本文關(guān)鍵詞： 鐵電材料 第一性原理計(jì)算 相場(chǎng)模擬 無(wú)約束薄膜 納米管 二維電子氣 隧穿結(jié)　出處：《浙江大學(xué)》2015年博士論文　論文類型：學(xué)位論文

【摘要】：鐵電材料在功能器件、智能結(jié)構(gòu)等領(lǐng)域有著廣泛的應(yīng)用。隨著器件的小型化和微型化,鐵電材料納米結(jié)構(gòu)的研究受到了學(xué)術(shù)界和工業(yè)界越來(lái)越多的關(guān)注。其中,退極化場(chǎng)的消除機(jī)理關(guān)系到鐵電材料的尺寸效應(yīng)(區(qū)別于塊體材料的特殊性質(zhì),例如,納米尺寸下的電疇形貌),臨界尺寸(小于臨界尺寸鐵電性將消失)等問(wèn)題。因此,對(duì)退極化場(chǎng)消除機(jī)理的研究不僅對(duì)物理、材料領(lǐng)域的基礎(chǔ)研究有著重要的學(xué)術(shù)意義,而且對(duì)工業(yè)應(yīng)用等也可以給出重要的指導(dǎo)。本文主要利用第一性原理計(jì)算,在不同的力學(xué)、電學(xué)邊界條件下,探索和討論了多種退極化場(chǎng)的屏蔽機(jī)理。同時(shí),以相場(chǎng)模擬方法作為第一性原理計(jì)算的外延,討論較大尺寸的鐵電納米結(jié)構(gòu)。(1)本文討論的第一種退極化場(chǎng)屏蔽機(jī)理為“多疇結(jié)構(gòu)”。多疇結(jié)構(gòu)盡可能的減少了極化在自由表面的不連續(xù)性,降低了退極化場(chǎng)的強(qiáng)度,因而可以在電疇內(nèi)部保持鐵電極化。通過(guò)對(duì)PbTiO3無(wú)約束薄膜的模擬,我們討論了其穩(wěn)定的閉合電疇結(jié)構(gòu)。并且通過(guò)和BaTiO3薄膜的比較,解釋了PbTi03納米薄膜中鐵電臨界厚度消失的機(jī)理。研究發(fā)現(xiàn),應(yīng)變梯度起了重要的作用。為了驗(yàn)證這一點(diǎn),我們?cè)O(shè)計(jì)了BaTiO3納米管的算例,并在其中驗(yàn)證了上述機(jī)理所預(yù)測(cè)的撓曲電效應(yīng)。(2)在無(wú)約束鐵電薄膜中,本文進(jìn)一步利用相場(chǎng)模擬方法,在更大尺寸范圍內(nèi)針對(duì)鐵電薄膜中帶狀180。電疇結(jié)構(gòu)的Kittel law和generalized scaling law,做了探索性的研究。相場(chǎng)模擬方法作為一種連續(xù)介質(zhì)理論,高度依賴于其采用的能量形式和參數(shù)。然而,其梯度能系數(shù)在以往的文獻(xiàn)中并不確定,只是限于一個(gè)取值范圍。本文結(jié)合以上兩點(diǎn),在帶狀180。電疇結(jié)構(gòu)的模擬中考慮了不同的梯度能系數(shù),以揭示其對(duì)Kittel law的影響。并且,通過(guò)進(jìn)一步考慮疇壁厚度,在數(shù)值計(jì)算的層面上驗(yàn)證并討論了generalized scaling law。(3)本文闡述的第二種退極化場(chǎng)屏蔽機(jī)理為“二維電子氣”�；贚aAlO3/SrTiO3界面上二維電子氣的實(shí)驗(yàn)發(fā)現(xiàn),本文嘗試將其與鐵電薄膜相結(jié)合,利用鐵電極化來(lái)代替極性材料(例如LaAlO3)中的內(nèi)在極化,并且從熱力學(xué)模型預(yù)測(cè)出了該機(jī)理的可行性。根據(jù)熱力學(xué)模型,當(dāng)鐵電薄膜的厚度大于臨界厚度時(shí),二維電子氣可以穩(wěn)定存在于薄膜的界面或表面,并幾乎完全抵消了退極化場(chǎng),從而穩(wěn)定了鐵電薄膜中的面外單疇構(gòu)型。根據(jù)這一預(yù)測(cè),本文對(duì)PbTiO3/SrTiO3異質(zhì)結(jié)構(gòu)進(jìn)行了第一性原理計(jì)算,計(jì)算結(jié)果證實(shí)了該機(jī)理。(4)本文討論的第三種退極化場(chǎng)屏蔽機(jī)理為“金屬電極”。已有的實(shí)驗(yàn)就多鐵性隧穿結(jié)Fe/BaTiO3/Fe的界面原子結(jié)構(gòu)給出了不同的報(bào)道。本文利用第一性原理計(jì)算,結(jié)合不同的界面結(jié)構(gòu),對(duì)兩種隧穿結(jié)進(jìn)行了系統(tǒng)的研究,包括鐵電性、鐵磁性和界面磁電耦合效應(yīng)。并揭示了由于不穩(wěn)定化學(xué)鍵的形成所導(dǎo)致的結(jié)構(gòu)整體鐵電性的提高和界面磁電耦合效應(yīng)的增強(qiáng)。
[Abstract]:Ferroelectric materials have been widely used in functional devices, intelligent structures and other fields. With the miniaturization and miniaturization of devices, the research of ferroelectric nanostructures has attracted more and more attention in academia and industry. The elimination mechanism of depolarization field is related to the size effect of ferroelectric materials (different from the special properties of bulk materials, such as domain morphology in nanometer size). Therefore, the study of the mechanism of depolarization field elimination is of great academic significance not only to the basic research in the field of physics and materials. In this paper, we mainly use the first principle calculation, under different mechanical and electrical boundary conditions, explore and discuss the shielding mechanism of a variety of depolarization fields. The phase field simulation method is used as the extension of first-principle calculation. The first kind of depolarization field shielding mechanism discussed in this paper is "multi-domain