以脈衝雷射鍍膜儀成長Pt/ZnO/YIG三層薄膜結構，並以RHEED、XRD、AFM檢測薄膜品質，再將該三層薄膜應用至自旋幫浦系統中，並量測其產生之反轉自旋霍爾效應電壓。將此電壓訊號及鐵磁共振訊號一併討論，並與Pt/YIG雙層薄膜量測出之反轉自旋霍爾效應電壓比較，可利用模型推演出氧化鋅之自旋擴散長度，該模型特色為周全考慮於同樣條件下成長之薄膜可能存在樣品差異性。而推導出之氧化鋅自旋擴散長度為1.01~6.77 nm，並由實驗結果得知氧化鋅之自旋擴散長度可藉由改變氧化鋅之成長氧壓以調控。

Study of Spin Diffusion Length in ZnO via Spin Pumping Mechanism

Author : Chih-Hua Liu
Advisor : J. C. A. Huang
Department of Physics, National Chen Kung University

SUMMARY

Spin diffusion length λ stands for the characteristic length which spin current maintains its original spin state. λ of heavy spin orbital coupling material such as Platinum, Tantalum etc. are known and are about just few nanometers. If we are able to realize λ of other materials that are longer than few nanometers, then we can utilize the material as a transmission layer and transport spin current through it. ZnO is a promising semiconductor with wide band gap and large exciton binding energy. Besides, it is transparent, costs less and can be widely applied. Therefore, we would like to discuss the spin diffusion length of ZnO. In this work, our sample preparation are YIG(Y3Fe5O12)/ZnO/Pt trilayer with different thickness of ZnO(0, 2, 4, 10, 40 nm). Through comparing the inverse spin hall effect voltage of YIG/Pt and YIG/ZnO/Pt generated by spin pumping, one can derive the spin diffusion length of ZnO.

Key words：YIG, ZnO, spin pumping, spin diffusion length

INTRODUCTION

Spin current possesses a lot more benefits than charge current such as lower energy consuming, higher storage of quantum information and faster velocity. The spin pumping mechanism enables spin current generation from ferromagnetic resonance of ferromagnetic layer and injection into normal metal. Due to the strong spin-orbit coupling of normal metal, the spin current will be converted into charge current, which is so-called inverse spin hall effect. Spin transport has been studied intensively since the spintronic had been developed. Through inserting a layer into FM/NM bilayer, the transmission properties of the middle layer might be studied by spin pumping.
One of the transmission properties is the spin diffusion length, which stands for the characteristic length that spin current maintains its original spin state. The spin diffusion length might be the key to other transmission properties. Therefore, discussing spin diffusion length might reveal more information of spin current transmission. ZnO is a promising semiconductor with wide band gap and high optical transmittance, besides its carrier concentration can be tuned easily through defect or oxygen vacancies. For industrial use, ZnO is a perfect material since it is low cost and eco-friendly. From above, we know that ZnO could be applied in multiple ways. Since no one has ever discussed the spin diffusion length of ZnO through spin pumping. I think it would be an important event if we find out the spin diffusion length of ZnO.

MATERIAL AND METHODS

In our study, it requires Pt/YIG bilayer and Pt/ZnO/YIG trilayer samples. First, we grow these samples in-situ in pulsed laser deposition. The growth parameters of these materials including temperature, laser energy, repetition, oxygen pressure are 860℃, 1.412 J/cm2, 5 Hz, 10-1 torr for YIG respectively. For ZnO, the parameters are 600℃, 1 J/cm2, 2 Hz, 10-7 torr. Finally, we grow Pt at room temperature with laser energy 4.68 J/cm2 and in ultrahigh vacuum chamber. Then confirm the crystal quality by RHEED, XRD and AFM. After checking the quality, we place our samples in spin pumping system and measured the inverse spin hall effect voltage. Through comparing the voltage of bilayer and trilayer samples and considering the sample difference due to slightly difference of operation, we define the sample difference factor
f=(V_ISHE (d_ZnO))/(V_ISHE (d_ZnO=0)) (R_Pt (R_Pt^'+R_ZnO))/(R_Pt^' R_ZnO ) L/L' (tanh⁡(d_Pt/(2λ_Pt )))/(tanh⁡((d_Pt^')/(2λ_Pt ))) (J_S1^0)/J_S1^0',
and we assume that the spin current density injected from YIG to ZnO J_s1^0 decays exponentially while transmit through ZnO. Therefore the spin current density at the Pt/ZnO interface J_s2^0 is equal to J_s1^0 exp⁡(-d_ZnO⁄λ_ZnO ). Combining the sample difference factor f and the concept of spin current density decay, we are able to derive the spin diffusion length of ZnO though the equation below

Here,V_ISHE (d_ZnO) is ISHE voltage of trilayer, ,V_ISHE (d_ZnO=0) is ISHE voltage of bilayer, R_Pt is the resistance of Pt, of bilayer, R_Pt' is the resistance of Pt of trilayer R_ZnO is the resistance of ZnO, L is the length of bilayer, L' is the length of trilayer, d_Pt is the thickness of Pt, λ_Pt is the spin diffusion length of Pt, J_S1^0' is spin current density injected from YIG to ZnO in trilayer sample and J_S1^0' is the spin current density injected from YIG to Pt in bilayer sample. In Eqn. (1), the spin current density J_s1^0,J_s1^0' could be calculated by the ferromagnetic resonance result. For the spin diffusion length of Pt, we utilize the result derived by E. Saitoh and his group, and its spin diffusion length is 7.7±0.3 nm.

