Magnetic And Electrical Properties Of The BiFeO₃ Based Multiferroic Heterostructures

BiFeO₃,which is one of the single phase multiferroics,has high ferroelectric Curie temperature,high Néel temperature and large remanent polarization.Thus,it has potential applications in the non-volatile memory devices.In this dissertation,BiFeO₃ thin films and BiFeO₃-based heterostructures were fabricated by magnetron sputtering.The microstructure,magnetic properties and electrical properties were investigated.In BiFeO₃/LaAlO₃ heterostructures,as the thickness of the BiFeO₃ thin films increases,the BiFeO₃ phase undergoes a series of phase transition processes that are the highly strained tetragonal-like BiFeO₃ phase?tetragonal-like BiFeO₃phase?tilted M_C phase?intermediate monoclinic phase?rhombohedral bulk-like BiFeO₃ phase.In the 17-nm-thick BiFeO₃ thin film,the BiFeO₃ phase is the highly strained tetragonal-like BiFeO₃ phase.The net magnetic moment in the highly strained tetragonal-like BiFeO₃ phase results from the spin canting,which is induced by the asymmetric Fe O₅ pyramid.Thus,the net magnetic moment exists in the17-nm-thick BiFeO₃ thin film.In Pt/Fe/BiFeO₃/SrRuO₃ heterostructures,the mechanisms of the resistive switching and capacitance switching behaviors were investigated.The resistive switching behavior comes from the ferroelectric polarization modulated depletion layer width around the BiFeO₃/SrRuO₃ interface.The conductivity mechanisms are the Fowler-Nordheim tunneling mechanism under the negative bias and the space-charge-limited-conduction mechanism under the positive bias.The capacitance switching behavior is attributed to the electric field modulated the ferroelectric domains(including number,domain wall and polarization direction),the depletion layer width and the rearrangement of the carrier density in the depletion layer.Under the positive bias,the capacitance of the Pt/Fe/BiFeO₃/SrRuO₃ heterostructures is in the low capacitance states.Under the negative bias,the capacitance of the Pt/Fe/BiFeO₃/SrRuO₃ heterostructures is in the high capacitance states.Moreover,when the light shines on the Pt/Fe/BiFeO₃/SrRuO₃ heterostructures with the low capacitance state,the capacitance evidently increases.When the light shines on the Pt/Fe/BiFeO₃/SrRuO₃ heterostructures with the high capacitance state,the capacitance evidently decreases.The photo-capacitance behavior is mainly related to the variation of the depletion layer width,induced by the ferroelectric polarization modulated accumulation of the photogenerated carriers around the BiFeO₃/SrRuO₃ interface.In addition,the influence of the uniaxial strain on the magnetic properties of the Fe₃O₄/muscovite and Fe₃O₄/BiFeO₃/Pt heterostructures was investigated.When the uniaxial compressive strain is applied to the Fe₃O₄/muscovite heterostructures,the Verwey transition temperature increases and the saturation magnetization is enhanced.When the uniaxial tensile strain is applied to the Fe₃O₄/muscovite heterostructures,the Verwey transition temperature decreases and the saturation magnetization is enhanced.These phenomena result from the charge transfer between the Fe _A³⁺and Fe _B²⁺ions in the Fe₃O₄ thin films,induced by the uniaxial strain.Further,when the uniaxial strain is applied to the Fe₃O₄/BiFeO₃/Pt heterostructures,the exchange bias field decreases.This phenomenon stems from the decrease of the interfacial superexchange interaction,induced by the variation of the Fe _A-O-Fe _BiFeO3 bond length and bond angle at the Fe₃O₄/BiFeO₃ interface and the decrease of the Fe _A³⁺ions at the Fe₃O₄ layer in bending states.