Spin orbit interaction induced spin separation in platinum nanostructures

SPIN-ORBIT INTERACTION INDUCED SPIN-SEPARATION IN PLATINUM NANOSTRUCTURES KOONG CHEE WENG (B Eng (Hons.), NUS) A Thesis Submitted for the Degree of Doctor of Philosophy NUS Graduate School for Integrative Sciences and Engineering National University of Singapore 2009 Acknowledgements I would like to express my gratitude to all those who contribute in one way or another in the completion of this thesis I want to thank the A∗ STAR Graduate Academy for the scholarship, and the support of my school, NUS Graduate School for Integrative Sciences & Engineering I am deeply indebted to my supervisors Prof Berthold-Georg Englert from the National University of Singapore and Prof Chandrasekhar Natarajan from the Institute of Materials Research and Engineering I would also like to show my appreciation to the members of the Thesis Advisory Committee, Prof Christian Miniatura, Prof Ong Chong Kim and Dr Adekunle Adeyeye, whose stimulating suggestions and encouragement has helped me in my research and the writing of this thesis My thanks go out especially to Dr Deng Jie, Ms Teo Siew Lang, and Mr Chum Chan Choy for making the e-beam lithography available for me I would also want to thank Dr Nikolai Yakovlev for doing the SIMS analysis and Ms Doreen Lai for doing the EDX analysis I would also like to thank Ms Ma Han Thu Lwin for helping to coat the silicon wafers with an oxide layer Furthermore I would also like to thank countless other colleagues from IMRE and NUS, without i Acknowledgements ii their support, this thesis would not have been possible Finally, I would like to give my special thanks to my parent and my wife Linda, whose patience and love enabled me to complete this work Contents 1 1.1 Background 1.2 Introduction Outline of Thesis Theory 12 2.1 Spin-Orbit Interaction 13 2.2 Extrinsic Spin-Orbit Interaction Effect 15 2.2.1 Skew Scattering 15 2.2.2 Side-Jump 18 Intrinsic Spin-Orbit Interaction Effect 20 2.3.1 Dresselhaus and Rashba Spin-Orbit Interaction 20 2.3.2 Berry Phase 22 2.4 Spin Current 26 2.5 Spin Accumulation 31 2.6 Non-Local Geometry 32 2.7 Spin Relaxation 34 2.8 Spin Hall Effect 37 2.3 iii CONTENTS 2.9 iv 38 2.9.1 Platinum 41 2.10 Generation/Detection of Spin 44 2.10.1 Generation of Spin Currents via SHE 45 2.10.2 Detection of Spin Currents via ISHE 46 2.11 Summary Spin Hall Effect in Metals and Semiconductors 47 Sample Design and Fabrication 51 3.1 Design of Experiment (Motivation) 52 3.1.1 Proposed Design 54 Fabrication 57 3.2.1 Outline of Lithography Procedure 58 3.2.2 Fabrication of Contact Pads with Ultraviolet Lithography 59 3.2.3 E-Beam Lithography 63 3.2.4 Polymethylmethacrylate (PMMA) 63 3.2.5 Bilayer Mask 64 3.2.6 Fabrication of Device with E-Beam Lithography 65 3.3 Measurement Setup 68 3.4 Errors from Electrical Measurements 70 3.5 Electromigration 75 3.6 Transient Protection 76 3.6.1 Shunting For Electrostatic Discharge Protection 77 Summary 78 3.2 3.7 CONTENTS v Experimental Results and Analysis 79 4.1 Background 80 4.2 Spin Transport Equation in a Diffusive Conductor 80 4.2.1 Drift-Diffusion Model 82 4.2.2 Conversion of Spin Current into Hall Voltage 85 4.2.3 Theoretical Model of Device Geometry 88 4.3 The Samples 90 4.4 Characterization of Sample 90 4.4.1 X-Ray Diffraction (XRD) 91 4.4.2 Secondary Ion Mass Spectrometry (SIMS) 92 4.4.3 Energy-Dispersive X-Ray Spectroscopy (EDX) 95 4.5 Sample Testing Procedures 96 4.6 Experimental Data 99 4.6.1 4.7 The Spin Hall Resistance RsH and the Misalignment Voltage103 Discussions 104 4.7.1 RsH /RD versus Temperature 105 4.7.2 Measurement of Spin Diffusion Length λs 106 4.7.3 Intrinsic versus Extrinsic SHE 110 4.7.4 Comparison of RsH with Temperature 111 4.7.5 Skew Scattering due to Fe Impurities 113 4.7.6 Comparison of RsH of Pt with Au and Al 113 4.7.7 RsH in Different Current Ranges 115 4.8 Gold and Platinum — A Comparative Analysis 116 4.9 Device Failure 119 CONTENTS vi 4.10 Shunting For Electrostatic Discharge Protection 121 4.11 Summary 123 Conclusion and Outlook 125 REFERENCES 129 A Experimental results: VsH against Iy 143 B Spin-Orbit Interaction Induced Spin-Separation in Platinum Nanostructures C Giant spin Hall conductivity in platinum at room temperature 147 148 Summary We have demonstrated electrical generation and detection of spin polarization by the spin Hall effect in platinum The spin Hall effect refers to the generation of a transverse spin current, and the subsequent non-equilibrium spin accumulation near sample boundaries This occurs when a longitudinal electrical current is applied to materials with spin-orbit interaction Spin polarization in metals is usually small and requires ferromagnetic metals to create and detect spin polarization This ferromagnetic-based approach