Magnetic and Electric Properties of Nanobased Bi2Fe4O9
Supervisor:lv xiao mei wang ye ning
In recent years,there is considerable interest in studying systems in which the magnetism(magnetic/spin)and electricity(electric field/dielectric constant)are coupled.It involves not only the origin of magnetoelectric coupling in multiferroics,but also an important property of magnetic insulators,magneto-dielectric effects.Materials with magneto-dielectric effect are widely used in tunable devices,magnetic sensors,transducers and so on.For example,the impedance with a specific frequency range can be changed according to the applied magnetic field in tunable devices.However,the magneto-dielectric effects in most magnetic insulator are too small or the magnetic transition temperature possessing the magnetoelectric effect is much lower than room temperature,which limits its application.In ternary oxide materials,especially those with pentagonal lattice structure of magnetic elements,the special lattice structure leads to the magnetic frustration and strong spin-lattice coupling,such as BiMn2O5,NdMn2O5 and BihFe4O9.As this kind of material,antiferromagnetic Bi2Fe4O9 has been widely studied recently.Dielectric anomalies near antiferromagnetic transition have been reported in single crystal indicating strong spin-lattice coupling;the results of ceramic samples show its Neel temperature(250K)close to room temperature,weak ferroelectric polarization and room temperature magneto-dielectric effect.It is considered that Bi2Fe4O9 is an excellently magneto-dielectric material.In addition,a excellently magnetic properties also observed in Bi2Fe4O9 ceramics.So,we investigated the magnetic and magneto-dielectric properties of nano-based Bi2Fe4O9,discussed the origin of magneto-electric coupling and improved the magneto-dielectric properties by Mn doping and alloying with ferroelectric material in this dissertation.The specific contents are as follows:1.We investigated the magnetic and dielectric properties of 300 nm Bi2Fe4O9 nano-crystals.The 2 μm × 2 μm × 300 μm Bi2Fe4O9 nano-crystals show AFM transition at about 235 K and weak FM properties in the temperature range of 10-310 K.An applied magnetic field gives rise to a large MD effect about-13%at 50 K and 0.45 T;besides,an obvious magneto-dielectric anomaly is observed near TN,and the anomaly moves to higher temperatures with increasing magnetic field.Furthermore,by comparing the results on crystals with different sizes,we think that the FM properties in nanobased Bi2Fe4O9 are mainly related to the local FM exchange interactions induced by Fe valence change,and the MD effects are originated from both the AFM and FM properties.This work confirms large magneto-dielectric properties in Bi2Fe4O9 nano-crystals which is much higher than that of ceramics.2.We investigate the influence of Mn doping amount x on antiferromagnetic Bi2Fe4O9 nano-crystals(Bi2Fe4(1-x)Mn4xO9,x=0.00,0.02,0.04,0.06 and 0.10).Our results show that the doped samples have much stronger ferromagnetic properties while a relatively low TN.With increase of x,Mn ions are substituted at Fe2 sites in trivalent form firstly and then simultaneously at Fe2 sites and Bi sites in trivalent form for x=0.04 sample;finally,most Mn ions enter Fe2 sites in mixed valent form(trivalent and tetravalent)for x=0.06 and 0.10 samples.Among those samples,the x=0.04 sample has the largest magneto-dielectric effect with Δε/ε being about-4.9%at 220 K and 0.6 T,while Δε/ε=-1.69%for x=0.00 sample.Combined with the results of magneto-striction measurement,we believe that the magneto-dielectric effect are mainly from spin-lattice coupling in BFMO samples with antiferromagnetic long-range order.The Mn-doped BFMO samples own the improved magnetic and magneto-dielectric properties at room temperature,which is a great advantage for designing electromagnetic devices.3.We investigated the magnetic and magneto-dielectric properties of Bi2Fe4O9/BaTiO3 nano-composite ceramic.(1)Magnetic measurement shows a large ferromagnetic properties with Ms about 0.69 emu/g,and the XRD results indicates that the large magnetization are induced by the third phase BaFe12O19.(2)The dielectric result shows a large magneto-dielectric effect compared with pure Bi2Fe4O9 ceramic,with Δε/ε being about-2.49%at 310 K and 0.4 T;a relaxation related to the two-site polaron hoping process of the charge transfer is observed at about 200 K,and with increase of the applied magnetic field,the relaxation peaks shift to low temperatures.The above two evidences indicate strong coupling between the spin and dipolar ordering.(3)In this system,the BaFe12O19 has a large magnetostrictive effect,meanwhile,Bi2Fe4O9 and BaTiO3 are better dielectric materials.So,with application of magnetic field,a lattice distortion and large magneto-dielectric effect would occur at the interface.