鑭鍶錳氧－鋯鈦酸鉛異質結構系統之複鐵交互耦合作用之研究

A Study of Multiferroic Exchange Coupling in La1-xSrxMnO3–Pb(Zr0.5Ti0.5)O3 Heterostructure Systems

廖政華(Jeng-Hwa Liao)

指導教授 ： 吳泰伯

國立清華大學/工學院/材料科學工程學系/博士(2007年)

https://doi.org/10.6843/NTHU.2007.00432

摘要

複鐵材料（Multiferroic）是將鐵電與鐵磁結合在一體，此材料具有無法由單一鐵電或鐵磁所達成之新穎的特性。在本研究中，複鐵材料是由鑭鍶錳氧（La1-xSrxMnO3）(LSMO)之鐵磁性與鋯鈦酸鉛（Pb(Zr1-xTix)O3）(PZT)之鐵電性所合成，當PZT在無外加偏壓或外加偏壓為±7V時，其複鐵交互耦合作用將在本研究中做詳細的討論。
磊晶之LSMO薄膜（x = 0.25）鍍製在鈦酸鍶(SrTiO3)(STO)單晶基板上，其居禮溫度（TC）隨膜厚的減少而降低。死層的膜厚為5 nm，具絕緣性質，且經熱活化能躍遷理論證實相分離的存在。超高真空系統之導電性原子力顯微鏡所量測之影像亦證實此一結果。另一方面，磊晶之LSMO薄膜（x = 0.1）在STO與鋁酸鑭（LaAlO3）(LAO)單晶基板上之磁傳導特性與應變效應之關係亦被詳細討論。薄膜應變鬆弛的結果會導致反鐵磁絕緣相的發生。隨著膜厚的增加，在LAO基板上之薄膜其反鐵磁絕緣相的特性會越明顯；在STO基板上之薄膜其磁化量與TC均會降低。接著我們鍍製磊晶PZT/La0.9Sr0.1MnO3之雙層結構在摻雜鈮之鈦酸鍶基板上(Nb-doped STO)。磁性量測顯示在場冷與零場冷間有一分離現象，其代表著磁性非均質結構存在於界面處，此結構組成為鐵磁晶粒鑲埋在非鐵磁基質中。當PZT極化時在自旋群內會引發一自旋子釘鎖效應(Spin-pinning effect)，此效應會造成零場冷之M/M(10K)比值、分離溫度與磁滯曲線的變化。此外，我們亦探討鐵電場效應對磊晶La0.75Sr0.25MnO3/PZT/La0.75Sr0.25MnO3三層結構的磁性性質之影響。PZT之極化造成的Spin-pinning effect會使三層結構內之LSMO層其磁化量與TC均下降，而矯頑磁場則是上升。在三層結構中，鐵磁層與自旋子釘鎖層之間的鐵磁-反鐵磁交互耦合作用的可能性亦在此研究中做探討。

關鍵字

複鐵材料

並列摘要

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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