Fabrication And Characterization Of Nanostructured Multiferroic Materials

Multiferroic materials with two or more types of ferroic orderings have attracted a great deal of attention in the past a few years. The magnetoelectnc coupling makes the multiferroic materials promising for a wide range of applications including multiple-state memory devices, sensors, tranformers, gyrators, drivers, optical devices and micro-electromechanical systems (MEMS), among others. In this dissertation, the nanostructured multiferroic materials have been systematically investigated. The current state of multiferroic materials and their potential applications were reviewed in the introduction first, including: (1) the fabrication methods, fundamental physical properties and characteristics of multiferroic thin film; (2) various techniques to synthesize and characterize one-dimensional nanofibers. Based on these reviews, the fabrication and characteristics of multiferroic composite films, composite nanofibers, single-phase nanofiber and nanostructured patterns were investigated systematically by the experimental methods. The main research findings are summarized as follows:1. Processing and properties of multiferroic composite filmsLead zirconnate titanate(PZT), cobalt ferrite(CFO) and nickel ferrite(NFO) thin films were deposited on Pt/Ti/SiO₂/Si substrate by chemical solution deposition (CSD) technique. On the basis of single-phase ferroelectric and ferromagnetic films, two kinds of random composite thin films, PZT-CFO and PZT-NFO, were synthesized by sol-gel process and spinning coating. Another four kinds of laminated composite thin films were also processed, including PZT-2CFO-PZT(PCP), CFO-2PZT-CFO(CPC), PZT-2NFO-PZT(PNP) and NFO-2PZT-NFO(NPN). The effects of various lead ions concentration, annealing temperatures and composite structure on the hysteresis loop, fatigue resistance and ferromagnetic properties were studied. The experimental results indicated:Good crystallinity and compact structure were observed in PZT thin film annealed at 750℃under the atmosphere with lead content exceeding 10%. The maximum remanent polarization of PZT film was as high as 55uC/cm2, whereas the fatigue resistance of the film was rather poor. CFO and NFO ferromagnetic thin films annealed at 750℃had homogeneous grain size, but loosen structure. The density of the two-phase random composite thin films was between the single-phase ferroelectric and ferromagnetic thin films. Owing to the interactions between the ferroelectric and ferromagnetic grains in the random composite thin film, the shape of electric and magnetic hysteresis loops were different from single-phase thin films. The fatigue resistance of random composite films was better than that of the PZT film. The corresponding remanent polarization Pr of PZT-CFO and PZT-NFO random composite films were 30 and 35uC/cm², coercive electric field Ec were 120 and 80kV/cm, remanent magnetization Mr were 5.0 and 0.25emu/cm³, coercive magnetic field Hc were 560 and 100Oe, respectively. Compared with PZT-NFO film, PZT-CFO composite thin film had better properties. The laminated composite thin films with sandwich structure, such as PCP and PNP were flat and smooth on the surface, and compact in the structure. The corresponding remanent polarization Pr of PCP and PNP composite films were 35 and 38uC/cm², coercive electric field Ec were 90 and 160kV/cm, remanent magnetization Mr were 11.2 and 3.75emu/cm³, coercive magnetic field Hc were 540 and 200Oe, respectively. However, the structure of CPC and NPN laminated sandwich composite thin films were very loose, which resulted in very large current leakage. In comparison with CPC and NPN, the multiferroic properties of PNP and PCP composite thin films were better.2. Syntheses and properties of ferroelectric and ferromagnetic nanofibersElectrospinning and sol-gel technique were used to synthesize the PZT, CFO and NFO nanofibers and nanoparticles, respectively. The effects of concentration of electrospinning solution, electric field strength and collecting distance on the shape, diameter, piezoelectric and ferromagnetic properties of nanofibers were studied. We found the optimized processing parameters for fabricating ferroelectric and ferromagnetic nanofibers with good morphology and dense structure. The experimental results showed:(1) PZT green fibers and dry gel calcined at 550℃for 2h in air with lead content excess 10% led to good crystallinity and pure perovskite PZT phase. PZT nanofibers with smooth surface and compact structure were composed of nanocrystalline grains with the size around 10-20nm; the diameter of each single nanofiber was very uniform. The hysteresis loop confirmed the ferroelectricity properties of PZT nanofibers, and the linear piezoelectric coefficient d₃₃ was estimated to be around 223.9pm/V, and the corresponding coercive electric field Ec was about 70kV/cm. PZT nanoparticles had regular round shape, whereas the grain size distribution of PZT nanoparticles was not as good as that of PZT nanofibers.