| Multiferroics materials simultaneously exhibit two or more types of ferroic orders(ferroelectricy, ferroelasticit y and ferromagnetism). Importantly, the coupling between the magnetic and ferroelectric order parameters can lead to magnetoelectric(ME) effects, in which the magnetization can be tuned by an applied electric field and vice versa. Recently, mult iferroic materials and devices with spin and polarization orders have attracted enormous interests and become a hotspot due to the fundamental science and the tantalizing technolo gical perspective. The peculiar properties of magnetoelectric coupling give more freedoms for design of materials and mult ifunctional devices, which lead to the potential applications in mult i-field data-read-and-write device, micro-electromechanical systems, transducers, sensors actuators and mult ifunctional devices.In this thesis, mult iferroic composite thin films, which were fabricated on basis of ferroelectric Pb(Zr0.52Ti0.48)O3(PZT), Bi3.15Nd0.85Ti3O12(BNT), and polyvinylidene fluoride(PVDF) and ferromagnetic(FM) La0.67Sr0.33 Mn O3(LSMO), La0.7Ca0.3Mn O3(LSMO) and Co40Fe40B20(Co Fe B) as the parents, have been systematically investigated. Firstly, the current state of multiferroic materials and their potential applications were reviewed in the introduction. And then, we mainly discussed:(1) the fabrication methods, fundamental physical properties and characteristics of multiferroic thin film;(2) the magnetoelectric effects(ME) and mechanism analysis of ME for the mult iferroic composite thin films. Our work are summarized as follows:1. Epitaxial thin films of Pb Zr0.52Ti0.48O3(PZT), La0.67Sr0.33 Mn O3(LSMO) and La0.67Sr0.33 Mn O3-Pb Zr0.52Ti0.48O3(LSMO-PZT) were deposited on Sr Ru O3(SRO)-buffered Sr TiO3(STO) and Pt/Ti/SiO2/Si(Pt) substrates by employing the pulsed laser deposition(PLD) method. The physical characteristics and magnetoelectric coupling properties of PZT, LSMO and PZT-LSMO films were investigated in detail. The layer sequences of each ferroic orders and the characteristics and ME were also demonstrated. The results indicated that:(1) the epitaxy and good crystallization of single-phase PZT and LSMO thin films on SRO-buffered STO subsrates were demonstrated. The ferroelectric behavior along with a remnant polarization(2 Pr) of 119 μC/cm2 for the single-phase PZT films on the SRO-buffered STO substrates was found. And the good magnetic properties for the single-phase LSMO films were demonstrated.(2) The magnetoelectric PZT-LSMO composite thin films were grown on STO substrates by PLD with different growth sequences of PZT and LSMO yielding the following la yered structures: PZT/LSMO/STO and LSMO/PZT/STO. The experimental results show that these composite films exhibit both good ferroelectric and magnetic properties, as well as magnetoelectric effects at room temperature. The results can be influenced by growth sequences due to substrate clamping and interface bonding of PZT and LSMO.2. Considering the effect of misfit of substrate, the oxide electrodes were used to suppress defects and enhance ferroelectricit y and magnetoelectric effects. The epitaxial composite thin films of La0.67Sr0.33 Mn O3/Pb Zr0.52Ti0.48O3(LSMO/PZT) were deposited on Sr TiO3 substrates with a SRO buffer layer by employing PLD method. The LSMO/PZT/SRO/STO composite films exhibit both good ferroelectric and magnetic properties, and an improved magnetoelectric effect(ME) voltage coefficient of 92 mV/cm·Oe were obtained at RT due that the buffer layer can reduce the clamping effect of substrate and the misfit resulting in stress.3. Considering environmental protection and lattice matching, Bi3.15Nd0.85Ti3O12 thin films(one of the most promising lead-free ferroelectric materials) and the 0-3 type composite thin films of Bi3.15Nd0.85Ti3O12-La0.7Ca0.3Mn O3 were deposited on Pt substrates by Sol-Gel technique. The technological parameter of BNT, and the electrical properties, leakage mechanisms and