【摘要】：近些年來,以鎳酸鹽為代表的鈣鈦礦關(guān)聯(lián)氧化物材料一直是凝聚態(tài)物理和材料科學(xué)研究的熱點(diǎn)。在鈣鈦礦鎳酸鹽異質(zhì)結(jié)構(gòu)中,晶格、電荷、自旋和軌道自由度之間強(qiáng)烈的相互作用使得該類材料有著復(fù)雜的物理性質(zhì)。通過應(yīng)變、尺寸、超晶格結(jié)構(gòu)設(shè)計(jì)等手段調(diào)控這些自由度,可以實(shí)現(xiàn)對(duì)材料的電、磁特性的調(diào)控。本論文以鎳酸鹽異質(zhì)結(jié)構(gòu)為研究對(duì)象,制備了 LaNiO_3(LNO)超薄膜和LaNiO_3/LaMnO_3(LNO/LMO)超晶格結(jié)構(gòu),研究了其電、磁特性,并通過應(yīng)變、尺寸效應(yīng)、緩沖層的插入以及超晶格結(jié)構(gòu)的設(shè)計(jì)等手段對(duì)其輸運(yùn)特性、磁性進(jìn)行了調(diào)控,具體工作如下:(1)利用脈沖激光沉積技術(shù)(PLD)和反射高能電子衍射儀(RHEED)在(001)方向的SrTiO_3(STO)和LaAlO_3(LAO)單晶襯底上分別外延生長了一系列不同厚度的LNO超薄膜。X射線衍射(XRD)表征表明STO襯底上的LNO薄膜受拉應(yīng)力作用。原子力顯微鏡(AFM)表征表明超薄膜具有原子級(jí)別平整的表面。輸運(yùn)特性測(cè)試表明,隨著薄膜厚度的減小,薄膜電阻逐漸增大。當(dāng)厚度小于某一臨界值時(shí),LNO由金屬態(tài)變?yōu)榻^緣態(tài)。在STO和LAO襯底上這一臨界厚度分別為5和4個(gè)晶胞(u.c.)。對(duì)面電阻-溫度曲線的擬合結(jié)果表明該轉(zhuǎn)變的起因是LNO薄膜中載流子的局域化效應(yīng)。(2)在5u.c.厚的LNO薄膜與STO襯底間插2u.c.厚的STO薄膜作為緩沖層后,LNO薄膜的輸運(yùn)特性發(fā)生了明顯的變化,溫度高于100 K時(shí)LNO由絕緣態(tài)變?yōu)榻饘賾B(tài)。X射線光電子能譜儀(XPS)分析表明低氧壓下(10-3mbar)生長的STO薄膜中存在氧空位。LNO薄膜輸運(yùn)特性的變化可歸因于STO薄膜中的氧空位。氧空位提供了大量電子,電子從STO層轉(zhuǎn)移到LNO層,抑制了LNO層中載流子的局域化。通過對(duì)STO薄膜進(jìn)行退火處理以及提高生長氧壓,STO薄膜中的氧空位大大減少,5u.c.厚的LNO薄膜又恢復(fù)了絕緣態(tài)。(3)利用PLD在(111)方向的STO單晶襯底上生長了[(LNO)m/(LMO)n]x超晶格薄膜。磁性測(cè)試結(jié)果表明在低溫下這一包含順磁鐵磁界面的體系中存在交換偏置現(xiàn)象,其原因可能是界面處電荷轉(zhuǎn)移導(dǎo)致的鐵磁相互作用。改變超晶格周期數(shù)x和LNO組元的層數(shù)m后,超晶格的交換偏置場(chǎng)HE和矯頑場(chǎng)HC的大小均發(fā)生了變化,具體表現(xiàn)為:隨著周期數(shù)x和LNO組元的層數(shù)m的減小,交換偏置場(chǎng)HE和矯頑場(chǎng)HC均變小。
[Abstract]:In recent years, perovskite-related oxide materials, represented by nickel, have been a hot topic in condensed matter physics and material science. In perovskite nickel heterostructure, the strong interaction between lattice, charge, spin and orbital degree of freedom makes the material have complex physical properties. These degrees of freedom can be regulated by means of strain, size and superlattice structure design, which can control the electrical and magnetic properties of materials. In this thesis, LaNiO_3 (LNO) ultrathin films and LaNiO_3/LaMnO_3 (LNO/LMO) superlattices have been prepared, and their electrical and magnetic properties have been studied by means of strain and size effects. The transport characteristics and magnetic properties of the buffer layer are regulated by the insertion of the buffer layer and the design of the superlattice structure. The main works are as follows: (1) A series of LNO ultrathin films with different thickness have been epitaxially grown on (001) direction SrTiO_3 (STO) and LaAlO_3 (LAO) single crystal substrates by pulsed laser deposition (PLD) and reflection high energy electron diffractometer (RHEED). The LNO film on STO substrate is subjected to tensile stress. Atomic force microscopy (AFM) (AFM) characterization shows that the ultrathin film has a flat surface with atomic level. The transport characteristic test shows that the film resistance increases with the decrease of film thickness. When the thickness is less than a certain critical value, LNO changes from metal state to insulating state. The critical thickness on STO and LAO substrates is 5 and 4 (u. C.), respectively. The fitting results of the surface resistance-temperature curve indicate that the reason of the transition is the localization effect of carriers in the LNO film. (2) at 5u. C. The thick LNO film is intercalated with the STO substrate by 2u. C. The transport characteristics of thick STO films as buffer layer have changed obviously. When the temperature is above 100K, the (XPS) analysis of LNO changes from insulating state to metal state. (XPS) analysis shows that the oxygen vacancy in STO films grown under low oxygen pressure (10-3mbar) can be attributed to the oxygen vacancies in STO films. Oxygen vacancies provide a large number of electrons, which transfer from the STO layer to the LNO layer, which inhibits the localization of carriers in the LNO layer. The oxygen vacancies in STO thin films were greatly reduced by annealing and increasing oxygen pressure. (3) [(LNO) m / (LMO) n] x superlattice films were grown on (111) direction STO single crystal substrates by PLD. The magnetic measurement results show that the exchange bias exists in the system containing paramagnetic interface at low temperature, which may be due to the ferromagnetic interaction caused by charge transfer at the interface. After changing the number of layers of x and LNO components, the exchange bias field HE and coercive field HC of superlattice change, which is shown as follows: with the decrease of the number of layers of x and LNO component m, the exchange bias field of superlattice changes. The exchange bias field HE and coercive field HC both become smaller.
【學(xué)位授予單位】：南京大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TB383.2
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本文編號(hào)：2146441