六角铁氧体的制备及其物理特性的研究

发布时间：2018-04-01 19:26

本文选题：六角铁氧体　切入点：固相烧结　出处：《华东师范大学》2014年博士论文

【摘要】：作为一种单相磁电材料,六角铁氧体(Hexaferrites)的铁电极化能够在室温以及小磁场(～30mT)诱导下实现,因而受到了研究者们的广泛关注,也使其在多铁器件的应用方面有着良好的前景。高性能六角铁氧体材料的制备及其相关物理特性的调控一直是该领域的研究热点。本论文主要研究了Y型和M型结构六角铁氧体的制备,以及金属离子掺杂对其晶格、形貌、磁学特性的调控规律,系统阐释了Y型结构六角铁氧体的介电响应及其产生机制,在此基础上进一步研究了Y型结构六角铁氧体的磁热效应。本文内容主要包括以下几个方面：首先,利用固相反应方法制备了Al离子掺杂的Y型Ba0.5Sr1.5Zn2(Fe1-xAlx)12O22和Co、Ti离子共掺杂的M型结构BaFe12-2xCoxTixO19六角铁氧体,研究了上述金属离子掺杂对铁氧体物理特性的影响。(1)由于Y型与其它型六角铁氧体成相温度存在重叠,因此单相样品的制备有待进一步研究,本文通过调控Al离子掺浓度可以实现单相Y型Ba0.5Sr1.5Zn2(Fe1-xAlx)12O22六角铁氧体样品的制备,而且发现当Al离子掺杂浓度增大到一定程度时,六角形晶粒结构被破坏；对于Co、Ti离子共掺杂的M型结构BaFe12-2xCoxTixO19六角铁氧体,结果表明随掺杂浓度增大,六角形晶粒逐渐增大；六角铁氧体晶粒形状和尺寸的改变可以归咎于金属离子掺杂改变了生长基元之间的表面互连状况,从而改变了晶粒成核能；(2)通过研究金属离子掺杂对六角铁氧体的磁学特性(磁相变温度、饱和磁矩、矫顽场)的影响,发现金属离子掺杂可以改变Fe-O-Fe之间的超交换作用,进而诱导磁自旋结构由共线(或纵向圆锥)到非共线螺旋(或横向圆锥)排列转变,从而影响其磁学特性。其次,通过测试体系变温介电特性,利用修正等效电路模型拟合以及Maxwell-Wagner理论分析,阐明了体系中存在的三种介电响应及其产生机制,即晶粒、晶界和接触电极：(1)低温下,由于热激发小,晶粒的介电响应在体系中起主要作用,即本征的介电响应；随着温度升高,晶格振动增强,体系热激发增大,晶界处电荷积累增多,电极化增强,晶界介电响应显现；温度进一步升高,接触电极的介电响应逐渐表现出来；对介电弛豫活化能的拟合分析表明二价Fe离子和三价Fe离子之间跳跃的电子是Y型六角铁氧体介电响应的主要导电载流子类型；(2)Al离子掺杂的体系中,由于Al离子取代了部分Fe离子,从而改变了晶粒的电导特性,进而改变了界面处电荷积累状况,最终导致晶界和接触电极引起的介电弛豫峰向高频移动。第三,Y型六角铁氧体作为一个室温下具有磁电耦合效应的多铁材料,其磁矩随温度变化曲线表明该体系在非常窄的温度区间内产生了剧烈的磁相变,基于此,我们结合麦克斯韦关系计算了纯相Y型结构六角铁氧体中的磁热效应。研究表明：(1)高于50K时,该体系表现出正磁热效应,对于场变7T,磁熵变(-△SM)和相对制冷能力(RCP)可以达到,～1.53JKg-1K-1和～280JKg-1,最大磁熵变出现在室温附近(远离居里温度),这主要是源于体系中发生的圆锥自旋相到混合螺旋自旋相(二级磁相)的贡献,其大小由自旋磁化矢量的螺旋程度决定；(2)低于50K时,系统的第一磁相变导致了反磁热效应,这主要由磁场诱导螺旋自旋结构产生了更大的自旋失措引起。此外,本论文的附录部分介绍了利用分子束外延法生长SrTiO3同质外延薄膜的相关工作。(1)摸索了SrTiO3衬底预处理方法,通过腐蚀与退火相结合的方法,得到了完美的Ti02截止面,在此基础上制备了SrTiO3同质外延薄膜；(2)通过在不同氧分压下的退火工艺,实现对SrTiO3薄膜中氧空位浓度的控制,并研究了氧空位对其光学和磁学性质的影响,发现氧空位对红/蓝光辐射和磁性都有较大的贡献。
[Abstract]:As a single - phase magneto - electric material , the ferroelectric polarization of hexaferrite can be realized under the induction of room temperature and small magnetic field ( ~ 30mT ) . The preparation of hexagonal ferrite with high performance and the regulation of its related physical properties have been a hot spot in this field .

