Anisotropic Magnetoelectric Effects In Fe₃O₄/BiFeO₃Epitaxial Heterostructures

Multiferroics is a versatile material posing both magnetism and ferroelectricity.Spintronic properties of the spin polarized materials can be modulated by compositingwith multiferroics, which shows potential in applications. As the representative of thetype I multiferroics, BiFeO3shows diversity in the lattice structure with correspondingmagnetic properties. More complex coupling would exist at the interface when BiFeO3is epitaxially combined with the half metal Fe3O4. In this dissertation, epitaxial BiFeO3and Fe3O4/BiFeO3bilayers were fabricated by using radio frequency and facing targetreactive sputtering. The structures, magnetic and transport properties weresystematically analyzed.Epitaxial BiFeO3of rhombohedral and tetragonal structures were grown on SrTiO3(100),(111) and LaAlO3(100) substrates, respectively. The epitaxial growth of Fe3O4layers can be optimized by moderately decreasing the substrates’ temperature anddepositing rate. When the thickness of BiFeO3increases, the surface roughness firstgrows and then decreases, following the Volmer Weber mode.An inversely proportional increase of the saturated magnetization was obtained inthe epitaxial Fe3O4/BiFeO3bilayers with decreasing thicknesses of the ferromagneticlayers, indicating an obvious interfacial coupling effect. An increasing exchange biascan be observed in the bilayers below~100K. It is supposed that the magnetic couplingmainly originates from the Fe O Fe superexchange interaction at the [Fe2O4]/[FeO2]interface. When the temperature of measurement is growing, the thermal activationwould offset the influence of superexchange, leading to a lower exchange bias.The low field peaks of the butterfly shaped MR curves in the epitaxialFe3O4/BiFeO3bilayers are related to the coercivity of hysteresis loops. The position andvalue of the peaks are asymmetric in the magnetization reversal process due to theinfluence of interfacial pinning effect on spin scattering. The unidirectional anisotropywas enhanced in the anisotropic magnetoresistance of the bilayers with a growingmagnetic field. It is due to a considerable scattering effect induced by the increasingrotation of magnetic moments pinned at the interface. According to the AMR fitted, theantiferromagnetic residual moments providing pinning would approach the ferromagnetic easy axis along with the magnetization process under the exchangecoupling effect.Enhancement of exchange bias effect was observed in the tetragonal likeFe3O4/BiFeO3bilayers compared with the rhombohedral bilayers. An increment ofexchange bias from160Oe (R bilayer) to320Oe (T bilayer) was obtained at3K. It isconsidered to originate from the stronger superexchange interaction induced by thedistorted [FeO2] face of T like BiFeO3at the interface.