(2) The effect of polymer and ion concentration, electric field strength and collecting distance on the appearance and structure of PZT nanofibers were investigated. The average diameter of PZT nanofibers increases with increasing polymer and ion concentration. If the polymer and ion concentration were too high or too low, continuous PZT nanofibers with uniform diameter and regular shape couldn't be formed. Straight continuous PZT nanofibers with homogeneous diameter could be obtained when solution was electrospinned at proper electric field. However, if the electric field was too high, it was easy to form curved nanofibers. If it was too low, no nanofibers could be produced. In addition, with the appropriate collecting distance, regular round shape PZT fibers could be obtained; while with too short distance, it was easy to have conglutination and collapse in the nanofibers, and with too long collecting distance electrospinning does not reduce the diameter size further. The following optimal processing conditions have been identified: the PZT concentration approximately 0.15～0.30mol/L, polymer concentration being 0.02～0.035g/mL, electric field strength about 1.0～1.4kV/cm, and collecting distance close to 18～24cm, under which large amount of continuous PZT nanofibers with uniform diameter and regular shape could be fabricated by electrospinning.(3) Similar microstructure and appearance were observed in the CFO and NFO ferromagnetic nanofibers and nanoparticles. Ferromagnetic nanofibers with regular straight shape were composed of nanocrystalline grains with its size between 20 to 40nm, the diameter of each single nanofiber was very uniform with the diameters around 100～300nm. In addition, the loose structure and many pores were observed in ferromagnetic nanofibers, with slightly larger grain size than PZT nanofibers. The grains size distribution of ferromagnetic nanofibers was more uniform than that of nanoparticles. The remanent magnetization of the CFO nanofibers and nanoparticles were 31.8 and 35.1emu/g, respectively, and the corresponding coercive magnetic fields were 1700 and 1260Oe, respectively. The remanent magnetization of the NFO nanofibers and nanoparticles were 13.8 and 11.6emu/g, and the corresponding coercive magnetic fields Hc were 200 and 130Oe, respectively.3. Syntheses and properties of multiferroic composite nanofibersA series of PZT-CFO and PZT-NFO composite nanofibers with different ferroelectric and ferromagnetic molar radio were synthesized by sol-gel process and electrospinning. The effects of solution ion concentration, solution component on the appearance, grain size and component distribution, and piezoelectric and ferromagnetic properties of composite nanofibers were studied as well. The experimental results indicated:(1) The appearance and structure of two-phase hybrid PZT-CFO and PZT-NFO composite nanofibers were similar. The dense multiferroic nanofibers with smooth surface were composed of nanocrystalline PZT and CFO (or NFO) grains with random distribution and grain size in the range of 10～30nm. Furthermore, the diameter of each single nanofiber was rather uniform and straight. The effects of the solution concentration and the molar ratio of PZT to CFO or NFO on the shape, structure and properties of composite nanofibers were investigated. The average diameter of nanofibers decreased with reducing ion concentration. For composite nanofibers with large difference of molar ratio between the two phases, large variation in fiber diameter was observed, and the diameter of the largest fiber was about ten times of the fine ones. Phase segregation was observed in PZT phase frequently, while ferromagnetic CFO and NFO phases were relative stable.(2) The piezoelectric coefficient d₃₃ of the composite nanofibers decreased with increasing content of ferromagnetic phase, while the coercive magnetic fields Hc increased with it. The effective piezoelectric coefficient d₃₃ were estimated to be around 60, 125, 140, 157 and 190pm/V for the composite nanofibers with PZT-CFO molar ratios being 1:1.5, 1:1.25, 1:1, 1:0.75 and 1:0.5, respectively, the remanent magnetization Mr were measured to be around 15.0, 11.3, 8.6, 3.3 and 2.7emu/g, and the coercive magnetic fields Hc were approximately 1700,730,576,386 and 365Oe, respectively.