is suitable to be used as a laboratory investigation of spin transport phenomenon, but it limits the performance and scalability of the spintronics devices Therefore, we designed and experimentally demonstrated using a non-local lateral geometry structure to investigate the generation and detection of spin polarization based on the spin Hall effect, without the need for magnetic materials, external magnetic field, or bulky optical systems The geometry made use of the spin Hall effect effect to generate spin polarization and its reciprocal effect, the inverse spin Hall effect, to detect spin polarization A large spin Hall effect signal was observed from 10 K up to room temperature, which was the largest value reported so far in the literature The measurements were also done using gold and aluminum samples Aluminium failed vii Summary viii to demonstrate any signal while gold showed weak signals compared to platinum in spite of similar atomic number This suggested that the spin Hall effect in platinum was unusual The drift-diffusion model was found to be adequate to model the spin transport in platinum Based on the experimental results, the spin Hall effect in platinum was expected to be extrinsic However, the extrinsic contribution to the observed effect was not fully understood and further investigation is needed List of Tables 2.1 Theoretical Prediction of Spin Hall Conductivity σ xy 42 2.2 Experimental Measurement of Spin Hall Conductivity σxy 43 4.1 Measurement of Spin Hall Resistance RsH 100 4.2 Measurement of Misalignment Resistance Rmis and the Adjusted RsH 103 4.3 Summary of λs , σxy , and σyy 106 4.4 The effects of Shunt Resistance in Measurements 122 ix REFERENCES 134 [58] Wang, J., Wang, B., Ren, W., and Guo, H Physical Review B 74, 155307 (2006) [59] Sun, Q.-F., Xie, X C., and Wang, J W Physical Review B 77, 035327 (2008) [60] Silsbee, R H Bulletin of Magnetic Resonance 2, 284–285 (1980) [61] Jedema, F J., Filip, A T., and van Wees, B J Nature 410, 345–348 (2001) [62] Valenzuela, S O and Tinkham, M Nature 442, 176–179 (2006) [63] Fan, J and Eom, J Applied Physics Letters 92, 142101 (2008) [64] Elliott, R J Physical Review 96, 266–279 (1954) [65] Yafet, Y Solid State Physics Advances in 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against Iy 143 APPENDIX A EXPERIMENTAL RESULTS: VSH AGAINST IY 144 100 K 100 K 4.5 290 K 4.0 290 K 4.0 3.5 3.5 ( V) 2.5 sH 2.0 V V sH ( V) 3.0 1.5 3.0 2.5 2.0 1.5 1.0 1.0 0.5 0.5 0.0 0.0 I y 10 ( A) I y (a) 72 nm 10 ( A) (b) 74 nm 100 K 100 K 4.5 3.0 290 K 4.0 2.5 ( V) 3.0 2.0 1.5 sH 2.5 2.0 V V sH ( V) 3.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0 I y 10 ( A) I y (c) 82 nm 10 ( A) (d) 125 nm 100 K 0.7 100 K 290 K 290 K 1.2 0.6 1.0 ( V) 0.4 sH 0.3 0.8 0.6 V V sH ( V) 0.5 0.2 0.4 0.1 0.2 0.0 0.0 I y ( A) (e) 165 nm 10 I y 10 ( A) (f) 170 nm Figure A.1: Experimental results spin Hall voltage versus current for various length L APPENDIX A EXPERIMENTAL RESULTS: VSH AGAINST IY 145 100 K 100 K 290 K 1.2 290 K 1.2 ( V) 1.0 0.8 0.8 0.6 V V sH 0.6 sH ( V) 1.0 0.4 0.4 0.2 0.2 0.0 0.0 I y 10 ( A) I y (g) 193 nm 10 ( A) (h) 199 nm 100 K 100 K 0.45 290 K 1.4 0.30 ( V) 0.35 1.0 sH 0.8 0.6 V sH ( V) 1.2 V 290 K 0.40 0.4 0.25 0.20 0.15 0.10 0.05 0.2 0.00 0.0 I y 10 ( A) I y (i) 203 nm 10 ( A) (j) 314 nm 100 K 100 K 0.30 290 K 0.50 0.45 0.25 ( V) 0.35 0.30 sH 0.25 0.20 V V sH ( V) 0.40 0.15 0.20 0.15 0.10 0.05 0.10 0.05 0.00 0.00 I y ( A) (k) 323 nm 10 I y 10 ( A) (l) 332 nm Figure A.1: Experimental results spin Hall voltage versus current for various length L (cont.) APPENDIX A EXPERIMENTAL RESULTS: VSH AGAINST IY 100 K 146 100 K 0.14 0.5 0.12 0.4 ( V) ( V) 0.10 0.08 0.06 V sH 0.2 V sH 0.3 0.1 0.04 0.02 0.00 0.0 -0.02 I y 10 ( A) I y (m) 425 nm (n) 439 nm 100 K 0.10 100 K 0.10 Linear Fit of D 0.08 0.08 0.06 ( V) ( V) 0.06 0.04 sH 0.04 0.02 V sH V 10 ( A) 0.02 0.00 0.00 -0.02 -0.02 I y ( A) (o) 440 nm 10 I y 10 ( A) (p) 450 nm Figure A.1: Experimental results spin Hall voltage versus current for various length L (cont.) Appendix B Spin-Orbit Interaction Induced Spin-Separation in Platinum Nanostructures 147 Appendix C Giant spin Hall conductivity in platinum at room temperature 148 ... 129 A Experimental results: VsH against Iy 143 B Spin- Orbit Interaction Induced Spin- Separation in Platinum Nanostructures C Giant spin Hall conductivity in platinum at room temperature 147 148... which includes the spin- spin interactions will lead to the proper definition of the spin current It is pointed out that the spin- spin interactions provide a microscopic understanding of the spin. .. broken by inducing an internal electric field in the device structure by means of artificial heterostructures or quantum wells 2.2 Extrinsic Spin- Orbit Interaction Effect The extrinsic spin- orbit interaction