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姓名 沈宗緯(Tsung-Wei Shen) 查詢紙本館藏 畢業系所 物理學系 論文名稱
(First-principles study in structural and elec-tronic properties of FeBaTiO3Fe multiferroic tunneling junction)相關論文 檔案 [Endnote RIS 格式]
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摘要(中) 多鐵異質接合的結構是由鐵磁與鐵電材料相接而成之系統，為了使傳輸記憶體之耗能降低與增加邏輯電路元件的可調變量，在原有的鐵磁傳輸接合的基礎上加了能利用外加電場翻轉電偶極的鐵電材料，使其成為能夠由外加磁場與電場雙重調控的裝置。在我們的理論模擬中，利用第一原理計算鐵/鈦酸鋇的超晶格結構，並計算中間層層數在四、六、八層鈦酸鋇的超晶格結構，並在接合結構中定義了一個鈦氧平面位移參數，藉此了解電偶極在層數不同中的變化，同時也計算了投影態密度，藉此深入了解鐵磁與鐵電材料在界面處所發生的鐵磁鐵電的交互作用，其重點在該界面上的鐵與鈦酸鋇產生之軌道耦合主要貢獻在費米能量附近的旋轉偏極態密度以及穿隧波譜，兩者皆是影響自旋傳輸的主要原因。在最後一部分的討論中，我們推論了在外加電場下自旋向下態密度的變化以及兩邊電極透過外加磁場翻轉磁矩後對於費米能量附近態密度的變化，透過兩種外加場的影響，鐵/鈦酸鋇接合有機會成為一個擁有四電阻態的裝置。 摘要(英) Multiferroic tunnel junction (MFTJs) is the combination of ferromag-netic (FM) and ferroelectric (FE) material system. To push for low energy consumption memory and logical spintronic devices, a reversal of magnet-ization requiring only the application of an electric field can be achieved via a ferroelectric (FE) barrier as an active role in a magnetic tunnelling junction. In this study, the first-principle calculation is employed to inves-tigate the atomic structure and electronic properties of (Fe2)7-(BaTiO3)m-(Fe2)6 superlattice with m=4, 6, 8. To better understand the magnetoelectric coupling between the magnetic moment of Fe electrode and the electric dipole moment of central ferroelectric BaTiO3 barrier, which is decided the direction of electric dipole (Δz???), we calculate the spin-polarized density of states. The hybridization between Fe and Ti ions at Fe/TiO2 interface not only contribute to the spin-polarized density of states and transmission spectrum near the Fermi energy but also may dominate the spin transport properties. In the end, we infer the change of spin down interfacial state under the external electric field and the anti-parallel case which is flipped magnetic moment of electrode with external magnetic field. With two ways to affect the resistance, Fe/BaTiO3 junction may be a four-resistance states device. 關鍵字(中) ★ 多鐵異質接合
★ 鐵電材料
★ 第一原理
★ 鐵/鈦酸鋇
★ 鐵磁材料關鍵字(英) ★ Multiferroic tunnel junction
★ first-principle calculation
★ Fe/BaTiO3
★ ferroelectric
★ ferromagnetic論文目次 Chapter 1.Introduction ................................................................................ 1
Chapter 2. Theory ........................................................................................ 4
2.1 Density Function Theory ................................................................... 4
2.1.1 Born-Oppenheimer Approximation ............................................ 4
2.1.2 Hartree-Fock Approximation ...................................................... 5
2.1.3 The Hohenberg-Kohn Theorem .................................................. 7
2.1.4 The Kohn-Sham Equation ........................................................... 8
2.1.5 Exchange-Correlation Energy Function ...................................... 9
Local Density Approximation (LDA) ................................................ 10
Exchange Generalized Gradient Approximation (GGA) ................... 11
2.2 Pseudopotential Method for DFT calculation ................................. 12
2.2.1 Bloch Theorem and Cutoff Energy ........................................... 12
2.2.2 Projector Augmented-Wave Method......................................... 13
2.3 Crystal Field Model ......................................................................... 15
2.3.1 Transition Metal ........................................................................ 15
2.3.2 Octahedral Complexes............................................................... 15
Chapter 3. Computational details .............................................................. 17
3.1 Structural Geometry ........................................................................ 17
3.2 Parameter for Structure Relaxation ................................................. 19
Chapter 4. Discussions .............................................................................. 22
4.1 Structures of Junction for m=4 to 8 in Cubic and Tetragonal Phases ............................................................................................................... 22
4.2 Projected Density of States .............................................................. 26
4.2.1 PDOS for m=4 ........................................................................... 26
4.2.2 Eg and T2g Band Density of States and Local Density of States (LDOS) ............................................................................................... 29
4.2.3 Charge Transfer in the Interface ................................................ 32
4.2.4 Layer Dependent PDOS ............................................................ 35
4.3 Multiferroic Effect ........................................................................... 38
4.3.1 Electrostatic Potential Energy ................................................... 38
4.3.2 Simple Model for FTJ ............................................................... 42
4.3.3 Tunneling Electroresistance (TER) Effect in MFTJ ................. 44
4.3.4 Tunneling Magnetoresistance (TMR) Effect in MFTJ ............. 49
Chapter 5. Summary .................................................................................. 52
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