最近幾年，透過電流將磁化方向翻轉的技術有了明顯的突破，其中在自旋軌道矩-磁性記憶體(SOT-MRAM)的應用上，透過自旋軌道矩(spin-orbit torques)翻轉磁矩方向的方式，來實現低耗能，高密度的邏輯電路。自旋軌道矩最初認為是來自於塊材的自旋霍爾效應和界面的Rashba效應。目前為止，大部分針對這兩個現象的研究是透過重金屬和不同鐵磁層的系統來完成。當外加電流通過該系統時，重金屬的自旋霍爾效應提供了垂直方向上的自旋電流，而這個自旋電流對鐵磁層產生了自旋軌道矩，導致了磁性的翻轉。 我們簡單介紹了重金屬/反鐵磁/鐵磁材料 (HM/AFM/FM)三層結構的系統。在此結構中，最大的區別是重金屬產生的自旋電子流會先穿過反鐵磁層再到達鐵磁層。在於這個新穎的反鐵磁結構，不僅能夠在未來提供另一個探討自旋軌道矩的系統，對於跟MTJ的相容性來說也相當有利，因為反鐵磁跟重金屬都在鐵磁層的同一側。可以期待將來能夠更順利的整合入SOT-MRAM。
由於反鐵磁不易於調控和偵測，因此我們將目標轉移到亞鐵磁(Ferrimagnets)材料上。在這篇工作中，我們成功在CoTb和CoGd上找到非常好的垂直異相性和磁矩補償點。並在CoTb單層結構中展示了標準自旋軌道力矩的翻轉。現階段的成果對於未來博士生活的研究是個非常好的墊腳石。

Magnetization reversal from the electric current is an area which has made rapid advancements in recent time. In particular, SOT-MRAM, where magnetization is reversed by spin-orbit torques. Spin-orbit torque technique offers energy efficient magnetization switching, and also high scalability in logic circuits. Spin-orbit torques are originated from the bulk spin Hall effect (SHE) and interfacial Rashba effect. So far, these effects are mainly studied in heavy metal and ferromagnet bilayer system. Where the heavy metal exhibits bulk spin Hall effect (SHE). When charge current is injected in the HM/FM bilayer system, a spin current is generated in the transverse direction and this spin current exerts the spin-orbit torques on ferromagnet which results in magnetization switching.
We introduced the heavy metal, antiferromagnet, and ferromagnet (HM/AFM/FM) tri-layer structure. In this structure, the spin current generated by the heavy metal first went in the antiferromagnet and then to a ferromagnet, which is the major difference between the tri-layer structure study mainly studied in the past (HM/FM/AFM). For the complex structure of the antiferromagnet opens a new path to explore the spin-orbit torques in this structure. Moreover, the HM/AFM/FM structure is also compatible with full MTJ structure since AFM and HM both are one side of FM. Therefore, this structure can be easily integrated into SOT-MRAM.
It is hard to harness and detect the antiferromaget, so we move on to the ferrimagnet (FIM) material which is more flexible than AFM in terms of the manipulation. Here, we showed the thin films of CoTb and CoGd with good PMA. We also performed the SOT switching of CoTb samples and it seems a good starter for the further research.

1. 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)
2. Hirsch JE. Spin Hall effect. Phys Rev Lett 83, 1834 – 1837 (1999)
3. Y. K. Kato, R. C. Myers, A. C. Gossard, D. D. Awschalom. Observation of spin hall effect in semiconductors. Science 306, 1910-1913 (2004).
4. Néel L. Anisotropie magnétique superficielle et surstructures d’orientation. J. Phys. Radium 15, 225-239 (1954).
5. Carcia PF, Meinhaldt AD, Suna A. Perpendicular Magnetic Anisotropy in Pd/Co thin film layered structure. Appl Phys Lett 47, 178-180 (1985).
6. Denbroeder FJA, Kuiper D, Vandemosselaer AP, Howing W. Perpendicular magnetic anisotropy of Co-Au multilayers induced by interface sharpening. Phy Rev Lett 60, 2769-2772 (1988).
