臺灣博碩士論文加值系統

近來來，磁性隨存記憶體已獲得極大的關注。相對於自旋轉換力矩的磁性隨存記憶體而言，自旋軌道力矩的磁性隨存記憶體擁有較低的寫入電流，也因此在邏輯電路中進一步的改善了能量效率和可縮放性。自旋軌道力矩的產生主要有兩種來源，其一是源自於自旋霍爾效應產生的自旋電流，另一個則是異質結構中的介面產生的有效電場所導致。在一由鐵磁層和一鄰近重金屬層所組層的基本結構中，當一水平方向的電流流入時，會產生一垂直於膜面方向的自旋電流作用於鐵磁層的磁矩。一般來說，在一具有垂直異向性的鐵磁材料裡，我們需要一個額外的水平磁場來達到決定性的翻轉行為。即便這一水平外加場可能可以透過後端製程如封裝，來整合作用於元件中。但此種方式對於實際產品來說，會提高製程的複雜度且增加了額外的步驟。除此之外，在大部分具有垂直異向性的材料系統中，當我們想要縮小磁性元件的體積時，熱穩定的下降是一個很大的問題。

我們已經瞭解到，對於自旋軌道力矩的磁性隨存記憶體而言，在工程上於元件結構裡提供一內建場或其他作用力可能是解決外加場問題的方式。因此在本文中，我們將利用一反鐵磁材料鄰近於鐵磁層旁，透過彼此的交互作用力來提供一水平方向的偏場解決外加場的問題。透過在水平方面的後端場退火，我們成功地達成了無外加場的二元翻轉。且令人驚訝的是，我們發現位於鐵磁層及反鐵磁層介面中的自旋電子也隨著鐵磁層的磁矩透過自旋軌道力矩產生了翻轉行為。經由電流脈衝過後，這磁滯曲線上產生的偏移於無零場中展示了單一方向的異向性。此種新穎的用來翻轉交換偏壓場的方式大大地增加了應用產品中磁化態的熱穩定性。就結果而論，我們的研究對於自旋軌道力矩的磁性隨存記憶體和其他自旋電子元件而言，不但提供了很好的改進方法，也有驚人的突破。

Magnetic random access memory (MRAM) has got intensive attention during these years. For spin-orbit-torque (SOT) MRAM, it possesses lower write current compared to spin-transfer-torque (STT) MRAM, which improves the energy efficiency and the scalability in logic circuit. The spin-orbit-torque arises mainly from two difference sources, one is the spin current generated by bulk spin Hall effect (SHE), the other one is the effective electrical field induced by the heterostructure interfaces. With the basic structure composed of a ferromagnetic (FM) layer and an adjacent heavy mental layer, when the in-plane current flowing into the structure will generate the pure spin current perpendicular to the film acting on the magnetization of FM. In the FM materials performing the perpendicular magnetic anisotropy (PMA), generally, we need an additional in-plane magnetic field to allow the deterministic switching. Despite this in-plane field may can be integrated into the devices by back-end process, like package. It still rises the complexity and provides additional steps for practical application. Besides, in most material system with PMA, the degradation of thermal stability is a huge problem when we want to scale down the magnetic devices.

We have known that acquiring a build-in field or other forces in the structure may be a way to solve the issue of external field for engineering in SOT-MRAM. So in this work, we used an antiferromagnetic (AFM) materials adjacent the FM layer to provide an in-plane bias field by interlayer coupling. The field-free binary switching was accomplished successfully after the post field-annealing along the in-plane direction. Surprisingly, we found that the interfacial spins at FM/AFM interface were also reversed by the SOT with the magnetization of FM layer. This shift in perpendicular hysteresis loop displayed the unidirectional anisotropy at zero external field after the electric pulses. This innovative method to switch the direction of exchange bias tremendously increased the thermal stability for magnetic state in practical applications. As a result, our researches offer a great improvement and a significant breakthrough for SOT-MRAM, as well as other spintronic devices.

