Magnetic Doping And Multiferroic Properties Of Bismuth Layer Structured Ferroelectrics

The single-phase (SP) multiferroics exhibit ferroelectric and magnetic orders simultaneously. Due to the atomic level combination of ferroelectricity and magnetism in SP multiferroics, tuning the ME effect in the quantum level can be realized. Compared with the single traditional magnetic or ferroelectric materials, the SP multiferroics with ME coupling effect have attracted remarkable interest because of their potential use in and new multiplestage memories and ME devices. This dissertation aims at magnetic doping and multiferroic properties of bismuth layer structured ferroelectrics. Room-temperature (RT) multiferroic was achieved. The discipline of magnetic doping in BTO and BTF materials was found based on lots of experimental results. The physical mechanism of ME coupling was also discussed. Some main contents and innovations in the dissertation are listed in the following:(l)The influence of doping content on RT multiferroic in A or B-site doped Bi4Ti3O12(BTO) and RT ME coupling effect of part samples were studied:A-site doped Bi3.15-xNixNd0.85Ti3O12, Bi3.15-xMnxNd0.85Ti3012-δ, and B-site doped Bi4FexTi3-xOi2-δ, Bi3.15Nd0.85NixTi3-xO12-δ, Bi3.15Nd0.85Mn3-δO12and Bi3.15Nd0.85CoxTi3-xOi2-δ ceramic samples were prepared by conventional solid state reaction. It can be inferred that magnetic ions entered into the A or B sites of perovskite-like layers successfully. The samples of A or B-site doping of Ni or Mn give a nearly linear relationship, while for Fe or Co-doped sample, the typical S-type ferromagnetic hysteresis for ferromagnetic materials were observed. The ME coupling in Fe-doped sample x=2at RT was demonstrated by measuring the effect of magnetic poling on ferroelectric hysteresis loops and the effect of DC electric field poling on magnetic hysteresis loops.(2) The physical mechanism of magnetic enhancement and RT positive ME coupling in B-site co-doping three-layered perovskite BNT was discussed:(Bi3.15Nd0.85)(Ti2FexCoi-x)O12-δ ceramic samples were prepared by conventional solid state reaction. The co-doping of Fe and Co did not change the three-layered perovskite structure of BNT. Compared with the samples in section (1), the co-doping of Fe and Co greatly improved the RT magnetic properties of the samples. The measurement of x-ray photoelectron spectroscopy indicated the coexistence of Fe3+and Fe2+ions in the samples. According to the results of analysis of the valence, the mechanism of significant improvement of RT magnetic property was discussed. A clearly identified positive magnetocapacitance (MC) effect was observed with a large MC value of14.2%at30kHz.(3) The mechanism of magnetic modification of four-layered perovskite Bi5Ti3FeOi5(BTF) was investigated:In this section, we study the microstructure and performance of BTF, BisTi3Feo.sNio.sOis(BTFN) and Bi4NdTi3Fe0.5Co0.5O15(BNTFC) ceramics. The XRD results demonstrated the presence of four-layered Aurivillius phase. Dielectric constant of the three samples all have relatively strong dielectric dispersion phenomenon. As for magnetism, magnetic doping improves the RT magnetism of the samples. The mechanism of enhancement in2Mr was discussed.(4)RT multiferroic properties of B-site magnetic doping of four-layered perovskite Bi4NdTi3FeO15(BNTF) were systematically studied. The phenomenon of "x=0.3" was found and the intrinsic physical mechanism was discussed:In this section, a systematic study was carried out for B-site Ni, Mn, Co and Cr-doped BNTF ceramics. Four-layered perovskite structure was formed in all of the four series samples. Co and Cr-doped BNTF exhibited single phase while little Ni2O3and Mn3O4existed in Ni and Mn-doped BNTF. For RT ferromagnetism, the typical S-type ferromagnetic hysteresis was observed in all of the samples. Magnetic doping enhanced the magnetic response greatly except Cr doping. Very interestingly, the sample of "x=0.3" exhibited the largest2Mr value for Ni, Mn and Co-doped samples. For the samples of "x=0.3" in Ni and Co-doped samples, RT ME coupling were observed.The innovations of the thesis are as follows:1. The best way of magnetic modification for three-layered perovskite BTO within experimental parameters was acquired, i.e. B-site magnetic co-doping. The sample exhibited the best RT multiferroic properties and large positive RT ME coupling effect when Fe:Co=1:1by B-site magnetic co-doping of Fe and Co.2. The phenomenon of "x=0.3" was found in magnetic modification of four-layered perovskite BTF, i.e. the effect of magnetic doping is the best when Fe:M≈7:3(M is doped magnetic ion). What’s more, in "x=0.3" samples, obvious ME coupling effect was also observed.