7. Carcia PF. Perpendicular Magnetic Anisotropy in Pd/Co and Pt/Co thin film layered structures. J Appl Phys 63, 5066-5073 (1988).
8. Denbroeder FJA, Janssen E, Mud A, Kerkhof JM. Co/Ni multilayers with perpendicular magnetic anisotropy. J Magn Magn Mater 126, 563-568 (1993).
9. Daalderop GHO, Kelly PJ, and FJA den Broeder. Prediction and ConSrmation of Perpendicular Magnetic Anisotropy in Co/Ni Multilayers. Phys Rev Lett, 68(05), 682 (1992).
10. Enlong Liu, Swerts J, Devolder T, Couet S, Mertens S, Lin T, Spampinato V, Franquet A, Conard T, Van Elshocht S, Furnemont A, De Boeck J, and Kar G. Seed Layer Impact on Structural and Magnetic Properties of [Co/Ni] Multilayers with Perpendicular Magnetic Anisotropy. J Appl Phys (2017).
11. Den Broeder FJA, Janssen E, Hoving W, and Zeper WB. Perpendicular Magnetic Anisotropy and Coercivity of Co/Ni Multilayers. IEEE Transactions on magnetics, 28(5), 2760-2765 (1992).
12. Zhang YB, Woollam JA, Shan ZS, Shen JX, and Sellmyer DJ. Anisotropy and magneto-optical properties of sputtered Co/Ni multilayer thin films. IEEE Transactions on magnetics, 30(6), 4440-4442 (1994).
13. Kurt H, Venkatesan M, and Coey JMD. Enhanced perpendicular magnetic anisotropy in Co/Ni multilayers with a thin seed layer. J. Appl. Phys. 108, 073916 (2010).
14. Yang E, Vincent M. Sokalski, Matthew T. Moneck, David M. Bromberg, and Jian-Gang Zhu. Annealing effect and under/capping layer study on Co/Ni multilayer thin films for domain wall motion. J. Appl. Phys. 113, 17C116 (2013).
15. L. You, R. C. Sousa, S. Bandiera, B. Rodmacq, and B. Dieny. Co/Ni multilayers with perpendicular anisotropy for spintronic device applications. Appl. Phys. Lett. 100, 172411 (2012).
16. W. H. Meiklejohn and C. P. Bean. New Magnetic Anisotropy. Phys. Rev. 102, 1413 (1956).
17. Ching Tsang. Magnetics of small magnetoresistive sensors. Journal of Applied Physics 55, 2226 (1984).
18. S.A.H. Shah, Vibrating Sample Magnetometer: Analysis and Construction, Department of Physics, Syed Babar Ali School of Science and Engineering, LUMS, Dec. 2013.
19. Tanaka, T. et al. Intrinsic spin Hall effect and orbital spin hall effect in 4d and 5d transition metal. Physical Review B77, 16 (2008)
20. Zhang, W. et al. Spin Hall effects in metallic antiferromagnets. Phys Rev Lett 113, 196602 (2014).
21. Miron, I. M. et al. Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection. Nature 476, 189–193 (2011).
22. Liu, L. et al. Spin–torque switching with the giant spin Hall effect of tantalum. Science 336, 555–558 (2012).
23. 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).
24. Pai, C.-F. et al. Spin transfer torque devices utilizing the giant spin Hall effect of tungsten. Appl. Phys. Lett. 101, 122404 (2012)
25. Berger, L. et al. Emission of spin waves by a magnetic multilayer traversed by a current Phys. Rev. B 54, 9353 (1996).
26. Slonczewski, J. C. Current-driven excitation of magnetic multilayers. J. Magn. Magn. Mater. 159, L1 (1996).
27. T. Min, Q. Chen, R. Beach, G. Jan, C. Horng, W. Kula, T. Torng, R. Tong, T. Zhong, D. Tang, P. Wang, M. m. Chen, J. Z. Sun, J. K. Debrosse, D. C. Worledge, T. M. Maffitt, and W. J. Gallagher, IEEE Trans. Magn. 46, 2322 (2010).
28. W. S. Zhao, Y. Zhang, T. Devolder, J. O. Klein, D. Ravelosona, C. Chappert, and P. Mazoyer, Microelectron. Reliab. 52, 1848 (2012).