RESULT AND DISCUSSION

After growing thin film, we have to check the crystal quality and surface morphology through RHEED, XRD and AFM. Figure 1 shows the RHEED pattern, XRD and AFM of YIG. In Figure 1., one can see that the thin film is really high quality and the roughness Rq of the YIG surface is 0.331 nm.

Figure 1. (a) RHEED pattern (b) XRD spectra (c) AFM of YIG
Also, the quality of ZnO must be confirmed. Therefore, the RHEED, XRD and AFM result is shown in Figure 2. In Figure 2., the RHEED pattern shows great crystal structure for ZnO growing on YIG. The XRD spectra shows there is no other crystal orientation except ZnO(0001) when ZnO grows on YIG, and the AFM roughness Rq is 0.575 nm.

Figure 2. (a) RHEED pattern on YIG (b) XRD spectra on YIG and Al2O3 (c) AFM of ZnO
samples with different ZnO thickness (2, 4, 10, 40 nm). We place all these samples into our spin pumping system, and measure the FMR spectra and ISHE voltage. We find out that only the ISHE voltage of the sample Pt/ZnO(2 nm)/YIG can be measured. Figure 3. shows the ISHE voltage of the Pt/ZnO(2 nm)/YIG sample under multiple microwave frequency and multiple microwave power. In Figure 1(a) one can obviously see the symmetric signal in positive and negative magnetic field, and Figure 1(b) shows the linear dependent relation between microwave power and ISHE voltage.
Figure 3 (a) ISHE voltage under different mw frequency (b) ISHE voltage under different mw power
We compare the voltage of this sample with Pt/YIG sample ISHE voltage and calculate the sample difference factor for both samples. Finally, we derive the spin diffusion length of ZnO under different frequency. Table 1. shows derived spin diffusion length of ZnO under different microwave frequency. Then we take average and get the average value of ZnO spin diffusion length is λ_ZnO=1.4163±0.08 nm .
Frequency (GHz) 2 3 4 5 Average
λZnO (nm) 1.51 1.38 1.54 1.33 1.42±0.08
Table 1. Spin diffusion length under different mw frequency
In addition to the spin diffusion length of pure ZnO grown in 10-7 torr oxygen atmosphere, we discuss the spin diffusion length of ZnO which is doped with other elements and which is grown in higher oxygen atmosphere. Table 2. shows the comparison between pure ZnO grown in 10-7- torr and ZnO with dopant. In Table 2. we can see that the spin diffusion length of ZnO:Al and ZnO:Bi both decrease. We speculate that the spin diffusion length of ZnO:Al decrease due to the carrier concentration increase. In ZnO:Bi sample, the spin diffusion length decrease due to strong spin-orbit coupling probably.
Material ZnO ZnO:Al 3% ZnO:Bi 5%
λZnO (nm) 1.42±0.08 1.11±0.24 1.01±0.24
Table 2. Spin diffusion length of ZnO with dopant
Table 3. shows the comparison between the spin diffusion length of ZnO grown in different oxygen pressure. In Table 3. the spin diffusion length of ZnO which is grown in 10-2 torr oxygen pressure is the highest. From this result we realize that spin diffusion length could be tuned by growing in higher oxygen atmosphere. However, the spin diffusion length of the one grown in 10-5 torr is the lowest. We speculate that 600℃ might not be the most suitable growth temperature when the thin films are grown in 10-5 torr.
Material ZnO (10-7 torr) ZnO (10-5 torr) ZnO (10-2 torr)
λZnO (nm) 1.42±0.08 1.21±0.15 6.77±2.82
Table 3. Spin diffusion length of ZnO grown in different O2 pressure
Later, we will keep investigate the factors that affect the transmission properties by discussing the spin diffusion length of ZnO with other dopants or the spin diffusion length of other semiconductors.

CONCLUSION

In our study, we successfully measure the ISHE voltage in Pt/ZnO/YIG trilayers an build a model to derive the spin diffusion length of ZnO, which is 1.42±0.08 nm for pure ZnO grown in 10-7 torr oxygen pressure. Besides, we find out that the spin diffusion length could be tuned by changing oxygen pressure. We think that the transmission properties might be affected by carrier concentration, band gap, resistance and so on. It’s quite interesting and worth for us to discuss deeply.

