Epitaxial Growth And Properties Of BiFeO₃Based Solid Solutions Multiferroic Thin Films

Multiferroic materials, one of the research focuses with rich physical content, have broadapplication prospects. As the only single-phase multiferroic material above the room temperature,bismuth ferrite (BiFeO3, BFO) has been extensively studied in the recent decade. The ferroelectricCurie temperature and antiferromagnetic Neel temperature of BFO are830℃and370℃, respectively,which are promising for potential applications in information storage and spintronic devices.Compared with the weak polarization observed earlier on bulk ceramics, BFO films exhibit largeroom temperature ferroelectric polarization and obvious ferromagnetism. In this thesis, pulsed laserdeposition (PLD) method was utilized to prepare0.9BiFeO3-0.1TbMnO3(0.9BFO-0.1TMO) and (1-x)BiFeO3-xYMnO3(BFO-YMO, x=0~0.15) thin films. Studies have been focused on the effect ofdifferent deposition conditions and YMO-doping on the crystalline phases，surface topography，ferroelectric and magnetic properties at room temperature.High-quality epitaxial and polycrystalline0.9BFO-0.1TMO thin films were successively grownonto (001)c oriented single crystal SrTiO3(STO) substrate and (111)-Pt/TiO2/SiO2/Si substrate usingPLD, respectively. The crystallization and flatness of the films was enhanced by increasing thesubstrate temperature. However, the grain size was also increased, which caused the reduction ofpiezoelectric and ferroelectric properties. It was revealed that epitaxial films deposited at650℃inthe ambient of13Pa oxygen pressure had better electric properties. Moreover, the upward ferroelectricself-poling, enhanced polarization switching and piezoelectric response characteristics wereconfirmed in epitaxial0.9BFO-0.1TMO films with compressive strains on LSMO and SRO bottomelectrodes buffered (001) STO substrates.Epitaxial (1-x) BFO-xYMO (x=0.05,0.10,0.15) thin films were also prepared via PLD, andX-ray diffraction showed that they are all single-phase perovskite, free of impure phases and texturedwith preferential orientation along the [001] direction. The films are dense and uniform with noobvious pores, both the grain size and root-mean-square (RMS) surface roughness increased with theincrease of YMO content. Almost saturated ferroelectric hysteresis loop was obtained from the samplewhen x=0.10, with the remnant polarization of2.02μC/cm2and low leakage current density. Thedependences of magnetization on field showed that all BFO-based films exhibit well-definedferromagnetic response with saturated magnetization at room temperature. Interestingly the additionof YMO fraction leads to a substantial increase in magnetization, unlike the moderate ferroelectric polarization. The doped BFO film containing15%of YMO is evident to possess the maximumsaturated magnetization of nearly26.35emu/cm3. Besides the current research, we also turn ourattention to the optical properties of BFO-based thin films and make some preliminary tests on directband gap.