Study On Electric Field Control Of Magnetism In BiFeO₃ Based Multiferroic Heterostructures

The materials which possess two or more ferroic properties in the same phase are called multiferroic materials. Multiferroic BiFeO3 (BFO) has a great potential to be used in devices because of its room temperature ferroelectricity (Curie temperature Tc～1100 K) and G type antiferromagnetism (Neel temperature TN～ 640 K). This paper studied the exchange bias effect at the interface between BFO and ferromagnetic thin films, in order to achieve the reversible vertical-electric-field control of magnetic order at room temperature in BFO based multiferroic heterostructures. Finally we also try to expand our research field to the light control of magnetic order in BFO multiferroic heterostructures.The main research contents of this thesis:(1) We use pulsed laser deposition (PLD) system to prepar BFO thin films, the quality of the films greatly depends on the PLD parameters. The light path is one of the important PLD factors, which affect the laser energy density and laser imaging on the target. We found that the laser energy density has obvious effect on the quality of the sample and the exchange bias effect.(2) Polycrystalline antiferromagnetic BFO thin films have been grown on Si/SiO2/Ti/Pt (111) substrates by PLD, and then ferromagnetic films Co2Fe(Al0.5Sio.s) by magnetron sputtering. After fabrication, the films were vacuum annealed under a 1 kOe magnetic field at different temperatures from 150℃ to 500℃. The exchange bias field can be tuned by the annealing temperature for the heterostructures and the electric domain size can be controlled by the crystal grain size. A large exchange bias of about 50 Oe has been observed in one of the samples. It can be speculated that the crystal grain sizes are the key element in determining the exchange bias and coercivity of the films annealed at the temperature higher than Neel temperature of BFO.(3) SrTiO3 (111)/SrRuO3/BFO/NiFe/Ta heterostructure has been fabricated by PLD and magnetic sputtering. The BFO film is pure and highly (111)-oriented. A typical anisotropic magnetoresistance (AMR) effect has been observed at room temperature. When applying an out-of-plane electric field, remarkable reverses of the AMR phase have been observed several times from about -0.015% to +0.015% in our experiment. These results indicate that a reversible out-of-plane electric field control of magnetization in ferromagnetic layer could be achieved through multiferroic BiFeO3 at room temperature.(4) The electric field controlled exchange bias in a PMN-PT/SrRuO3/BFO/Co90Fe10 heterostructure has been investigated under different tensile strain states. The in-plane tensile strain of the BFO film is changed from +0.52% to +0.43%as a result of external electric-field applied to the PMN-PT substrate. An obvious change of exchange bias by the control of non-volatile strain has been observed. A magnetization reversal driven by electric field has been observed in the absence of magnetic field.(5) We expand our view from electric field control of magnetic ordering to using visible light to control the magnetic ordering in BFO based heterostructures. We found the resistance changes up to 2% in the illumination and basically determined that high resistance change is caused by the photostrictive effect of BFO.