学术报告题目：Spintronics with pure spin currents

报告人：Prof. Yi Ji（University of Delaware）

邀请人：王金兰
时间：9月24号（周二）上午10:00am
地点：田家炳南楼203

Abstract:

Spintronics relies on the functionalities of spin currents. A pure spin current arises when electrons with opposite spins flow in opposite directions. Pure spin currents are less invasive and less dissipative compared to spin polarized charge currents. A pure spin current can be generated in a nonlocal spin valve (NLSV) and transported along the nonmagnetic channel of the NLSV. The interactions between the pure spin currents and a nanoscale magnetic element (a spin detector) yield spin-dependent voltage signals as well as spin-transfer switching and dynamics. In this talk, I will discuss spin transport phenomena in submicron metallic NLSV structures.

The spin diffusion lengths and interfacial spin polarizations are crucial for high quality NLSV structures. I will show that the spin relaxation process in the nonmagnetic channel is dominated by surface slip scattering from magnetic impurities. The spin-flip rate can be reduced by oxidizing the surface impurities in air. An aluminum oxide barrier (nominally 2 nm thick) between the ferromagnet and the nonmagnetic channel provides a substantial interfacial polarization. The absorption of a pure spin current into the ferromagnet is asymmetric across the finite-size AlO_x junctions. This asymmetry leads to an unconventional approach for detecting nonlocal spin accumulation. Spin-transfer effects have been achieved by pure spin currents in a broad range of temperatures from 4.5 K to 200 K. Both full magnetization reversals and evidence for magnetization dynamics have been observed.

A nanometer-sized break-junction gap can be formed between the spin detector and the nonmagnetic channel in a NLSV by electromigration. Large spin signals with both signs (non-inverted or inverted) have been detected. The spin signals are truly nonlocal and are consistent with the delicate nature of a break junction. Theoretically a spin-charge coupling effect across the resistive break-junction leads to a large chemical potential split, which provides conduction channels for pure spin currents across the interface. Therefore an interface which is resistive to a charge current can be actually conductive to a spin current. The signs and magnitudes of the spin signals can be understood by considering the profiles of the electrochemical potentials across the interface on the scale of the charge screening length.

Yi Ji教授简介：

Education

・ Ph.D. in Physics, 2003, the Johns Hopkins University.

・ B.S. in Physics, 1997, Peking University.

Professional Experiences

・ Associate Professor of Physics, University of Delaware, Sept. 2012 - present

・ Assistant Professor of Physics, University of Delaware, Sept 2006 � Aug. 2012

・ CNM distinguished postdoctoral fellow, Argonne National Lab, July 2003 � Aug. 2006

・ Research assistant, Johns Hopkins University, 1998 � 2003

・ Teaching assistant, Johns Hopkins University, 1997 � 2000

Research interests

Spintronic materials, spin-dependent electron transport in nanoscale systems, and prototype spintronic devices.

Selected Publications

1. Han Zou and Yi Ji “The origin of high surface spin-flip rate in metallic nonlocal spin valves”, Applied Physics Letters 101, 082401 (2012).

2. Han Zou, Shuhan Chen, and Yi Ji “Reversal and excitations of a nanoscale magnetic domain by sustained pure spin currents", Applied Physics Letters 100, 012404 (2012).

3. Han Zou and Yi Ji “Temperature evolution of spin-transfer switching in nonlocal spin valves with dipolar coupling”, Journal of Magnetism and Magnetic Materials, accepted (2011).

4. Han Zou, S. T. Chui, X. J. Wang, and Yi Ji “Large and inverted spin signals in nonlocal spin valves”, Physical Review B 83, 094402 (2011).

5. X. J. Wang, H. Zou, and Y. Ji, "Increase of nonlocal spin signal at high d.c. bias due to a redistribution of the injection current", Physical Review B 81, 104409 (2010).

6. X. J. Wang, H. Zou, L. E. Ocola, and Y. Ji, "High spin injection polarization at an elevated dc bias in tunnel-junction-based lateral spin valves", Applied Physics Letters 95, 022519 (2009).

7. X. J. Wang, H. Zou, and Y. Ji, "Spin transfer torque switching of cobalt nanoparticles", Applied Physics Letters 93, 162501 (2008).

8. Y. Ji, A. Hoffmann, J. E. Pearson, and S. D. Bader, “Enhanced spin injection polarization in Co/Cu/Co non-local lateral spin valves”, Applied Physics Letters, 88, 052509 (2006).

9. Y. Ji, A. Hoffmann, J. S. Jiang, and S. D. Bader, “Spin injection, diffusion and detection in lateral spin-valves”, Applied Physics Letters 85, 6218 (2004).

10. T. Y. Chen, Y. Ji, C. L. Chien, and M. D. Stiles, “Current-driven switching in a single exchange-biased ferromagnetic layer”, Physical Review Letters 93, 026601 (2004).

11. T. Y. Chen, Y. Ji, and C. L. Chien, “Switching by point-contact spin injection in a continuous film”, Applied Physics Letters, 84, 380 (2004).

12. Y. Ji, C. L. Chien, and M. D. Stiles, "Current-induced spin-wave excitations in a single ferromagnetic layer", Physical Review Letters 90, 106601 (2003).

13. Y. Ji, C. L. Chien, Y. Tomioka, and Y. Tokura, "Measurement of spin polarization of single crystals of La_0.7Sr_0.3MnO₃ and La_0.6Sr_0.4MnO₃", Physical Review B. 66, 012410 (2002).

14. Y. Ji, G. J. Strijkers, F. Y. Yang, and C. L. Chien, "Comparison of two models for spin polarization measurements by Andreev reflection", Physical Review B. 64, 224425 (2001).

15. Y. Ji, G. J. Strijkers, F. Y. Yang, C. L. Chien, J. M. Byers, A. Anguelouch, Gang Xiao, and A. Gupta, "Determination of the spin polarization of half-metallic CrO₂ by point contact Andreev reflection", Physical Review Letters 86, 5585 (2001).

16. G. J. Strijkers, Y. Ji, F. Y. Yang, C. L. Chien, and J. M. Byers, "Andreev reflections at metal/superconductor point contacts: Measurement and analysis", Physical Review B. 63, 104510 (2001).