Nanomagnetism and Spintronics:
Since mid-1990s the majority of Buhrman’s research activities have been in spintronics and thin film nanomagnetism. Contributions in this area include the pioneering, definitive demonstration of reversible spin transfer torque switching of thin film nanomagnets[i], the ability of spin transfer torque to excite nanoscale magnetic structures, including magnetic vortices[ii], into persistent microwave precession, and the early demonstration that spin torque can be successfully utilized with magnetic tunnel junctions[iii], despite the rather high tunnel current densities that are required. These works played a seminal role in the initiation of the current worldwide industrial effort to realize a new high performance magnetic memory replacement for CMOS memory based on spin transfer torque.
In the course of this spintronics research, several years ago the Buhrman research group and its collaborators discovered a giant spin Hall effect in Pt[iv], Ta and W thin films[v],[vi] where spin-orbit interactions result in the generation of robust pure spin currents transverse to the direction of an applied electrical current, and successfully applied it in a novel 3-terminal spin torque device that has potential for high performance magnetic memory and other spintronics applications. Buhrman’s group continues to study and works to enhance spin-orbit generated torques in hybrid ferromagnet/normal metal thin film systems and to further develop these phenomena for technological applications.
[i] J. A. Katine, F. J. Albert, R. A. Buhrman, E. B. Myers, D. C. Ralph, Current-driven magnetization reversal and spin-wave excitations in Co/Cu/Co pillars. Phys Rev Lett 84, 3149 (2000)
[ii] V. S. Pribiag, I. N. Krivorotov, G. D. Fuchs, P. M. Braganca, O. Ozatay, J. C. Sankey, D. C. Ralph, R. A. Buhrman, Magnetic vortex oscillator driven by d.c. spin-polarized current. Nat Phys 3, 498 (2007)
[iii] G. D. Fuchs, N. C. Emley, I. N. Krivorotov, P. M. Braganca, E. M. Ryan, S. I. Kiselev, J. C. Sankey, D. C. Ralph, R. A. Buhrman, J. A. Katine, Spin-transfer effects in nanoscale magnetic tunnel junctions. Appl Phys Lett 85, 1205 (2004)
[iv] L. Q. Liu, T. Moriyama, D. C. Ralph, R. A. Buhrman, Spin-Torque Ferromagnetic Resonance Induced by the Spin Hall Effect. Phys Rev Lett 106, 036601 (2011)
[v] L. Q. Liu, C. F. Pai, Y. Li, H. W. Tseng, D. C. Ralph, R. A. Buhrman, Spin-Torque Switching with the Giant Spin Hall Effect of Tantalum. Science 336, 555 (2012)
[vi] C. F. Pai, L. Q. Liu, Y. Li, H. W. Tseng, D. C. Ralph, R. A. Buhrman, Spin transfer torque devices utilizing the giant spin Hall effect of tungsten. Appl Phys Lett 101, 122404 (2012)
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