 shows a linear change at room temperature,and the sample with x=2.4 shows a twisted butterfly shape.The study of the temperature dependence of the magnetodielectric effect of SrFe12-xInxO19 ceramics(x=0 and 2.4)shows that the mechanism of the magnetodielectric effect of x=0 and x=2.4 is different.(2)Sr1-xLaxFe12O19(x=0,0.05,0.1 and 0.15)ceramic samples were successfully synthesized by solid state reaction method.The structural analysis shows that La doping mainly affects the structure of the samples.The dielectric constant increases greatly and the loss is relatively large,which is mainly due to the Maxwell-Wagner interface polarization effect,while the magnetic properties are improved with the lanthanum content.The change of the dielectric constant with magnetic field shows that the temperature of positive and negative magnetic dielectric transition is different.The transition temperature for x=0 is between 50 and 75 K,and the transition temperature for x=0.05 to 0.15 is between 25 and 50 K.At the same time,it is observed that the magnetodielectric coefficient is relatively large between 75 and 200 K,due to the polarization effect of the Maxwell-Wagner interface,which is an extrinsic phenomenon.(3)Lanthanum doping can greatly enhance the dielectric constant of the ceramic but the loss is also large,while indium doping can well control the dielectric loss of the ceramic sample.Therefore,the La-In co-doped Sr0.95La0.05Fe12-xInO19(x=1.2,1.8,2.0 and 2.4)ceramic samples were synthesized by solid state method.The structural analysis shows that La-In co-doping has a certain effect on the structure and morphology of the material.The dielectric constant of the doped ceramic sample is larger and the loss of the sample is smaller in comparison with the undoped samples,and the dielectric relaxation phenomenon caused by the interface polarization effect exists in the low frequency range.The magnetic properties of the ceramic samples analysis shows that magnetic properties of the doped materials gradually decrease.According the magnetodielectric results,there are two different mechanisms affecting the magnetodielectric properties in the tested temperature range.The positive and negative changes of magnetodielectric coefficient at low temperature may be due to the change of magnetic structure caused by ion doping,while the changes above 150 K are mainly caused by the giant dielectric phenomenon.Although the magnetic properties of La-In co-doped ceramics are relatively weak,the improvement of dielectric properties provides a feasibility for the subsequent exploration of dielectric functional materials.