Study On Performance Control Of Spinel Based Multiferroic Materials

Multiferroic materials play an important role in our theoretical research and practical application,and spinel materials have special magnetic structure,which greatly enhance the research value of spinel based multiferroic materials.In view of the limitations of single-phase class I multiferroic materials and the lack of research on the dielectric properties of class II multiferroic materials,this paper mainly studies the magnetic and dielectric properties of spinel based class I multiferroic composites and class II multiferroic materials(1)Bi₄Ti₃O₁₂/NiFe₂O₄ nanocomposite films have been prepared via sol-gel spin-coating route.The successful phase formation of orthorhombic Bi₄Ti₃O₁₂ and cubic NiFe₂O₄ has been confirmed using X-ray diffraction without any detectable extra phase.The SEM with EDS measurement reveals homogeneous and dense growth of both phases accompanied by clear interface between the two layers.Magnetic measurements show that the films have room temperature ferromagnetic properties.At the same time,we also measured the room temperature ferroelectric properties,good fatigue endurance properties and low leakage current density.The frequency dependence of dielectric constant displays the monotonous decreasing tendency with low dielectric loss.Moreover,evident magnetodielectric behavior is achieved in the nanocomposite films at room temperature.The coexistence of ferromagnetic and ferroelectric behavior including the appearance of magnetodielectric effect proves the room-temperature multiferroic properties in Bi₄Ti₃O₁₂/NiFe₂O₄ nanocomposite films.(2)The(1-x)Bi₂Fe₄O₉-x MgFe₂O₄ composite ceramics were synthesized by sol-gel method,crystal structure,morphology,magnetic,and dielectric properties of composite ceramics bicomponent ceramics are investigated in detail.X-ray diffraction analysis shows that the composite ceramics are orthorhombic phase Bi₂Fe₄O₉ and spinel MgFe₂O₄ coexist,and there is no third phase.SEM results show that the homogeneous grain distribution with different grain sizes of Bi₂Fe₄O₉ and MgFe₂O₄.The magnetic measurement shows decreasing coercivity and increasing values of remnant and saturation magnetization with the increase in MgFe₂O₄.Temperature-dependent dielectric properties and electrical modulus are conducted in the temperature range of 27–527°C with the frequency ranging from 1 to 30 k Hz.Two abnormal peaks are,respectively,observed at low and high temperature ranges.The variation of electric properties is closely related to the composition of MgFe₂O₄.The results of dielectric and electrical modulus indicate the appearance of dielectric relaxation in(1-x)Bi₂Fe₄O₉–x MgFe₂O₄.Combined with the study of activation energy,the dielectric relaxation is mainly induced by different electric responses of various charge carriers intrinsically dominating in a thermally activated process with the incorporation of MgFe₂O₄.The combination of Bi₂Fe₄O₉ and MgFe₂O₄ enhances the coexistence of magnetic and electric properties in the present composites.The combination of Bi₂Fe₄O₉ and MgFe₂O₄ enhances the coexistence of magnetic and electric properties in the present composites.It is quite promising from application point of view.(3)NiCr₂O₄ samples with uniform morphology and tetragonal phase were successfully prepared by sol-gel method,and the samples had higher room temperature dielectric constant.The relaxation behavior below room temperature is confirmed through measurements of dielectric constant,dielectric loss,electric modulus and impedance depended on frequency or temperature.The analysis through equivalent circuit simulation demonstrates the influence of grains at low temperature and the common contribution of grains and grain boundaries at high temperature.Moreover,the change in dielectric response indicates the presence of magnetodielectric coupling effect.Two different mechanisms are considered to control the magnetodielectric coupling,yielding the positive and negative coupling coefficient at different temperature ranges.The intrinsic magnetoelectric coupling gives rise to the frequency independent magnetodielectric behavior at low temperature.As temperature increases above 150K,the large magnetodielectric coupling coefficient depended on frequency is contributed by the Maxwell-Wagner effect.The negative value of magnetodielectric coupling coefficient is attributed to the leakage current.Therefore,both mechanisms of Maxwell-Wagner and leakage are competing as a function of frequency,which results in the coefficient crossover at room temperature.Such features bring NiCr₂O₄ a step closer to potential applications in areas such as magnetic field sensors,actuators and others.Such features bring NiCr₂O₄ a step closer to potential applications in areas such as magnetic field sensors,actuators and others.(4)In order to further study NiCr₂O₄,nanocrystalline NiCr₂O₄ samples with different annealing temperatures were successfully prepared by coprecipitation method.X-ray diffraction analysis shows that the crystalline phase of NiCr₂O₄ changes from cubic phase to tetragonal phase with the increase of annealing temperature.SEM and TEM results show that the average particle size increases from?80 nm to?280 nm with the increase of annealing temperature.By testing the dielectric properties of the samples annealed at different temperatures,we found that the dielectric properties of samples are similar to those of the samples annealed at 1100°C.Exchange bias behavior is detected at 10 K accompanied by the shift of magnetic hysteresis loops along field axis.This effect is gradually weakening as annealing temperature increases.The magnetization depending on temperature under zero-field-cooled and field-cooled conditions suggests the ferrimagnetic transition at?72 K and the spiral magnetic order at?25 K.Furthermore,the ferrimagnetic transition point shifts slightly to low temperature with increasing annealing temperature.Based on the measurement results,the appearance and evolution of exchange bias effect in NiCr₂O₄ nanoparticles with improving annealing temperature should be simultaneously derived from the lattice transition and the surface spin effects.