Table of Contents
Abstract II
摘要 IV
致謝 V
Table of Contents VI
Table of Figures VIII
Chapter 1. Introduction 1
1.1 Introduction 1
1.2 Motivation 1
Chapter 2. Background 3
2.1 Cobalt with perpendicular magnetic anisotropy 3
2.1.1 Origin of the perpendicular magnetic anisotropy at the interface 3
2.1.2 Magnetic and structure property of Co/Pt multilayers 5
2.1.3 Interface structure and perpendicular anisotropy in Co/Pt multilayers 7
2.2 The exchange bias of antiferromagnet/ferromagnet bilayer system 10
2.2.1 Antiferromagnetism 10
2.2.2 The interaction between AFM/FM layer 11
2.2.3 Perpendicular exchange bias 14
2.3 Spin-orbit-torque 18
2.3.1 The origin of spin-orbit-torque 18
2.3.2 The experiment method to distinguish field-like and damping-like torque 25
2.3.3 Factors that affect the strength of the spin orbit torque 27
Chapter 3. Experiment techniques 31
3.1 Sample preparation 31
3.1.1 Magnetron sputtering 31
3.1.2 Post field-annealing system 32
3.2 Micro-fabrication 32
3.2.1 Photolithography 32
3.2.2 lift-off process 33
3.2.3 Inductively coupled plasma enhanced ion etching (ICP-RIE) 34
3.3 Analysis Techniques 34
3.3.1 Atomic force microscopy (AFM) 34
3.3.2 Vibration sample magnetometer (VSM) 35
3.3.3 Focused polar magneto-optical Kerr effect 36
3.3.4 Anomalous Hall effect (AHE) 38
Chapter 4. Results and Discussions 39
4.1 Field-free magnetization switching by spin orbit torque with inserting AFM layer 39
4.1.1 As-deposited magnetic property 39
4.1.2 Post field-annealing to set in-plane exchange-bias 40
4.1.3 Switching phase diagram 42
4.1.4 Summary 46
4.2 Innovative method to reverse the direction of exchange bias 46
4.2.1 Exchange bias switching by spin orbit torque 46
4.2.2 Joul heating effect and temperature test 48
4.2.3 Other test and verification 51
4.3 The characteristic of spin current in Pt/Co/IrMn system 53
4.3.1 Observation of Spin orbit torque efficiency 53
4.3.2 The enhanced effective field with increasing bottom Pt layer 55
4.3.3 The different trend with increasing tIrMn for the samples without top Pt layer 57
4.3.4 Summary 58
Chapter 5. Conclusions 59
References: 60

1. Parkin, S. S. GIANT MAGNETORESISTACE IN MAGNETIC NANOSTRUCTURES. Annu Rev Mater Sci 25, 357-388 (1995).
2. Yuasa, S. & Djayaprawira, D. D. Giant tunnel magnetoresistance in magnetic tunnel junctions with a crystalline MgO(0 0 1) barrier. Journal of Physics D: Applied Physics 40, R337-R354 (2007).
3. Hoffmann, A. Spin Hall Effects in Metals. IEEE Transactions on Magnetics 49, 5172-5193 (2013).
4. Chappert, C., Fert, A. & Van Dau, F. N. The emergence of spin electronics in data storage. Nat Mater 6, 813-823 (2007).
5. Lee, K.-S., Lee, S.-W., Min, B.-C. & Lee, K.-J. Threshold current for switching of a perpendicular magnetic layer induced by spin Hall effect. Applied Physics Letters 102, 112410 (2013).
6. Kim, J. et al. Layer thickness dependence of the current-induced effective field vector in Ta|CoFeB|MgO. Nat Mater 12, 240-245 (2013).
7. Liu, L., Lee, O. J., Gudmundsen, T. J., Ralph, D. C. & Buhrman, R. A. Current-induced switching of perpendicularly magnetized magnetic layers using spin torque from the spin Hall effect. Phys Rev Lett 109, 096602 (2012).
8. Jamali, M. et al. Spin-orbit torques in Co/Pd multilayer nanowires. Phys Rev Lett 111, 246602 (2013).
9. Nakayama, H. et al. Spin Hall magnetoresistance induced by a nonequilibrium proximity effect. Phys Rev Lett 110, 206601 (2013).
10. Fujiwara, K. et al. 5d iridium oxide as a material for spin-current detection. Nat Commun 4, 2893 (2013).
11. Kimura, T., Otani, Y., Sato, T., Takahashi, S. & Maekawa, S. Room-temperature reversible spin Hall effect. Phys Rev Lett 98, 156601 (2007).
