摘要
目前,磁性材料与铁电材料的发展应用已经渗透于现代科技的各个领域中。譬如：电子信息产品一般会利用铁磁体来存储信息。相应地,传感器工业则强烈地依赖于一类被称为铁电体的材料,这类材料具有自发电极化,而且它的电极化方向可以随着外加电场的方向的变化而改变。多铁材料是一类有趣的兼具电、磁和结构序参的化合物,因此导致在同一个相中它可能同时存在铁电、铁磁和铁弹性。随着大规模集成电路的发展,器件小型化成了必然的发展趋势,而这种趋势促使人们开始寻找集铁电性和铁磁性于一身的多功能材料。
本文采用了基于密度泛函理论(DFT)的第一性原理计算方法,研究了有机磁体[M(N3)2(HCOO)][(CH3)2NH2] (M=Fe and Co)的电子结构,电荷转移及磁性相互作用,发现该材料的基态呈现反铁磁性,而铁磁态为亚稳态,其铁磁相互作用主要是通过叠氮基进行传递的。计算的能带结构表明现其在铁磁态呈现半导体性质,而在反铁磁态呈现金属性。对于闪锌矿结构的ZnO,研究了Co和Cu掺杂后的电子结构和磁性质。计算的能带结构表明Co掺杂的ZnO显示半导体性质,而Cu掺杂的ZnO显示有趣的半金属性。
对LiNbO3类型的ZnSnO3,采用密度泛函微扰理论(DFPT)研究了它的铁电和非线性光学性质,结果表明ZnSnO3是一个直接带隙的半导体材料,有着较大的自发极化。大的介电常数和非线性光学系数显示了它是一个高竞争力的介电及非线性光学材料。此外,计算的非线性光学系数和电光张量显示虽然LiNbO3类型的ZnSnO3和LiNbO3晶体属于同样的3m点群,但是二者的对称性是不同的。对于单相多铁材料BiMn2O5,通过DFT和Berry phase方法,分别计算了其电子结构和自发极化,计算结果显示该材料的基态为反铁磁态,自发极化主要来自于Bi 6s孤对电子诱发的结构扭曲和非共线磁序两个因素。
At present, the development of magnetic and ferroelectric materials has penetrated in all areas of modern science and technology. For example, electronic information products usually use the ferromagnets to store information. Accordingly, the sensor industry is strongly dependent on a class known as ferroelectric materials, which have a spontaneous polarization, and the direction of its polarization with the electric field can be changed. Multiferroics are an interesting group of compounds that have electric, magnetic and structural order parameters that result in simultaneous ferroelectrics, ferromagnetism and ferroelasticity in the same phase. With the development of large scale integrated circuits, device miniaturization has become an inevitable trend, and this trend promotes people to look for multi-functional magnetoelectric materials.
In this paper, we have studied the electronic structure, charge transfer and magnetic coupling of the two chains compounds of [M(N3)2 (HCOO)][(CH3)2NH2] (M= Fe and Co) by employing the first-principles method based on density functional theory (DFT). The results reveal that AFM state is the ground state and FM state is the metastable one for the two compounds, and the ferromagnetic interaction is mainly transmitted by end-on azido. The two compounds exhibit semiconductor character with small gap in the FM state, while metallic in the AFM state. Using the same method, we have studied the electronic structure of Co and Cu doped zinc-blende ZnO, band structure calculations show that the Co-doped ZnO is semiconductor, while the Cu-doped ZnO shows an interesting half-metallic character.
For LN-type ZnSnO3, we have investigated the ferroelectric and nonlinear optical properties by using DFPT method. The results show that the LN-type ZnSnO3 is a direct-band-gap semiconductor and has a large spontaneous polarization. The large dielectric constants and NLO susceptibilities indicate that the LN-type ZnSnO3 would be a candidate for high-performance dielectric and nonlinear optical material. In addition, by comparing the NLO susceptibilities and EO coefficients of LN-type ZnSnO3 with that of LiNbO3, although the two compounds belong to the same point group (3m), it is found that the symmetry of them is different. For mutiferroic BiMn2O5, we have calculated its electronic structure and spontaneous polarization. The results reveal that antiferromagnetic state is the ground state. The ferroelectricity of BiMn2O5 arises from two factors:the role of Bi lone pair electrons and the noncollinear magnetic ordering.
引文
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