magnetoelectric coupling properties of BNT and BNT-LCMO films were investigated in detail. The experimental results show that ferroelectric behavior along with a remnant polarization(2 Pr) of 46 μC/cm2 and a low leakage current of <1?10-6 A/cm2 were found for BNT thin films at the annealed temperature of 750 oC. And the 0-3 type composite films of BNT-LCMO exhibited a saturated magnetization around 56 emu/cm3 and magnetoelectric effect voltage coefficient(αME) of 33 mV/cm Oe at 100 K.4. Considering the disadvantages of Sol-Gel technique, the 2-2 type of mult iferroic composite BNT-LCMO thin films with different growth sequences were fabricated on the La NiO3-buffered STO substrates by RF magnetron sputtering. The results indicated that:(1) the ferroelectric behaviors along with remnant polarization(2Pr) of 50 μC/cm2 and 40 μC/cm2 at room temperature, saturated magnetizations around 206 emu/cm3 and 192 emu/cm3 at 100 K were measured for BNT/LCMO and LCMO/BNT composite thin films, respectively.(2) The temperature dependence of magnetoelectric coupling(ME) effect was investigated in detail and the ME voltage coefficients of 63 mV/cm·Oe and 60 mV/cm·Oe at 100 K were obtained, and the ferroelectric domains were measured by PFM. The results show that the films exhibit both good ferroelectric and ferromagnetic properties, as well as a substantial ME effect. Moreover, it is demonstrated that the layer sequences and temperature have an obvious influence on the magnetoelectric coupling behavior of these double-layered thin films, which may be caused by the magnetic-mechanical-electric interaction, substrate clamping and interface coupling.(3) The electrical, magnetic and magnetoelectric coupling properties can be influenced by temperature field: the magnetoelectric effect voltage coefficient is weak at room temperature and it can be improved at low temperatures, which can be attributed to the temperature dependence of structures in LCMO-BNT multiferroic composite ?lms.5. Considering the application to flexible electronics and the improvement of magnetoelectric effect by using flexible substrates, we fabricated magnetostrictive Co40Fe40B20(Co Fe B) films and the exchange bias heterojunction of Ta/Ir Mn/Co Fe B on flexible polyethylene terephthalate(PET) and ferroelectric polyvinylidene fluoride(PVDF) substrates by DC magnetron sputtering, in which a significant uniaxial magnetic anisotropy was observed. The magneto-mechanical coupling effect and thickness effect of PET/Co FeB/Ta, the magneto-mechanical-electrical coupling of the heterojunction of PVDF/Ta/Co FeB/Ta and PVDF/Ta/Ir Mn/Co Fe B were investigated in detail. The results indicated that:(1) the hysteresis loops were measured under various compressive or tensile strains by inward or outward bending of the films. The orientation and strength of the uniaxial magnetic anisotropy can be tuned by the external stress. Consequently, the easy axis can be tuned to the hard axis, or vice versa. A modified Stoner-Wohlfarth model with considering the distribution of easy axes was developed to account for the mechanically tunable magnetic properties in flexible Co40Fe40B20 films. The simulation results indicate that the competition between stress-induced anisotropy and the uniaxial anisotropy together with its distribution of Co40Fe40B20 films are the key factors in mechanically tunable anisotropy.(2) The magnetoelectric coupling behavior of Co Fe B/PVDF composites were investigated in detail, and the results demonstrated that the ferroelectric properties can be tuned under magnetic field and vice versa.(3) The coercive forces of 47 Oe and 61.5 Oe, and the exchange bias field of 102 Oe and 40 Oe were measured for the exchange bias heterojunction of Ta/Ir Mn/Co Fe B on Si and PVDF substrates, respectively. The exchange bias fields can be tuned by temperature and electric field on basis of the thermal expansion and piezoelectric effect. |
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