12. Yu, G. et al. Switching of perpendicular magnetization by spin-orbit torques in the absence of external magnetic fields. Nat Nanotechnol 9, 548-554 (2014).
13. Huang, K.-F., Wang, D.-S., Lin, H.-H. & Lai, C.-H. Engineering spin-orbit torque in Co/Pt multilayers with perpendicular magnetic anisotropy. Applied Physics Letters 107, 232407 (2015).
14. Carcia, P. F. PERPENDICULAR MAGNETIC-ANISOTROPY IN PD/CO AND PT/CO THIN-FILM LAYERED STRUCTURES. Journal of Applied Physics 63, 5066-5073 (1988).
15. Hashimoto, S., Ochiai, Y. & Aso, K. PERPENDICULAR MAGNETIC-ANISOTROPY AND MAGNETOSTRICTION OF SPUTTERED CO/PD AND CO/PT MULTILAYERED FILMS. Journal of Applied Physics 66, 4909-4916 (1989).
16. Johnson, M. T., Bloemen, P. J. H., denBroeder, F. J. A. & deVries, J. J. Magnetic anisotropy in metallic multilayers. Rep. Prog. Phys. 59, 1409-1458 (1996).
17. Weller, D. et al. ORBITAL MAGNETIC-MOMENTS OF CO IN MULTILAYERS WITH PERPENDICULAR MAGNETIC-ANISOTROPY. Physical Review B 49, 12888-12896 (1994).
18. Nakajima, N. et al. Perpendicular magnetic anisotropy caused by interfacial hybridization via enhanced orbital moment in Co/Pt multilayers: Magnetic circular x-ray dichroism study. Phys. Rev. Lett. 81, 5229-5232 (1998).
19. Lin, C. J. et al. MAGNETIC AND STRUCTURAL-PROPERTIES OF CO/PT MULTILAYERS. Journal of Magnetism and Magnetic Materials 93, 194-206 (1991).
20. Weller, D. et al. in Magnetic Ultrathin Films: Multilayers and Surfaces, Interfaces and Characterization Vol. 313 Materials Research Society Symposium Proceedings (eds B. T. Jonker et al.) 791-797 (Materials Research Soc, 1993).
21. Oh, H. S. & Joo, S. K. Enhancement of coercivity by underlayer control in Co/Pd and Co/Pt multilayers. Ieee Transactions on Magnetics 32, 4061-4063 (1996).
22. Bertero, G. A. & Sinclair, R. STRUCTURE-PROPERTY CORRELATIONS IN PT/CO MULTILAYERS FOR MAGNETOOPTIC RECORDING. Journal of Magnetism and Magnetic Materials 134, 173-184 (1994).
23. Li, Z. G. & Carcia, P. F. MICROSTRUCTURAL DEPENDENCE OF MAGNETIC-PROPERTIES OF PT/CO MULTILAYER THIN-FILMS. Journal of Applied Physics 71, 842-848 (1992).
24. Zhang, Z., Wigen, P. E. & Parkin, S. S. P. PT LAYER THICKNESS DEPENDENCE OF MAGNETIC-PROPERTIES IN CO/PT MULTILAYERS. Journal of Applied Physics 69, 5649-5651 (1991).
25. Bertero, G. A., Sinclair, R., Park, C. H. & Shen, Z. X. INTERFACE STRUCTURE AND PERPENDICULAR MAGNETIC-ANISOTROPY IN PT/CO MULTILAYERS. Journal of Applied Physics 77, 3953-3959 (1995).
26. Bandiera, S., Sousa, R. C., Rodmacq, B. & Dieny, B. Enhancement of perpendicular magnetic anisotropy through reduction of Co-Pt interdiffusion in (Co/Pt) multilayers. Applied Physics Letters 100, 142410 (2012).
27. Das, T. et al. Anomalous enhancement in interfacial perpendicular magnetic anisotropy through uphill diffusion. Sci Rep 4, 5328 (2014).
28. Meiklejohn, W. H. & Bean, C. P. New Magnetic Anisotropy. Physical Review 105, 904-913 (1957).
29. Weller, D. & Moser, A. Thermal effect limits in ultrahigh-density magnetic recording. Ieee Transactions on Magnetics 35, 4423-4439 (1999).