1. Gregg, J.F., Michael Ziese, and Martin J. Thornton, Spin electronics. 2001.
2. J.E., H., Spin Hall effect. Phys Rev Lett, 1999. 83(9).
3. Kato, Y.K., et al., Observation of the Spin Hall Effect in Semiconductors. Science, 2004. 306(1910).
4. Kimura, T., et al., Room-temperature reversible spin Hall effect. Phys Rev Lett, 2007. 98(15): p. 156601.
5. Liu, L.a.P., Chi-Feng and Li, Y and Tseng, HW and Ralph, DC and Buhrman, RA, Spin-torque switching with the giant spin Hall effect of tantalum. Science, 2012. 336: p. 555-558.
6. Ando, K. and E. Saitoh, Inverse spin-Hall effect in palladium at room temperature. Journal of Applied Physics, 2010. 108(11): p. 113925.
7. Kato, Y.K., et al., Observation of the spin Hall effect in semiconductors. Science, 2004. 306(5703): p. 1910-3.
8. Oiwa, A., et al., Effect of optical spin injection on ferromagnetically coupled Mn spins in the III-V magnetic alloy semiconductor (Ga,Mn)As. Phys Rev Lett, 2002. 88(13): p. 137202.
9. Kimura, T., Y. Otani, and J. Hamrle, Switching magnetization of a nanoscale ferromagnetic particle using nonlocal spin injection. Phys Rev Lett, 2006. 96(3): p. 037201.
10. Vlietstra, N., et al., Simultaneous detection of the spin-Hall magnetoresistance and the spin-Seebeck effect in platinum and tantalum on yttrium iron garnet. Physical Review B, 2014. 90(17).
11. Mosendz, O., et al., Detection and quantification of inverse spin Hall effect from spin pumping in permalloy/normal metal bilayers. Physical Review B, 2010. 82(21).
12. Inoue, H.Y., et al., Detection of pure inverse spin-Hall effect induced by spin pumping at various excitation. Journal of Applied Physics, 2007. 102(8): p. 083915.
13. Saitoh, E., et al., Conversion of spin current into charge current at room temperature: Inverse spin-Hall effect. Applied Physics Letters, 2006. 88(18): p. 182509.
14. Dushenko, S., et al., Experimental Demonstration of Room-Temperature Spin Transport in n-Type Germanium Epilayers. Phys Rev Lett, 2015. 114(19): p. 196602.
15. Du, C.H., et al., Probing the spin pumping mechanism: exchange coupling with exponential decay in Y3Fe5O12/barrier/Pt heterostructures. Phys Rev Lett, 2013. 111(24): p. 247202.
16. Ozgur, U., D. Hofstetter, and H. Morkoc, ZnO Devices and Applications:A Review of Current Status and Future Prospects. IEEE, 2010. 98(7).
17. Opel, M., et al., Laser molecular beam epitaxy of ZnO thin films and heterostructures. Journal of Physics D: Applied Physics, 2014. 47(3): p. 034002.
18. Lee, J.-C., et al., Inverse spin Hall effect induced by spin pumping into semiconducting ZnO. Applied Physics Letters, 2014. 104(5): p. 052401.
19. 許庭維, 氧化鋅摻雜鈷在低載子濃度下磁性、電性及光學性質研究, in 物理學系碩博士班. 2010, 國立成功大學: 台南市. p. 94.
20. Sokolov, N.S., et al., Thin yttrium iron garnet films grown by pulsed laser deposition: Crystal structure, static, and dynamic magnetic properties. Journal of Applied Physics, 2016. 119(2): p. 023903.
21. Sun, Y., Yttrium Iron Garnet Nano Films: Epitaxial Growth, Damping, Spin, Pumping, and Magnetic Proxmity Effect. 2014.
22. Hirsch, J.E., Spin Hall Effect. Phys Rev Lett, 1999. 83(9).
23. 戴貝珊, 鎳鐵合金自旋幫浦引發氧化鋅之逆自旋霍爾效應研究, in 物理學系碩博士班. 2013, 國立成功大學: 台南市. p. 72.
24. Azevedo, A., et al., Spin pumping and anisotropic magnetoresistance voltages in magnetic bilayers: Theory and experiment. Physical Review B, 2011. 83(14).
25. 陳金宏, 鈷鎵共摻雜氧化鋅薄膜覆蓋於氮化鎵發光二極體之光、電特性研究, in 物理學系碩博士班. 2012, 國立成功大學: 台南市. p. 73.
26. 賴柜宏, 自組織奈米結構與氧化鋅表面現象之掃瞄式探針顯微術研究, in 物理學系碩博士班. 2010, 國立成功大學: 台南市. p. 173.
27. 黃舜漁, 鉛鈀氧摻雜鈷薄膜之磁電性研究, in 物理學系碩博士班. 2011, 國立成功大學: 台南市. p. 69.
28. Nakayama, H., et al., Geometry dependence on inverse spin Hall effect induced by spin pumping in Ni81Fe19/Pt films. Physical Review B, 2012. 85(14).
29. Watanabe, S., et al., Polaron spin current transport in organic semiconductors. Nature Physics, 2014. 10(4): p. 308-313.