| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 95 === Abstract
Multiferroics, which combine ferromagnetism and ferroelectricity in one body, exhibit novel characteristics and could not be achieved separately in either ferroelectric or ferromagnetic (FM) materials. In this study, the multiferroic exchange coupling between the ferromagnetic, La1-xSrxMnO3 (LSMO), and ferroelectric, Pb(Zr1-xTix)O3 (PZT), materials was demonstrated under the condition that the PZT was unpolarized or polarized with an applied voltage, Va = ±7V.
Epitaxial LSMO films (x = 0.25) deposited on SrTiO3 (STO) substrate display a decrease in Curie temperature (Tc) with reducing the film thickness down to 5 nm. The 5-nm-thick film, i.e. the thickness of the dead layer, displays an insulative characteristic and the phase-separation phenomenon was demonstrated by the thermally-activated hopping transport model. The conductive atomic force microscopy images also confirm the results. On the other hand, the strain effect on the magnetotransport properties of epitaxial LSMO films (x = 0.1) on STO and LaAlO3 (LAO) substrate, respectively, is also demonstrated. The strain relaxation of films results in the formation of spin-canted antiferromagnetic (AFM) insulative phase. The characteristics of AFM insulative phase become apparent with increasing the film thickness, which leads to a clear AFM transition in the films grown on LAO and a reduction of magnetization and Tc in those on STO. In chapter 6, the magnetic properties of Pb(Zr0.5Ti0.5)O3/La0.9Sr0.1MnO3 bilayers epitaxially grown on Nb-doped STO show a divergence between field-cooled (FC) and zero-field-cooled (ZFC) magnetization measurements, which suggests the presence of a magnetic inhomogeneity composing of ferromagnetic grains embedded in non-ferromagnetic matrix at the interface. The polarization state of the PZT induces a spin-pinning effect on the spin clusters, causing a variation of the M/M(10K) ratio in ZFC, the divergent temperature, and the hysteresis loop characteristics. In chapter 7, the ferroelectric field effect on modulating the magnetic properties of La0.75Sr0.25MnO3/PZT/La0.75Sr0.25MnO3 trilayers epitaxially deposited on Nb-doped STO was investigated. The polarization of the PZT leads to a spin-pinning effect, which decreases the Tc and the magnetization of the LSMO layers and increases the coercive field for magnetic switching. A possible presence of a FM-AFM exchange coupling between the FM and the spin-pinned layers is also demonstrated.
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