Synthesis, Analysis and Applications of Cuprate/Manganite Heterostructures

本論文展示以傳統固態反應法及脈衝雷射沈積系統成長高溫超導體及龐磁阻錳氧化合物之異質結構，並研究其超導-鐵磁之間的鄰近效應、磁釘扎效應對臨界電流的影響，及電阻受電場而改變的效應，並利用其特性來製作記憶體元件。 首先，我們討論在釹鈣錳氧化物/釔鋇銅氧化物的雙層膜結構裡不同電流對於鄰近效應的影響，我們的試驗數據顯示超導轉變溫度的抑制比率在釹鈣錳氧化物/釔鋇銅氧化物雙層膜裡比在單層釔鋇銅氧化物薄膜所受到的效應大，當輸入的電流大於一毫安培時，超導轉變溫度降低的速率相對大了十倍以上。此效應歸因於在超導體裡的庫柏對被自旋極化的準粒子所破壞，亦為超導-鐵磁彼此之間磁性交互作用的結果。 接著，我們討論在釹鈣錳氧化物/釔鋇銅氧化物雙層膜結構裡的磁釘扎作用。在大於三特斯拉的高磁場下，相對於單層釔鋇銅氧化物薄膜其低於十毫安培以下的臨界電流，雙層膜結構的臨界電流依然可以保持在一百毫安培左右，這可歸因於具有不同磁疇的鐵磁層對超導體中的漩渦有釘扎的效應，使其不易移動而致超導臨界電流可以提升。 最後，在電極、釹鈣錳氧化物及釔鋇銅氧化物的三層膜結構裡，我們討論其在室溫下的電滯曲線及不同金屬電極對其電阻變化率的影響，其中在銅電極的三層膜結構裡，其所具有的電場改變導致的電阻變化率大約在二十五百分比左右，但對於銀電極的三層膜結構在五伏特的電場下，其改變的比率卻可以提升到十倍以上的的效率，這可證明在不同的電極結構裡，其所具有的電阻改變機制是不同的。In this dissertation, we fabricate the devices incorporating cuprate/manganite heterostuctres and study their proximity effect, magnetic pinning effect and the behavior of electric-pulse-induced-resistance (EPIR) change. The process of deposition of such oxide ferromagnet/superconductor (FM/SC) multilayers using the pulsed laser deposition (PLD) technique is also demonstrated. First of all, the current dependent proximity effects of Nd0.7Ca0.3MnO3/YBa2Cu3O7 (NCMO/YBCO) bilayers are investigated. Our experimental data show that the suppression rate of superconducting transition temperature (Tc) under the applied current (> 1 mA) is enhanced by one order of magnitude in NCMO/YBCO compared with that in pure YBCO. The enhanced Tc-suppresion is attributed to pair-breaking via the interactions with the spin-polarized quasiparticles and the magnetic exchange in association with a current-induced melting of charge-order state in NCMO. Secondly, the magnetic pinning effect in NCMO/YBCO bilayers is investigated. It is found that by increasing the magnetic field from 0 to 5 Tesla, the critical current Ic of pure YBCO film at 50 K is suppressed by three orders of magnitude. On the other hand, in the NCMO/YBCO bilayer Ic is less sensitive to the field and maintains at the level of 100 mA at high field. This result indicates that the magnetic pining effect of NCMO is much more efficient than the conventional routes, which may be related to the fine domain structure of NCMO film. Lastly, the electrical hysteresis in current-voltage characteristics of metal/NCMO/YBCO heterostructure, and the effect of metal/insulator interface on the resistance switching behavior are investigated at room temperature. When applying pulsed voltage of ±3 V, the Cu/NCMO/YBCO heterostructure shows resistance switching from a high- to low-resistance state with an EPIR (= (HR-LR)/LR) ratio of 25 %. On the other hand, a huge ratio as large as 1350 % is observed with ±5 V for Ag/NCMO/YBCO. Correlating the switching behavior with their current-voltage characteristics, our experimental results provide clear evidences that different metal/NCMO interfaces provide different mechanisms for the resistance switching.Acknowledgement ii Abstract (in Chinese) iv Abstract (in English) v List of publications vi Contents viii List of figures xi List of tables xxi Ch1 Introduction 1 1.1 History ……………………………………………………………………….1 1.2 Materials ……………………………………………………………………..10 1.2.1 YBa2Cu3O7 …………………………………………………………...10 1.2.2 Re0.7Ca0.3MnO3 Re=Nd, La, Pr ………………………………………15 1.3 Motivation and layout ………………………………………………………19 Ch2 Theoretical background 22 2.1 Superconductivity in cuprates ….