(3) The effective piezoelectric coefficient d₃₃ were estimated to be around 52, 55, 112, 128, and 150pm/V for the composite nanofibers with PZT-NFO molar ratios being 1:1.5, 1:1.25, 1:1, 1:0.75 and 1:0.5, respectively, the remanent magnetization were measured to be about 4.30, 1.58, 0.907, 0.776 and 0.712emu/g, and the corresponding coercive magnetic fields were around 203, 111, 90, 82 and 60Oe, respectively.4. Synthesis and properties of multiferroic BFO nanofibersBiFeO₃ (BFO) nanofibers were synthesized by electrospinning. Ordinary optical microscopy, x-ray diffraction(XRD), scanning electron microscopy(SEM), energy dispersive spectrum(EDS), transmission electron microscopy(TEM), high resolution transmission electron microscopy(HRTEM), atom force microscopy(AFM), high voltage piezoresponse force microscopy(PFM), and vibrating sample magnetometer(VSM) were used to characterize the microstructure and macro-properties of BFO nanofibers. The effects of various bismuth ions content, different polymer and annealing temperatures on the appearance, component, piezoelectric and ferromagnetic properties were studied. The main experimental results showed that:(1) BFO nanofibers were annealed at 550℃for 2h with bismuth content excess 5%. If Ar was used as protective atmosphere instead of air and nitrogen, all the impurity phases disappear and phase pure perovskite BiFeO₃ was obtained; otherwise BFO nanofibers would contain different amount of impurity. The polymer concentration of electrospinning BFO nanofibers needs to be high, with poly(vinyl-pyrrolidone) (PVP) being 0.04～0.08g/mL and poly(2-hydroxyethylmethacrylate)(PHEMA) 0.05～0.12g/mL, respectively. When PVP was used as polymer for electrospining, BFO nanofibers with straight, uniform diameter, and regular round shape were obtained easily. But if PHEMA was used as polymer, and the conglutination and collapse of BFO nanofibers were formed usually.(2) BFO nanofibers were composed of rather dense nanocrystalline grains with its size around 20 nm, though the surface of BFO nanofibers was rough. The polycrystalline nature and rhombohedral perovskite structure of BiFeO₃ nanofibers were confirmed by the selected area electron diffraction(SAED) and HRTEM patterns. The butterfly loop and phase hysteresis loop confirmed the piezoelectric and ferroelectricity properties of BFO nanofibers. The maximum piezoelectric coefficient d₃₃ was estimated to be around 69.3pm/V, and the coercive field of BFO nanofiber was around 150kV/cm. It was also observed that many of the ferroelectric domains were over 100 nm in size, though some domains as small as 10 nm were also present. The saturation magnetization of BFO nanofibers was as large as 4.0emu/g, with the remanent magnetization about 1.7emu/g and the coercive field around 200Oe, respectively.(3) Three different methods were attempted to fabricate aligned BFO nanofibers. Only the technique using rotating disc as cathode collecting device resulted in aligned BFO nanofibers in region of tens micrometer in size. The other two methods, one using aligned electrode plate as collecting device, and the other using BFO nonaparticles as eletrospinning raw material, did not lead to aligned nanofibers.5. Fabrication and properties of multiferroic nanostructures by soft lithographyThe fabrication of multiferroic nanostructures was explored. The PZT, two-phase random composite PZT-CFO and PZT-NFO, laminated composite PZT-CFO and PZT-NFO, and BFO micro- and nano-structure were synthesized by soft lithography. In this process, different kinds of patterned moulds by microfabrication were used as original moulds, and soft polydimethylsiloxane(PDMS) was used as elastomeric stamps. Ordinary optical microscopy, AFM and VSM were used to characterize the microstructure and macroscopic properties of the fabricated micro- and nano-structures. The preliminary experimental results indicated that:The patterned structure of soft PDMS stamps were the same as the initial moulds. Straight and regular PZT micro- and nano-structures with uniform stripe gap space were fabricated successful by nanolithography. Due to the similar fabrication process, the pattern morphology of random composite and laminated composite PZT-CFO and PZT-NFO were almost identical, and some micro- and nano-scale particles were distributed homogeneously on the stripes of random composite patterns. The remanent magnetization of the random composite PZT-CFO pattern was about 1.65emu/cm³, and the corresponding coercive magnetic field was 180Oe. Because the solution concentration of ferromagnetic precursor was rather low, the morphology of laminated PZT-CFO and PZT-NFO composite patterns, in which the ferromagnetic thin films were synthesized by spinning coating on top of patterned PZT film, were similar to those of random composite patterns. When the ferromagnetic phases were coated with the help of flat PDMS, the resulted laminated PZT-CFO and PZT-NFO patterns were regular wavy framework with uniform space between the straight stripes, since the ferromagnetic solution is deposited at the bottom of the stripes of PZT pattern. The width of the stripe was about 2².2um, and the corresponding depth around 68～85nm. The remanent magnetization of the laminated composite PZT-CFO pattern was about 22.3emu/cm³, and the corresponding coercive magnetic fields Hc was around 1000Oe. Single-phase multiferroic BFO patterns were also manufactured by nanolithography, and the patterns were the stripes with uniform space gap and regular straight shape.