30. Finazzi, M., Duo, L. & Ciccacci, F. Magnetic properties of interfaces and multilayers based on thin antiferromagnetic oxide films. Surf. Sci. Rep. 64, 139-167 (2009).
31. Pratt, G. W. Theory of Antiferromagnetic-Ferromagnetic Transitions in Dilute Magnetic Alloys and in the Rare Earths. Physical Review 108, 1233-1242 (1957).
32. Nogues, J. & Schuller, I. K. Exchange bias. Journal of Magnetism and Magnetic Materials 192, 203-232 (1999).
33. Ali, M. et al. Exchange bias using a spin glass. Nat. Mater. 6, 70-75 (2007).
34. Baltz, V., Rodmacq, B., Zarefy, A., Lechevallier, L. & Dieny, B. Bimodal distribution of blocking temperature in exchange-biased ferromagnetic/antiferromagnetic bilayers. Physical Review B 81, 4 (2010).
35. Baltz, V., Sort, J., Landis, S., Rodmacq, B. & Dieny, B. Tailoring Size Effects on the Exchange Bias in Ferromagnetic-Antiferromagnetic <100 nm Nanostructures. Phys. Rev. Lett. 94, 117201 (2005).
36. Lechevallier, L. et al. Structural analysis and magnetic properties of (Pt/Co)3/Pt/IrMn multilayers. Physical Review B 79, 174434 (2009).
37. Maat, S., Takano, K., Parkin, S. S. P. & Fullerton, E. E. Perpendicular exchange bias of Co/Pt multilayers. Phys. Rev. Lett. 87, 4 (2001).
38. Chen, J.-Y. et al. Magnetic Properties of Exchange-Biased Co/Pt]n Multilayer With Perpendicular Magnetic Anisotropy. IEEE Transactions on Magnetics 46, 1401-1404 (2010).
39. Sort, J., Baltz, V., Garcia, F., Rodmacq, B. & Dieny, B. Tailoring perpendicular exchange bias in [Pt/Co]-IrMn multilayers. Physical Review B 71 (2005).
40. van Dijken, S., Moritz, J. & Coey, J. M. D. Correlation between perpendicular exchange bias and magnetic anisotropy in IrMn/ Co/Pt (n) and Pt/Co (n)/IrMn multilayers. Journal of Applied Physics 97, 4 (2005).
41. Liu, L., Moriyama, T., Ralph, D. C. & Buhrman, R. A. Spin-torque ferromagnetic resonance induced by the spin Hall effect. Phys Rev Lett 106, 036601 (2011).
42. Miao, B. F., Huang, S. Y., Qu, D. & Chien, C. L. Inverse spin Hall effect in a ferromagnetic metal. Phys Rev Lett 111, 066602 (2013).
43. Miron, I. M. et al. Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection. Nature 476, 189-193 (2011).
44. Liu, L. et al. Spin-torque switching with the giant spin Hall effect of tantalum. Science 336, 555-558 (2012).
45. Fan, X. et al. Observation of the nonlocal spin-orbital effective field. Nat Commun 4, 1799 (2013).
46. Chernyshov, A. et al. Evidence for reversible control of magnetization in a ferromagnetic material by means of spin-orbit magnetic field. Nat. Phys. 5, 656-659 (2009).
47. Miron, I. M. et al. Current-driven spin torque induced by the Rashba effect in a ferromagnetic metal layer. Nat Mater 9, 230-234 (2010).
48. Miron, I. M. et al. Fast current-induced domain-wall motion controlled by the Rashba effect. Nat Mater 10, 419-423 (2011).
49. Hirsch, J. E. Spin Hall Effect. Phys. Rev. Lett. 83, 1834-1837 (1999).
50. Kato, Y. K., Myers, R. C., Gossard, A. C. & Awschalom, D. D. Observation of the spin hall effect in semiconductors. Science 306, 1910-1913 (2004).
51. Morota, M. et al. Indication of intrinsic spin Hall effect in4dand5dtransition metals. Physical Review B 83 (2011).
52. Tanaka, T. et al. Intrinsic spin Hall effect and orbital Hall effect in 4d and 5d transition metals. Physical Review B 77, 16 (2008).