structure". Multi-domain structure reduces polarization discontinuity on free surface as far as possible. The intensity of the depolarization field is reduced, so that the ferroelectrode can be kept in the domain. The simulation of the PbTiO3 unconfined film is carried out. The stable closed domain structure is discussed and the mechanism of the disappearance of ferroelectric critical thickness in PbTi03 nanocrystalline films is explained by comparison with BaTiO3 films. Strain gradient plays an important role. In order to verify this, we have designed an example of BaTiO3 nanotubes. It is verified that the flexural effect predicted by the above mechanism is further simulated in the unconstrained ferroelectric thin films by using the phase field simulation method. In a larger range, Kittel law and generalized scaling law with band-shaped 180. domain structure in ferroelectric thin films are obtained. The phase field simulation method, as a continuum theory, is highly dependent on the energy forms and parameters used. However, its gradient energy coefficient is uncertain in previous literatures. Considering the above two points, different gradient energy coefficients are considered in the simulation of the zonal 180. domain structure to reveal its influence on the Kittel law. The thickness of domain wall is further considered. In this paper, generalized scaling law. 3) is verified and discussed at the level of numerical calculation. The second depolarization field shielding mechanism described in this paper is "two-dimensional electron gas". Based on the experimental discovery of two-dimensional electron gas at LaAlO3/SrTiO3 interface. In this paper, we try to combine it with ferroelectric thin film and use iron electrode instead of internal polarization in polar materials (such as LaAlO _ 3). According to the thermodynamic model, when the thickness of ferroelectric film is larger than the critical thickness, the two-dimensional electron gas can stably exist on the interface or surface of the film. The depolarization field is almost eliminated, and the out-of-plane single-domain configuration in ferroelectric thin films is stabilized. According to this prediction, first-principles calculations of PbTiO3/SrTiO3 heterostructures are carried out. The calculated results confirm the mechanism. The third depolarization field shielding mechanism discussed in this paper is "metal electrode". Different reports have been given on the interface atomic structure of polyferroelectric tunneling junction (Fe/BaTiO3/Fe). First principle calculations are used in this paper. Combined with different interface structures, two tunneling junctions were systematically studied, including ferroelectric properties. The ferromagnetic and interface magnetoelectric coupling effects, and the enhancement of the ferroelectric properties of the structure and the magnetoelectric coupling effect of the interface due to the formation of unstable chemical bonds are also revealed.
【學(xué)位授予單位】：浙江大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2015
【分類號(hào)】：TB383.1

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