【作者】 陈大鹏；

【导师】 刘俊明；

【作者基本信息】 南京大学 ， 凝聚态物理学， 2017， 博士

【摘要】 铁电材料是指会自发极化而且在电场作用下会发生极化翻转的材料,由于这样的性质,铁电材料一直是研究者关注的热点。尤其是在计算机技术、电子通信技术飞速发展的现代,铁电材料因为其丰富的应用前景,使人们对它的兴趣日益增强。近年来发展起来的多铁性物理更加使传统的铁电物理学焕发了新的生机。铁电性的一个主要的应用是利用其可以翻转的极化进行诸如信息存储等相关应用,因此,对铁电极化翻转机制、铁电畴结构形态的研究就显得十分重要,铁电体也正是由于其在不同条件下丰富的畴结构、畴运动方式才成为具有丰富应用的功能材料。而近年来引起人们关注的单相多铁性材料因为其非共线螺旋序所导致的铁电性更引发了研究者对磁电相互调控的兴趣,这可以使微电子器件实现更丰富的功能。实验手段受限于技术手段的发展,对于一些极端情况下的铁电性质不易考察,因此,通过理论手段对特殊条件下的铁电体系进行模拟就成为铁电体研究的重要手段。在铁电体领域,从微观角度出发,利用第一性原理计算探询铁电体物理的微观本质是理论研究很重要的方面,但是对于铁电材料的应用来说,基于宏观唯象理论的铁电模拟具有更加实际的应用价值,本文就是基于宏观唯象理论,利用蒙特卡罗方法和相场方法对特定的铁电和多铁体系进行了模拟。我们的研究集中在三个方面,首先是铁电体系在外电场作用下极化翻转的动力学,也就是所谓的回线动力学,我们研究了在不同频率、不同大小的电场作用下,以及不同温度下的回线变化和畴结构运动,发现在低温和低场的情况下,铁电极化的翻转具有两种模式,对传统的单极化翻转模式做出了补充。而且在低频和高频方向,回线面积谱函数的幂函数标度规律仍然成立。其次,我们对应用广泛的铁电纳米岛的畴结构及其对称性进行了研究,我们发现纳米岛尺寸对畴结构有显著的影响,在某些尺度下,体系会陷入能量井而无法达到基态,从而呈现出90°畴和涡旋畴共存。当引入中心电荷缺陷后,畴的形态又呈现出新的涡旋态,根据我们引入的标度参数,我们对不同条件下的畴结构对称性也进行了研究,发现缺陷电荷的大小和符号对畴结构是如何影响的。最后,我们对多铁性skyrmion格子在外电场下的介电响应进行了研究,发现其极化分布和磁化分布呈现一一对应,而且都表现出六角对称性。在外加电场时,发现其介电响应表现出空间各向异性,而且当外电场在xy面内时,介电响应空间各向异性分布的轴线和外电场的方向垂直,当外电场沿z方向时,介电响应分布则成圆形。更多还原

【Abstract】 Ferroelectric materials refer to the kind of materials that has spontaneous polarization which can reverse under the stimulations of external electric field.Because of such a nature,ferroelectric materials have been a hot research topic for decades,especially in nowadays,when computer and communication technologies are under rapid development.Ferroelectric materials,because of their abundant application prospects,obtained continuous attentions from researchers,while multiferroic materials coming into the sights of people recently made the traditional ferroelectric physics to glow with new vitality.One of the main applications of ferroelectricity is to utilize its nature of dipole-switching under external electric field to realize convenient and reliable information storage as well as other related applications in microelectronic industy.Therefore,the mechanisms of ferroelectric polarization reversal are very important.On the other hand,various domain structures of ferroelectric materials under different conditions are of great importance for the ferroelectric applications.In fact,people name ferroelectric materials as functional materials,in some extent,just because of their abundant domain patterns.Except for traditional ferroelectric materials,single-phase multiferroic materials intrigued lots of researchers for the prospects of electric control of magnetic states and vice versa,which may enable more microelectric applications.For the above reasons,our studies are focused on three aspects of ferroelectrics:(1)dynamics of hysteresis of a two dimensional ferroelectric model,(2)domain patterns of two dimensional ferroelectric nanoislands,and(3)dielectric responses of improper ferroelectric induced by the inverse DM interactions of magnetic spins.The methods we used for the studies are phenomenological approachs,including Monte Carlo and phase field simulations.In the first area of our study,we put a square lattice of dipoles under external altenate electrical fields,and found double-peak behaviors in area-frequecy(A(ω)～ω)curves,which are different from previous studies.We also found the power law scaling behaviors still applicable in the two directions of low and high frequencies.In the model ferroelctric nanoislands,we investigated domain patterns of the islands under different conditions including various sizes of nanoislands,different magnitudes and signs of defect charges in the islands.We found that the sizes of nanoislands,the magnitudes and signs of electric charge defects affected structures and symmetries of ferroelectric domains collaboratively.In the third study of multiferroic skyrmion lattice,we applied electric fields inside the xy plane,then along the z axis.Under the two stimuli,we investigated the dielectric responses of the dipole skyrmion lattice,and observed some interesting dielectric response patterns as well as some intriguing physics.更多还原