Structural,Electrical And Magnetic Properties Of Bi₅Ti₃FeO₁₅ Multiferroic Compounds Modulated By Doping At B Sites

Bismuth layer ferroelectrics?BLFs?with high Curie point,strong ferroelectric,the fatigue-and lead-free behaviors,have great potential applications in ferroelectric memory,high temperature and high pressure electric brake etc.In addition,the natural layered perovskite systems composed of a typical three-layer Bi₄Ti₃O₁₂ and a typical multiferroic BiFeO₃,of which general formula is Bi_4+nTi₃Fe_nO_12+3n?n is a positive integer?,has attracted more and more attentions because of its ferroelectric,magnetic and magnetoelectric?ME?coupling effects.This kind of material system,with the magnetic ground state of an antiferromagnetic ordering,has several physical problems,such as the weak ME coupling effects and its unclear mechanism.Therefore,different methods,such as chemical doping,interface engineering and strain modulation,have used to adjust the magnetic order and enhance the magnetic transition temperature and ME coupling effect to bring about the application of magnetic devices based on the material system.On account of all,in this thesis,the electrical and magnetic properties of the four-layer Bi₅Ti₃FeO₁₅?BTFO?phase structure modulated by B-site doping have been researched.The main results are as follows:1.We have used the conventional solid-state sintering method to prepare the BTFO bulk materials with different Nb and Fe co-doping concentrations.The chemical formula is Bi₅Ti_3-2xFe_1+xNb_xO₁₅?BTFNO,x = 0,0.1,0.2,0.3,0.4?.The results show that the sintered BTNFO materials have typical Aurivillius sheet structure,of which the lattice constant a and c decrease linearly,while b increases linearly with the increase of the co-doping concentration of Nb and Fe.Moreover,it is found that with the increase of Nb and Fe concentration,the dielectric dispersion behavior is enhanced.When the concentration is higher than 0.2,the dielectric loss peak appears,and the relaxation peak shifts to the lower frequency with the increase of the concentration.The researches indicate that BTFNO material has the same magnetic order as the parent BTFO,that is,some antiferromagnetic order dominated by superparamagnetism?SPM?.Besides,the long-range order of antiferromagnetism?AFM?strengthens with the increasing of Nb and Fe co-doping concentration.Meanwhile,co-doping of Nb and Fe can inhibit the preferred orientation of the c axis of the platelet-shaped grain,and promote the growth along alb axis,thus,the piezoelectric properties can be improved.In addition,an abnormal ferroelectric switching behaviour?180°?was observed on the surface of BTFO ceramic samples.2.We have deposited the Mn-doped BTFO?BTFMO?thin films by chemical solution method,and then studied the effects of annealing temperature on microstructure,ferroelectric and magnetic properties.The results show that BTFNO have poor crystallinity when annealing temperature lower than 600 ? However,when the temperature is 750?,there are impurity phases as show in XRD map;When the annealing temperature was 650 ? and 700 ?,the samples show better crystallinity,and the films have full and dense grains.However,When the annealing temperature was 650 ?,BTFMO film shows strongest ferroelectricity with P.reach to-58.4?gC/cm2 when Emax= 650 kV/cm.In addition,the results show that BTFMO exhibits antiferromagnetic behavior,and an anomalous behavior occurs at about 80 K.