29. M. I. Dyakonov and V. I. Perel. Current-induced spin orientation of electrons in semiconductors. Phys. Lett. A 35, 459 (1971).
30. Asya Shpiro, Peter M. Levy, and Shufeng Zhang. Self-consistent treatment of nonequilibrium spin torques in magnetic multilayers. Phys. Rev. B 67, 104430 (2003).
31. S. Zhang, P. M. Levy, and A. Fert. Mechanisms of Spin-Polarized Current-Driven Magnetization Switching. Phys. Rev. Lett. 88, 236601 (2002).
32. Young-Wan Oh et al. Field-free switching of perpendicular magnetization through spin–orbit torque in antiferromagnet/ferromagnet/oxide structures. Nature Nanotechnology volume 11, pages 878–884 (2016).
33. A. van den Brink et al. Field-free magnetization reversal by spin-Hall effect and exchange bias. Nature Communications volume 7, Article number: 10854 (2016).
34. S. Fukami et al. Magnetization switching by spin–orbit torque in an antiferromagnet–ferromagnet bilayer system. Nature Materials volume 15, pages 535–541 (2016).
35. Paul M. Haney et al. Current induced torques and interfacial spin-orbit coupling: Semiclassical modeling. Phys. Rev. B 87, 174411 (2013).
36. Long you et al. Switching of perpendicularly polarized nanomagnets with spin orbit torque without an external magnetic field by engineering a tilted anisotropy. Proc. Natl. Acad. Sci. USA 2015, 112, 10310.
37. J. Kwon et al. Asymmetrical domain wall propagation in bifurcated PMA wire structure due to the Dzyaloshinskii-Moriya interaction. Appl. Phys. Lett. 110, 232402 (2017).
38. W. J. Kong et al. Field-free spin Hall effect driven magnetization switching in Pd/Co/IrMn exchange coupling system. Appl. Phys. Lett. 109, 132402 (2016).
39. H. Honjo, S. Fukami, T. Suzuki, R. Nebashi, N. Ishiwata, S. Miura, N. Sakimura, T. Sugibayashi,
40. N. Kasai, and H. Ohno. Domain-wall-motion cell with perpendicular anisotropy wire and in-plane magnetic tunneling junctions. J. Appl. Phys. 111, 07C903 (2012).
41. H. Ohno, T. Endoh, T. Hanyu, N. Kasai, and S. Ikeda. Magnetic tunnel junction for nonvolatile CMOS logic. 2010 IEEE International Electron Devices Meeting (IEDM), Technical Digest, pp. 9.4.1–9.4.4 (2010).
42. Guoqiang Yu et al. Spin-orbit torques in perpendicular magnetized Ir22Mn78 /Co20Fe60B20 /MgO multilayer. Appl. Phys. Lett. 109, 222401 (2016).
43. S. S. P. Parkin. Giant Magneto Resistance in Magnetic Nanostructure. Annu. Rev. Maler. Sci. 1995.
44. Qinli Ma, Yufan Li, D. B. Gopman, Yu. P. Kabanov, R. D. Shull, and C. L. Chien. Switching a Perpendicular Ferromagnetic Layer by Competing Spin Currents. Phys. Rev. Lett. 120, (2018).
45. Seung-heon C. Baek et al. Spin currents and spin–orbit torques in ferromagnetic trilayers. Nature Materials volume 17, pages509–513 (2018).
46. R. Q. Zhang et al. Current-induced magnetization switching in a CoTb amorphous single layer. Phys. Rev. B 101, 214418 (2020)
47. J. Finley et al. Spin-Orbit-Torque Efficiency in Compensated Ferrimagnetic Cobalt-Terbium Alloys Phys. Rev. A 6, 054001 (2016)
48. C.Pai et al. Determination of spin torque efficiencies in heterostructures with perpendicular magnetic anisotropy Phys. Rev. B 93, 14409 (2016)
49. L. Liu et al. Current-Induced Domain Wall Motion in a Compensated Ferrimagnet. Phys. Rev. Lett. 121, 057701 (2018).