………………………………………….22 2.1.1 Superconducting state ………………………………………………..22 2.1.2 The Meissner effect ……………….…………………………………..24 2.1.3 The Ginzburg-Landau theory ………………………………………...25 2.1.4 Bardeen-Cooper-Schrieffer theory …………………………………..29 2.1.5 Vortex state and pinning effect ………………………………………31 2.1.6 The pseudogap ………………………………………………………..33 2.2 Colossal magnetoresistance in manganites ……………………………..34 2.2.1 Magnetic order and ferromagnetism ………………………………...35 2.2.2 Colossal magnetoresistance ……………….………………………...38 2.2.3 The theoretical models ………………………………………………..41 2.2.3.1 Double-exchange (DE) ………………………………………41 2.2.3.2 Jahn-Teller effect (JT) ………………………………………….43 2.2.4 Phase separation and charge ordering ……………………………..45 2.3 Interaction of ferromagnetism and superconductivity …….…………47 2.3.1 Spin-injection into HTSC …………………………………………......47 2.3.2 Proximity effect ………………………….…………….………………49 2.3.3 Magnetic pinning effect ……………………………….………………52 2.4 Colossal electroresistance and RRAM …………………….……………..53 2.4.1 Electric-pulse-induced-resistance (EPIR) change effect ….….…..54 2.4.2 Resistive random access memory (RRAM) …..…………….………56 2.4.3 The theoretical models ………………………………………………60 2.4.3.1 Filamentary path …..……………………………………………60 2.4.3.2 Crystalline defect ……………………………………………….62 2.4.3.3 Schottky barrier …………………………………………………66 2.4.3.4 Mott transition ……..……………………………………………71 2.4.3.5 Charge transfer …………………………………………………74 2.4.3.6 Oxygen diffusion ………………………………………………..77 Ch3 Experimental and Analysis techniques 81 3.1 Pulsed laser deposition (PLD) …………………………………………….81 3.1.1 Historical development of PLD ……………………………………….81 3.1.2 The advantages of PLD ………………………………………………83 3.1.3 Mechanisms of PLD …………………..………………………………84 3.1.4 Key parameters of PLD ……………………………………………….87 3.2 Analysis techniques ………………………………………………………93 3.2.1 X-ray diffraction (XRD) system ………………………………………93 3.2.2 Atomic force microscope (AFM) ….…………………………………94 3.2.3 Scanning electron microscope (SEM) ………………………………95 3.2.4 Transmission electron microscope (TEM) …………………………99 3.2.5 Superconducting quantum interference device (SQUID) system 100 3.2.6 Physical property measurement system (PPMS) ………………101 3.2.7 Closed cycle refrigerator system …...........…….…………………101 3.3 Sample preparation………………………….…………………………….104 3.3.1 YBa2Cu3O7 and Re0.7Ca0.3MnO3 bulk ……………………………..104 3.3.2 YBCO and ReCMO thin film ………………………………………..104 3.3.3 ReCMO/YBCO bilayer ………………………………………………107 3.3.4 ME/ReCMO/YBCO trilayer …………………….……………………108 Ch4 Results and discussion 109 4.1 Basic characterization of Re0.7Ca0.3MnO3 with Re = Nd, La, Pr ……109 4.1.1 Bulk samples ……………………………………………….………...109 4.1.2 Thin films ……………………………………………………………...116 4.2 Bilayer systems of ReCMO/YBCO with Re = Nd, La, Pr ……………121 4.2.1 Current enhanced proximity effects ………………………………..126 4.2.2 Vortex pinning by magnetic domain ………………………………..140 4.3 Resistance switching behavior of ReCMO ……………………………..144 4.3.1 Effects of electrode on the electrical hysteresis …………………..151 4.3.2 Resistance switching effects in trilayers …………………………..154 Ch5 Conclusions 159 Bibliography 16