Preparation,Electric,Magnetic Properties And Dielectric Tunability Under Low Electric Field Of Doped BaFe_12-xM_xO₁₉?M=Zr/Ti/Nb? Ceramics

Nowadays,with the rapid development of electronic technology,electronic devices urge for the multifunctionality and miniaturization for meeting the requirements of scientific technology and marketing demands.Materials with excellent dielectric,tunable dielectric properties and magnetic property have great potential in the application of tunable devices,electric and magnetic devices.Herein,we focus on M-type barium ferrites BaFe12O19 and study its relations between formation,structure and properties systematically.The mechanisms of giant permittivity,high tunability driven by extremely low field and high saturation magnetization of the ferrites have also been explored.The concrete research content and main conclusions are listed as follows:(1)The Zr4+ doped barium ferrites ceramics BaFe12-xZrxO19 are prepared by sol-gel process.Zr4+ ions substitute for Fe3+ ions successfully and thus Zr4+ doped barium ferrites ceramics are formed.Fe2+ and oxygen vacancy are generated in the ferrites over low doping range of x<0.1,and are consumed over high doping range of x>0.1.Hence,the concentration of Fe2+ reaches the maximum in the critical doping content of x=0.1.Fe2+/Fe3+ defect dipoles are possibly formed in the ferrites through Zr4+ doping.The more the generated Fe2+ ions are in the system,the more the formed defect dipoles are in the ferrites.(2)Contributed by the defect dipoles and inhomogeneity conductivity,the Zr4+doped barium ferrites exhibit giant permittivity of 30k over wide frequency range of tens of megahertz.In addition,the activation energy based on the response of defect dipoles is extremely low to 0.245 eV,which reveals that the state of defect dipoles could be easily changed by low energy.(3)The effects of conductance and schottky junction capacitance to the dielectric tunability are excluded.Hence,with the contribution from the reorientation of defect dipoles and the decrease of dipole moment and susceptibility,and the enhancement from high content of Fe2+,high tunability of 60%is obtained under extremely low dc electric field of 25 V/cm in the ferrites.The energy loss is extremely low to 4.3×10-2 J/cm3·s under field of 25 V/cm,which accounts for 1/50?1/1000 of the energy loss of ABO3 system.(4)The generation of intrinsic Fe2+ in 4f2 site and the substitution of non-magnetic Zr4+ for Fe3+ in 4fi site increase the effective magnetic moments per molecular,and thus improve the saturation magnetization by about 17%.In addition,Zr4+ substituting for Fe3+ softens the magnetic properties of barium ferrites,in which the anisotropic field and coercivity are reduced dramatically.(5)The Ti4+ doped or Nb5+ doped barium ferrites BaFei2-xTixO19(x=0,0.2,0.4,0.6,0.8)and BaFei2-xNbxO19(x=0,0.1,0.2)are prepared.Ti4+ or Nb5+ ions are doped into the ferrites successfully by substituting for Fe3+ to form single phased doped barium ferrites.Fe2+ ions are produced in the ferrites over high substituted content in Ti4+ doped system,while Fe2+ ions are generated in the ferrites over low substituted content in Nb5+doped system and the content of Fe2+ reaches the maximum with Nb5+ of 0.1.The colossal dielectric response over wide frequency range is exhibited in both Ti4+ or Nb5+doped system.In addition,the dielectric tunability of 30%is obtained under extremely low field of 20 V/cm in Ti4+ doped system,while the dielectric tunability of 47%is achieved under extremely low field of<40 V/cm in Nb5+ doped system.Besides,substitution of Ti4+ for Fe3+ and generation of Fe2+ in 4f2 site increase the saturation magnetization by 33.3%,while the saturation magnetization increases by 21.7%in system with Nb5+ substituting for Fe3+ in down spin site,and both the non-magnetic Ti4+ or Nb5+ doping softens the magnetic properties of barium ferrites.