Tunnel magnetoimpedance effects and Kerr-microscopic domain imaging

「鐵磁/絕緣/鐵磁」層狀結構組成了磁性穿隧夾層的最基本元素，而其中所具有的穿隧磁電阻效應則強烈受到「鐵磁/絕緣」介面上的磁性及電子組態所影響。藉由特定的結構安排，鐵磁層的本性電子自旋極化率將可獲得調制。以雙重位壘為例，我們嘗試採用係數轉移矩陣方法來處理其自旋相關傳輸問題，並可推得隨中間層厚度而有振盪特徵的穿隧磁電阻效應。 在本篇論文中，準自旋閥及自旋閥架構之磁性穿隧夾層成它a藉由一特別設計建造的超高真空濺鍍系統所製備，其個別的穿隧磁電阻率可達到33%及42%。其特徵性的自旋相關傳輸可由磁光柯爾效應及四線磁電阻量測所檢證。此外結構分析及成分元素的擴散性質則各藉由穿透式電子顯微鏡及能量逸散X光譜儀鑑定。除了直流探測電源的量測之外，穿隧磁阻抗效應的研究也在一系列的磁性穿隧夾層中進行，其頻率範圍上達一百萬赫茲。觀察所得的實部阻抗被發現到可隨著探測頻率的增加而變號，並隨磁場掃描可得到巨大的比率變化。靠著一項與頻率相關的加權因子引入，此類的異常現象可由一頻率持助性的不均勻電流分佈效應所解釋。 考慮到交換耦合偏移作用及人工鐵磁耦合在自旋閥及人工自旋閥磁性穿隧夾層的關鍵性影響，氧化鎳/銅/鎳鐵三層膜中的層間磁性耦合及介面磁性結構的關聯性吸引了我們的研究。藉由磁光柯爾顯微鏡對NiFe層磁域演化的監測，我們發現了磁性耦合現象的穩定性與所施加磁場的方向及溫度有所關聯。旋轉場實驗的設計與進行使得此現象具體關聯到一臨界角度的測量，此一臨界角度對應的是鎳鐵層中不可逆磁域翻轉的發生。層間交換耦合作用產生的等效力矩對氧化鎳磁矩的磁化作用則被認為是此一效應的根源。Layered structure ferromagnet/insulator/ferromagnet (FM/I/FM) constitutes the very basic element of a magnetic tunnel junctions (MTJ) junction, and the tunnel magnetoresistance (TMR) effect in it is strongly influenced by the electronic and magnetic properties at the FM/I interface. With the help of specific structure arrangement, however, the intrinsic spin polarization of FM layers may be modulated. Taking double barrier tunnel junction for example, the spin dependent transport were studied here by means of transfer matrix calculation, and the oscillatory TMR effect was verified with ferromagnet of varying thickness inserted between two tunnel barriers. In this work, pseudo spin-valve (PSV) and spin-valve (SV) magnetic tunnel junctions were fabricated by an particularly designed ultra-high-vacuum (UHV) sputtering system. The TMR values of fabricated PSV and SV MTJs may reach 33% and 42%, respectively. The characteristic spin dependent transport was examined by invoking magneto-optical Kerr effect (MOKE) and 4-terminal magnetoresistance measurements. Moreover, the structural characterization and the monitoring of elements diffusion were accomplished by transmission electron microscopy (TEM) and energy dispersive X-ray spectrometer (EDS), respectively. In addition to the electric response under direct current (DC) source, the tunnel magnetoimpedance (TMI) effects were studied in a series of MTJs with sensing frequency up to 1 MHz as well. The observed real part was found to change sign as the sensing frequency increases and led to dramatic fractional change under sweeping magnetic field. By introducing a frequency dependent weighting factor to the lead resistance, such kind of anomalous behavior is explained in the picture of frequency assisted inhomogeneous current distribution effect. Because of the crucial influence of exchange bias coupling and the artificial ferromagnetic coupling (AFC) in SV and AFC-SV MTJs, the correlation between interfacial magnetic micro-structures and the interlayer magnetic coupling were studied in NiO/Cu/NiFe trilayers. By monitoring the evolution of magnetic domains in the NiFe layer with Kerr microscopy, the stability of the coupling was found to depend both on temperature and the direction of external magnetic field. By taking advantage of the rotational field experiments, the temperature dependent critical angle that defines the irreversible switching of magnetic domains may be obtained. It is believed that not the domain wall itself but the torque exerted by in-plane ferromagnetic moments re-magnetizes the antiferromagnetic NiO layer.Abstract iii 1 Introduction 1 2 General knowledge 3 2.1 Indirect exchange coupling via itinerant electrons 3 2.2 Exchange bias coupling . . . . . . . . . . . . . . . 