摘要
铁电/铁磁异质复合材料因其巨大的应用潜力而受到研究者的广泛关注。此类材料不仅同时具有铁电性和铁磁性,而且还具有磁电耦合效应,利用这一效应,可以设计磁探测器、新功能存储器、磁电马达等多种新型器件。一般认为磁电耦合效应的产生是由于复合体系中铁电相与铁磁相之间的应力作用,但由于普通的复合体系(如固熔块材复合体系、叠层复合体系等)中应力情况复杂,因此目前的研究对复合磁电材料中应力耦合的机理缺乏规律性的认识。然而在异质外延复合磁电薄膜中,薄膜的取向、厚度、界面粗糙度等都可以得到精确的控制,进而可以控制铁电相与铁磁相之间的应力作用情况,这为研究复合磁电材料的应力耦合机制奠定了基础。
本文从研究BaTiO_3/CoFe_2O_4双层复合磁电薄膜的外延生长过程出发,采用原位反射式高能电子衍射(RHEED)与后位高分辨X射线衍射(XRD)结合的方法,对异质外延复合磁电薄膜的界面应力情况进行充分表征。在获得复合薄膜中基本的应力作用情况后,进一步对应力引起的磁电性能变化进行研究。通过设计厚度系列、层数系列的实验,实现了利用结构变化调制应力变化,进而获得较优的铁电、铁磁性能。这一研究为进一步探索复合磁电材料的应力耦合机制奠定了基础。
在对BaTiO_3/CoFe_2O_4双层复合薄膜的生长过程、界面应力情况和界面应力效应的研究中发现,第一层BaTiO_3以层状模式生长,在其上生长的CoFe_2O_4受到压应力作用,在40nm以内主要以层状-岛状模式生长,厚度进一步增加时,生长模式转换为岛状生长模式。应力释放主要发生在生长模式转变阶段。CoFe_2O_4对BaTiO_3的张应力随着CoFe_2O_4厚度的增加而增大,增大的张应力使BaTiO_3的四方铁电性下降,导致BaTiO_3的Pr值下降,当CoFe_2O_4的厚度为120nm时,复合薄膜的2Pr值仅为3μC/cm~2。磁电效应研究表明,在静态磁场作用下,BaTiO_3/CoFe_2O_4复合薄膜的剩余极化值Pr值增大,增大的幅度随磁场强度的增强而增大,并且当极化电压达到5V时出现极大值。
在对多层系列Pb(Zr_(0.52)Ti_(0.48))O_3/NiFe_2O_4复合薄膜的研究中发现,在一定范围内复合层数的增加可以加强Pb(Zr_(0.52)Ti_(0.48))O_3与NiFe_2O_4之间的应力耦合作用,复合薄膜的Pr值也随之提高,当复合层数达到7层时,2Pr值达到最大值65μC/cm~2;但当复合层数进一步增加时,耦合应力通过界面缺陷等方式释放,造成Pr值的回落;应力变化对复合薄膜的铁磁性能影响较弱。
Magnetoeletirc composite material has drawn wide interest of researchers for its potential uses. In magnetoeletirc material, ferroelectrics and ferromagnetics coexist, and a dielectric polarization can be induced by an external magnetic field or conversely, a phenomenon known as the magnetoeletric (ME) effect. Materials systems with strong ME effect have potential applications as magnetic field and stress sensors, data storage and switching devices, actuators and etc. The ME effect in multiferroic composites comes from the direct strain-coupling between ferroelectric phase and ferromagnetic phase. Unfortunately, the physics mechanisms by which the different order parameters can be coupled are generally poorly understood, and in most of the composite systems, including ceramic composites, laminated composites and etc, the situation of residual strain is very complex. In epitaxial thin-film heterostructure, however, the studies of magnetoelectric composite could be permited precisely as crystallographic orientation, layer thickness and interfacial roughness may be controlled accurately, which makes the study of stress interaction in ME composites possible.
In this paper, we firstly studied the growth process of BaTiO_3/CoFe_2O_4 bilayer films. Reflection high-energy electron diffraction (RHEED) and high-resolution x-ray diffraction (HRXRD) were used to analyze situation of the interfacial stress. Changes of the ferroelectric and ferromagnetic qualities of the composites films induced by the interfacial stress were also studied. We achieved the control of the interfacial strain by adjusting the thickness of each layer and the number of layers in the composite films.
In the study of BaTiO_3/CoFe_2O_4 bilayer films, we found that the BaTiO_3 layer was grown on SrTiO3 substrate in a layer-by-layer growth mode. Within 40 nm, the CFO layer was grown in Stransky-krastonov growth mode, and when the thickness exceeded 40nm, the growth mode changed into island growth mode. The relaxation of the interfacial stress mainly happened during the period of growth mode transformation. The remnant polarization of the composite films decreases with the increasing of CFO thickness. This should be induced by the strengthened tensile stress which weakens the ferroelectric tetragonal phase of BTO. When the thickness of CFO attains to 120 nm, the 2Pr of the composite films decreased to 3μC/cm~2.The polarization had a response to the static magnetic field, which was strengthened with the increasing magnetic field. The response attained to maximum when the electric field was 5V.
In the study of the multilayer films of Pb(Zr_(0.52)Ti_(0.48))O_3/NiFe_2O_4, we found that the interfacial strain between the Pb(Zr_(0.52)Ti_(0.48))O_3 layer and the NiFe_2O_4 layer would be strengthened as the number of layers in the films increased in a certain range, which leading to the increases of remnant polarization of the films. When the number of layers was 7, the 2Pr attains to maximum of 65μC/cm~2. When the number of layer exceeded the range, the interfacial stress would be relaxed, causing the falling of the remnant polarization. The changes of the interfacial stress had a weak effect on the ferromagnetic qualities of the films.
引文
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