本文選題：六角鐵氧體　切入點(diǎn)：固相燒結(jié)　出處：《華東師范大學(xué)》2014年博士論文

【摘要】：作為一種單相磁電材料,六角鐵氧體(Hexaferrites)的鐵電極化能夠在室溫以及小磁場(chǎng)(～30mT)誘導(dǎo)下實(shí)現(xiàn),因而受到了研究者們的廣泛關(guān)注,也使其在多鐵器件的應(yīng)用方面有著良好的前景。高性能六角鐵氧體材料的制備及其相關(guān)物理特性的調(diào)控一直是該領(lǐng)域的研究熱點(diǎn)。本論文主要研究了Y型和M型結(jié)構(gòu)六角鐵氧體的制備,以及金屬離子摻雜對(duì)其晶格、形貌、磁學(xué)特性的調(diào)控規(guī)律,系統(tǒng)闡釋了Y型結(jié)構(gòu)六角鐵氧體的介電響應(yīng)及其產(chǎn)生機(jī)制,在此基礎(chǔ)上進(jìn)一步研究了Y型結(jié)構(gòu)六角鐵氧體的磁熱效應(yīng)。本文內(nèi)容主要包括以下幾個(gè)方面： 首先,利用固相反應(yīng)方法制備了Al離子摻雜的Y型Ba0.5Sr1.5Zn2(Fe1-xAlx)12O22和Co、Ti離子共摻雜的M型結(jié)構(gòu)BaFe12-2xCoxTixO19六角鐵氧體,研究了上述金屬離子摻雜對(duì)鐵氧體物理特性的影響。(1)由于Y型與其它型六角鐵氧體成相溫度存在重疊,因此單相樣品的制備有待進(jìn)一步研究,本文通過調(diào)控Al離子摻濃度可以實(shí)現(xiàn)單相Y型Ba0.5Sr1.5Zn2(Fe1-xAlx)12O22六角鐵氧體樣品的制備,而且發(fā)現(xiàn)當(dāng)Al離子摻雜濃度增大到一定程度時(shí),六角形晶粒結(jié)構(gòu)被破壞；對(duì)于Co、Ti離子共摻雜的M型結(jié)構(gòu)BaFe12-2xCoxTixO19六角鐵氧體,結(jié)果表明隨摻雜濃度增大,六角形晶粒逐漸增大；六角鐵氧體晶粒形狀和尺寸的改變可以歸咎于金屬離子摻雜改變了生長(zhǎng)基元之間的表面互連狀況,從而改變了晶粒成核能；(2)通過研究金屬離子摻雜對(duì)六角鐵氧體的磁學(xué)特性(磁相變溫度、飽和磁矩、矯頑場(chǎng))的影響,發(fā)現(xiàn)金屬離子摻雜可以改變Fe-O-Fe之間的超交換作用,進(jìn)而誘導(dǎo)磁自旋結(jié)構(gòu)由共線(或縱向圓錐)到非共線螺旋(或橫向圓錐)排列轉(zhuǎn)變,從而影響其磁學(xué)特性。 其次,通過測(cè)試體系變溫介電特性,利用修正等效電路模型擬合以及Maxwell-Wagner理論分析,闡明了體系中存在的三種介電響應(yīng)及其產(chǎn)生機(jī)制,即晶粒、晶界和接觸電極：(1)低溫下,由于熱激發(fā)小,晶粒的介電響應(yīng)在體系中起主要作用,即本征的介電響應(yīng)；隨著溫度升高,晶格振動(dòng)增強(qiáng),體系熱激發(fā)增大,晶界處電荷積累增多,電極化增強(qiáng),晶界介電響應(yīng)顯現(xiàn)；溫度進(jìn)一步升高,接觸電極的介電響應(yīng)逐漸表現(xiàn)出來；對(duì)介電弛豫活化能的擬合分析表明二價(jià)Fe離子和三價(jià)Fe離子之間跳躍的電子是Y型六角鐵氧體介電響應(yīng)的主要導(dǎo)電載流子類型；(2)Al離子摻雜的體系中,由于Al離子取代了部分Fe離子,從而改變了晶粒的電導(dǎo)特性,進(jìn)而改變了界面處電荷積累狀況,最終導(dǎo)致晶界和接觸電極引起的介電弛豫峰向高頻移動(dòng)。 第三,Y型六角鐵氧體作為一個(gè)室溫下具有磁電耦合效應(yīng)的多鐵材料,其磁矩隨溫度變化曲線表明該體系在非常窄的溫度區(qū)間內(nèi)產(chǎn)生了劇烈的磁相變,基于此,我們結(jié)合麥克斯韋關(guān)系計(jì)算了純相Y型結(jié)構(gòu)六角鐵氧體中的磁熱效應(yīng)。研究表明：(1)高于50K時(shí),該體系表現(xiàn)出正磁熱效應(yīng),對(duì)于場(chǎng)變7T,磁熵變(-△SM)和相對(duì)制冷能力(RCP)可以達(dá)到,～1.53JKg-1K-1和～280JKg-1,最大磁熵變出現(xiàn)在室溫附近(遠(yuǎn)離居里溫度),這主要是源于體系中發(fā)生的圓錐自旋相到混合螺旋自旋相(二級(jí)磁相)的貢獻(xiàn),其大小由自旋磁化矢量的螺旋程度決定；(2)低于50K時(shí),系統(tǒng)的第一磁相變導(dǎo)致了反磁熱效應(yīng),這主要由磁場(chǎng)誘導(dǎo)螺旋自旋結(jié)構(gòu)產(chǎn)生了更大的自旋失措引起。 此外,本論文的附錄部分介紹了利用分子束外延法生長(zhǎng)SrTiO3同質(zhì)外延薄膜的相關(guān)工作。(1)摸索了SrTiO3襯底預(yù)處理方法,通過腐蝕與退火相結(jié)合的方法,得到了完美的Ti02截止面,在此基礎(chǔ)上制備了SrTiO3同質(zhì)外延薄膜；(2)通過在不同氧分壓下的退火工藝,實(shí)現(xiàn)對(duì)SrTiO3薄膜中氧空位濃度的控制,并研究了氧空位對(duì)其光學(xué)和磁學(xué)性質(zhì)的影響,發(fā)現(xiàn)氧空位對(duì)紅/藍(lán)光輻射和磁性都有較大的貢獻(xiàn)。
[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 .

【學(xué)位授予單位】：華東師范大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2014
【分類號(hào)】：TM277

【參考文獻(xiàn)】

相關(guān)期刊論文 前1條

1 王恩哥;薄膜生長(zhǎng)中的表面動(dòng)力學(xué)(Ⅰ)[J];物理學(xué)進(jìn)展;2003年01期

，

本文編號(hào)：1696976