6 2.3 Magneto-optical effect . . . . . . . . . . . . . . . 10 2.4 Tunnel magnetoresistance effect .. . . . . . . . . . 11 2.4.1 Julli`ere’s model and spin dependent tunneling . 11 2.4.2 Slonczewski’s model and effective spin polarization . . . . . . . . . . . . . . . .. . . . . 13 2.4.3 Transfer matrix formalism . . . . . . . . . . . . 16 2.4.4 Magnetic tunnel junctions . . . . . . . . . . . . 21 3 Experimental 23 3.1 UHV sputtering system . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.1.1 Multi-functional chamber . . . . . . . . . . . . . 23 3.1.2 Contact masks . . . . . . . . . . . . . . .. . . . 24 3.1.3 Sample plate and the cartridge . . . . . . . . . . 24 3.1.4 Heater and LN Dewar . . . . . . . . . . . . . . . 24 3.1.5 Vacuum and processing gas flow management . . . . 26 3.1.6 Interlock for pumping system . . . . . . . . . . . 26 3.2 Magneto-optical Kerr effect. . . . . . . . . . . . ..29 3.3 Kerr microscope . . . . . . . . . . . . . . . . . . 30 3.4 4-terminal resistance . . . . . . . . . . . . . . . 32 3.5 Auto balancing bridge method . . . . . . . . . . . . 33 4 Tunnel magnetoresistance effects in magnetic tunnel junctions . . . . . . . . . . . . . . . . . . . . . . .35 4.1 Pseudo spin valve MTJ . . . . . . . . . . . . . . . 36 4.1.1 Sample preparation . . . . . . . . . . . . . . . . 36 4.1.2 Results . . . . . . . . . . . . . . . . . . . . . 36 4.2 MTJ with ferromagnet-superconductor composite electrode . . . . . . . . . . . . . . . . . . . . . . .47 4.2.1 Sample preparation . . . . . . . . . . . . . . . . 47 4.2.2 Results . . . . . . . . . . . . . . . . .. . . . . . . . 47 4.3 Spin valve MTJ . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.3.1 Sample preparation . . . . . . . . . . . . . . . . 55 4.3.2 Top spin valve . . . . . . . . . . . . . . . . . . 58 5 Tunnel magnetoimpedance effects in magnetic tunnel junctions . . . . . . . . . . . . . . . . . . . . . . .68 5.1 Sample preparation . . . . . . . . . . . . . . . . 68 5.2 Complex impedance based on the RC-in-parallel model .70 5.3 TMI effects on the magnitude and the imaginary par . 75 5.4 Tunnel magnetocapacitance effec . . . . . . . . . . 80 5.5 Abnormal tunnel magnetoimpedance effect on the real part . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.6 Nano-second accessing in MTJ based memory . . . . . .89 6 Evolution of magnetic domains in NiO/Cu/NiFe trilayer system . . . . . . . . . . . . . . . . . . . . . . . . 95 6.1 Sample preparation . . . . . . . . . . . . . . . . . 98 6.2 Domain imaging by Kerr microscopy . . . . . . . . . 99 6.3 Rotating field experiments . . . . . . . . . . . . 106 7 Conclusion . . . . . . . . . . . . . . . . . . . . .108 Bibliography . . . . . . . . . . . . . . . . . . . . .11