Doctor of Philosophy (Ph.D.)
Multifunctional oxides attract much attention recently. The strong correlated electron system involves the notable properties of colossal magnetoresistance, ferroelectric tunneling and spin transport, with the coupling of electron, spin and orbital degrees of freedom. Their rich functional behavior is of potential use for nanoelectronics and data storage. Particularly interesting are the mulitferroic materials, which exhibit simultaneously electric and magnetic ordering properties. Understanding the interface coupling mechanism of these two order parameters are critical to future development of high-performance spintronic devices. The goal of this dissertation is to elucidate the interfacial magnetoelectric (ME) coupling with optical characterization method -- magnetization-induced second-harmonic generation (MSHG), which is sensitive to the interface due to the broken spatial inversion symmetry. First, ME coupling at the interface of BaTiO3 (BTO)/La0.67Sr0.33MnO3 (LSMO) is observed by applying an external electric field. The voltage-dependent magnetic contrast reveals a sharp transition from ferromagnetic (FM) to antiferromagnetic (AFM) order occurring at positive voltage (applied to LSMO contact). This novel effect is attributed to interface ME coupling. Strain or ferroelectric (FE) polarization induced mechanisms do not play an important role in this system. A new mechanism is proposed -- minority spin injection -- to modulate the interface magnetization. The minority spin injection at the interface weakens the double-exchange coupling of nearby eg electrons, thereby weakening the FM ordering. Thus the dominant AFM superexchange coupling of localized t2g electrons causes the phase transition at positive voltage. The magnetic transition is shifted to higher voltage by reducing the carrier concentration of BTO. Second, a non-multiferroic heterostructure -- SrTiO3 (STO)/La0.5Ca0.5MnO3 (LCMO)/La0.67Sr0.33MnO3 (LSMO) -- is studied to elucidate further the interface ME effect. The magnetization transition is observed but shifted to negative voltage. The LSMO is pushed to higher hole doping level due to the STO layer which acts as a hole donating layer, while the LCMO interlayer at the medium doping level displays complicated CE-type AFM phase. Thus, a negative voltage is required to lower the hole doping level of LSMO to induce the FM phase. The magnetic contrast reappears at high positive voltage, indicating the occurrence of an A-type AFM phase, which is stable at high hole doping concentration. The results of this dissertation show that the interface magnetic phase of LSMO can be controlled by an applied electric field through modulation of the hole doping level.
© The Author
Fang, Fan, "Optical Characterization Of Interface Magnetization In Multifunctional Oxide Heterostructures" (2016). Dissertations, Theses, and Masters Projects. Paper 1477067969.