透過自旋軌道矩(spin-orbit torque, SOT)來翻轉鐵磁層的磁矩是新一代磁阻式隨機存取記憶體(magnetic random access memory, MRAM)的寫入機制，這樣的機制能夠使寫入電流不通過磁性元件本身，也能達成寫入的目的，能夠有效地增加元件耐久度與翻轉速度，是更具有潛力的非揮發性記憶體，可以應用於神經網路計算、邊緣運算等領域。但SOT-MRAM的選擇電晶體的面積與其翻轉電流密度成正比，如果翻轉電流太大，會導致記憶體的密度無法繼續微縮，因此研究如何降低翻轉電流密度是很重要的一個課題。
用來產生SOT的5d重金屬中，鎢擁有最大的自旋霍爾角，然而這樣的鎢必須要是β相才能有這麼大的自旋霍爾角，而要形成β相的鎢有一定的製程難度，其是需要在夠薄的厚度下才存在的亞穩態。
在本實驗中發現鎳鎢合金具有很強的自旋霍爾效應，在一系列SOT翻轉量測中的翻轉電流密度都落在106A/cm2的等級，而對其進行等效場量測得知鎳鎢合金的自旋霍爾角都大於β相的鎢，並且鎳鎢合金相較β相的鎢容易製成，種種實驗結果說明鎳鎢合金在SOT材料上面具有非常高的潛力，有助於提高SOT-MRAM的效能。

Reversing the magnetic moment of the ferromagnetic layer through spin-orbit torque (SOT) is the writing mechanism of the new generation of magnetic random access memory (MRAM). The purpose is to effectively increase the durability and turnover speed of components, and it is a more potential non-volatile memory that can be used in neural network computing, edge computing and other fields.
However, the area of the selection transistor of SOT-MRAM is proportional to its inversion current density. If the inversion current is too large, the density of the memory will not be able to continue to shrink. Therefore, it is very important to study how to reduce the inversion current density.
Among the 5d heavy metals used to generate SOT, tungsten has the largest spin Hall angle. However, such tungsten must be β-phase to have such a large spin Hall angle, and it is difficult to form β-phase tungsten, which is a metastable state that needs to exist at a sufficiently thin thickness.
In this experiment, it is found that the nickel-tungsten alloy has a strong spin Hall effect, and the inversion current density in a series of SOT inversion measurements falls at the level of 106A/cm2, and the effective field measurement of it shows that the spin Hall angle of nickel-tungsten alloys is larger than that of β-phase tungsten, and nickel-tungsten alloys are easier to make than β-phase tungsten. Various experimental results show that nickel-tungsten alloys have very high potential on SOT materials, which is helpful for improve the performance of SOT-MRAM.

1. Bychkov, Y. A. & Rashba, E. I. Oscillatory effects and the magnetic-susceptibility of carriers in inversion-layers. J. Phys. C 17, 6039–6045 (1984).
2. Edelstein, V. M. Spin polarization of conduction electrons induced by electric-current in 2-dimensional asymmetric electron-systems. Solid State Commun. 73, 233–235 (1990).
3. J. E. Hirsch, Spin Hall Effect. Phys. Rev. Lett. 83, 1834 (1999).
4. M. I. Dyakonov and V. I. Perel, Sov. CURRENT-INDUCED SPIN ORIENTATION OF ELECTRONS IN SEMICONDUCTORS. Phys. JETP 13, 467 (1971)
5. Bychkov, Y. A., & Rashba, E. I. (1984). Oscillatory effects and the magnetic susceptibility of carriers in inversion layers. Journal of physics C: Solid state physics, 17(33), 6039.
6. Miron, I. M., Moore, T., Szambolics, H., Buda-Prejbeanu, L. D., Auffret, S., Rodmacq & Gaudin, G. (2011). Fast current-induced domain-wall motion controlled by the Rashba effect. Nature Materials, 10(6), 419-423.
7. Bauer,G. E., Saitoh, E., & van Wees, B. J. (2012). Spin caloritronics. Nature materials, 11(5), 391-399.
8. Dyakonov, M. I., & Perel, V. I. (1971). Current-induced spin orientation of electrons in semiconductors. Physics Letters A, 35(6), 459-460.
9. M. I. Dyakonov and V. I. Perel, Pisma Zh. Eksp. Teor. Fiz. 13,657 (1971) , JETP Lett. 13,467
10. Hirsch, J. E. (1999). Spin hall effect. Physical Review Letters, 83(9), 1834.
11. Kim, J., Sinha, J., Hayashi, M., Yamanouchi, M., Fukami, S., Suzuki & Ohno, H. (2013). Layer thickness dependence of the current-induced effective field vector in Ta| CoFeB| MgO. Nature materials, 12(3), 240-245.
12. C. F. Pai, L. Liu, Y. Li, H.W. Tseng, D. C. Ralph, and R. A. Buhrman, Spin transfer torque devices utilizing the giant spin Hall effect of tungsten, Appl. Phys. Lett. 101, 122404(2012).
13. G. Y. Guo, S. Murakami, T.-W. Chen, and N. Nagaosa, Intrinsic Spin Hall Effect in Platinum: First-Principles Calculations, Phys. Rev. Lett. 100, 096401.(2008)
14. Jairo Sinova, Sergio O. Valenzuela, J. Wunderlich, C. H. Back, and T. Jungwirth,Spin Hall effects, Rev. Mod. Phys. 87, 1213 (2015.)
15. Y. Niimi, M. Morota, D. H. Wei, C. Deranlot, M. Basletic, A. Hamzic, A. Fert, and Y. Otani, Extrinsic Spin Hall Effect Induced by Iridium Impurities in Copper, Phys. Rev. Lett. 106, 126601. (2011)
16. Y. Niimi, Y. Kawanishi, D. H. Wei, C. Deranlot, H. X. Yang, M. Chshiev, T. Valet, A. Fert, and Y. Otani, Giant Spin Hall Effect Induced by Skew Scattering from Bismuth Impurities inside Thin Film CuBi Alloys, Phys. Rev. Lett. 109, 156602.(2012)
17. Y. Fan et al., Magnetization switching through giant spin–orbit torque in a magnetically doped topological insulator heterostructure, Nat. Mater. 13, 699 (2014).
18. M. Jamali, J. S. Lee, J. S. Jeong, F. Mahfouzi, Y. Lv, Z. Zhao, B. K. Nikolić, K. A. Mkhoyan, N. Samarth, and J.-P. Wang, Giant spin pumping and inverse spin Hall effect in the presence of surface spin-orbit coupling of topological insulator BiSe, Nano Lett. 15, 7126 (2015).
19. Liu, L., Pai, C. F., Li, Y., Tseng, H. W., Ralph, D. C., & Buhrman, R. A. (2012). Spin-torque switching with the giant spin Hall effect of tantalum.Science,336(6081), 555-558.
20. Pai, C. F., Liu, L., Li, Y., Tseng, H. W., Ralph, D. C., & Buhrman, R. A. (2012). Spin transfer torque devices utilizing the giant spin Hall effect of tungsten. Applied Physics Letters, 101(12), 122404.
21. Tanaka, T., Kontani, H., Naito, M., Naito, T., Hirashima, D. S., Yamada, K., & Inoue, J. (2008). Intrinsic spin hall effect and orbital Hall effect in 4 d and 5 d transition metals. Physical Review B, 77(16), 165117.