Firstly , BaFe12 - 2xCoxTixO19 hexagonal ferrite doped with Al ion doped Y - type Ba0.5Sr1 . 5Zn2 ( Fe1 - xAlx ) 12O22 and Co , Ti was prepared by solid - phase reaction .
BaFe12 - 2xCoxTixO19 hexaferrite co - doped with Co and Ti ions showed that hexagonal grains gradually increased with the increase of doping concentration .
changes in the shape and size of the hexagonal ferrite grains can be attributed to the change in the surface interconnection between the growth elements due to the doping of the metal ions , thereby changing the grain into nuclear energy ;
( 2 ) By studying the influence of metal ion doping on the magnetic properties of hexagonal ferrite ( magnetic phase transition temperature , saturation magnetic moment , coercive field ) , it is found that the doping of metal ions can change the superswitching effect between Fe - O - Fe and then induce the transformation of magnetic spin structure from collinear ( or longitudinal conical ) to non - collinear helical ( or transverse cone ) , thus affecting its magnetic properties .

Secondly , by using the modified equivalent circuit model fitting and Maxwell - Wagner theory analysis , three kinds of dielectric responses in the system and the generating mechanism , i.e . grain , grain boundary and contact electrode : ( 1 ) low temperature , the dielectric response of the crystal grains plays a major role in the system , that is , the intrinsic dielectric response .
As the temperature increases , the lattice vibration increases , the system thermal excitation increases , the charge accumulation at the grain boundary is increased , the electric polarization is enhanced , and the grain boundary dielectric response appears ;
the temperature is further increased , and the dielectric response of the contact electrode gradually appears ;
The analysis of the activation energy of the dielectric relaxation shows that the electrons jumping between the bivalent Fe ions and the trivalent Fe ions are the main conductive carrier types of the Y - type hexagonal ferrite dielectric response ;
( 2 ) In the system of Al ion doping , because Al ions replace some Fe ions , the conductance characteristic of the crystal grains is changed , and the charge accumulation state at the interface is changed , and finally , the dielectric relaxation peak caused by the grain boundary and the contact electrode is caused to move towards the high frequency .

Third , Y - type hexaferrite is used as a multiferroic material with magneto - electric coupling effect at room temperature . The magnetic moment of the system shows a strong magnetic phase transition in very narrow temperature range . Based on this , we calculate the magneto - caloric effect in the hexagonal ferrite of pure - phase Y - type structure . The results show that : ( 1 ) The maximum entropy change occurs near room temperature ( far away from Curie temperature ) , which is mainly derived from the contribution of the conical spin phase in the system to the mixed spiral spin phase ( secondary magnetic phase ) , whose size is determined by the degree of helix of spin magnetization vector ;
( 2 ) At lower than 50K , the first magnetic phase transition of the system leads to the diamagnetic thermal effect , which is mainly caused by the magnetic field - induced spiral spin structure .

In addition , the related work of the SrTiO3 homogeneous epitaxial film grown by molecular beam epitaxy was introduced in the appendix of this paper . ( 1 ) The method of pretreatment of SrTiO3 substrate was investigated . The perfect Ti02 cut - off surface was obtained by combining etching and annealing , and SrTiO3 homogeneous epitaxial film was prepared on the basis of the method .
( 2 ) Through the annealing process under different oxygen partial pressures , the oxygen vacancy concentration in SrTiO3 thin film was controlled , and the influence of oxygen vacancy on the optical and magnetic properties was studied .

【学位授予单位】：华东师范大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TM277

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