53. Gradhand, M., Fedorov, D. V., Zahn, P. & Mertig, I. Extrinsic Spin Hall Effect from First Principles. Phys. Rev. Lett. 104, 4 (2010).
54. Yamanouchi, M. et al. Three terminal magnetic tunnel junction utilizing the spin Hall effect of iridium-doped copper. Applied Physics Letters 102, 4 (2013).
55. Niimi, Y. et al. Giant Spin Hall Effect Induced by Skew Scattering from Bismuth Impurities inside Thin Film CuBi Alloys. Phys. Rev. Lett. 109, 5 (2012).
56. Niimi, Y. et al. Extrinsic spin Hall effects measured with lateral spin valve structures. Physical Review B 89, 10 (2014).
57. Zhang, W. et al. Spin Hall effects in metallic antiferromagnets. Phys Rev Lett 113, 196602 (2014).
58. Mendes, J. B. S. et al. Large inverse spin Hall effect in the antiferromagnetic metalIr20Mn80. Physical Review B 89 (2014).
59. Jungwirth, T., Marti, X., Wadley, P. & Wunderlich, J. Antiferromagnetic spintronics. Nat. Nanotechnol. 11, 231-241 (2016).
60. Fukami, S., Zhang, C., DuttaGupta, S., Kurenkov, A. & Ohno, H. Magnetization switching by spin-orbit torque in an antiferromagnet-ferromagnet bilayer system. Nat Mater (2016).
61. Huang, S. Y. et al. Transport magnetic proximity effects in platinum. Phys Rev Lett 109, 107204 (2012).
62. Zhang, W. F., Han, W., Jiang, X., Yang, S. H. & Parkin, S. S. P. Role of transparency of platinum-ferromagnet interfaces in determining the intrinsic magnitude of the spin Hall effect. Nat. Phys. 11, 496-+ (2015).
63. Qiu, X. et al. Spin-orbit-torque engineering via oxygen manipulation. Nat Nanotechnol 10, 333-338 (2015).
64. Wang, D. S. et al. High thermal stability and low Gilbert damping constant of CoFeB/MgO bilayer with perpendicular magnetic anisotropy by Al capping and rapid thermal annealing. Applied Physics Letters 104, 5 (2014).
65. van den Brink, A. et al. Field-free magnetization reversal by spin-Hall effect and exchange bias. Nat Commun 7, 10854 (2016).
66. Lombard, L. et al. IrMn and FeMn blocking temperature dependence on heating pulse width. Journal of Applied Physics 107, 09D728 (2010).
67. Vinai, G., Moritz, J., Bandiera, S., Prejbeanu, I. L. & Dieny, B. Enhanced blocking temperature in (Pt/Co)3/IrMn/Co and (Pd/Co)3/IrMn/Co trilayers with ultrathin IrMn layer. Journal of Physics D: Applied Physics 46, 322001 (2013).
68. Skinner, T. D. et al. Spin-orbit torque opposing the Oersted torque in ultrathin Co/Pt bilayers. Applied Physics Letters 104, 062401 (2014).
69. Reichlová, H. et al. Current-induced torques in structures with ultrathin IrMn antiferromagnets. Physical Review B 92 (2015).
70. Hin Sim, C., Cheng Huang, J., Tran, M. & Eason, K. Asymmetry in effective fields of spin-orbit torques in Pt/Co/Pt stacks. Applied Physics Letters 104, 012408 (2014).
71. Luqiao Liu, R. A. B., D. C. Ralph. Review and analysis of measurements of the spin Hall effect in platinum. arXiv:1111.3702.
72. Kondou, K., Sukegawa, H., Mitani, S., Tsukagoshi, K. & Kasai, S. Evaluation of Spin Hall Angle and Spin Diffusion Length by Using Spin Current-Induced Ferromagnetic Resonance. Applied Physics Express 5, 073002 (2012).
73. Ganguly, A. et al. Thickness dependence of spin torque ferromagnetic resonance in Co75Fe25/Pt bilayer films. Applied Physics Letters 104, 072405 (2014).
74. Rojas-Sanchez, J. C. et al. Spin pumping and inverse spin Hall effect in platinum: the essential role of spin-memory loss at metallic interfaces. Phys Rev Lett 112, 106602 (2014).