Spintronics is a branch of physics and nanoscience concerned with the storage and transfer of information by means of electron spins in addition to electron charge as in conventional electronics. The birth of spintronics can be traced to basic research on nanoscale ferromagnet/normal-metal multilayers in late 1980s, as recognized by the Nobel Prize in Physics 2007 awarded for the discovery of giant magnetoresistance (GMR). The GMR phenomenon also exemplifies one of the fastest transfers of basic research into applications where in less then ten years since its discovery it has revolutionized information storage technologies by enabling 100 times increase in hard disk storage capacity.
Unlike early non-coherent spintronics phenomena (such as GMR), the major themes of the second-generation spintronics are exploiting quantum-coherent spin states where spin component persists in the direction transverse to external or effective internal magnetic fields. Examples of such phenomena in metal spintronics are spin transfer torque (spin current of large enough density injected into a ferromagnetic layer either switches its magnetization from one static configuration to another or generates a dynamical situation with steady-state precessing magnetization) and spin pumping (moving magnetization generates pure spin current with no applied bias voltage and accompanying net charge current). In semiconductor spintronics recent second-generation experiments have succed in: transporting coherent precessing spins across 100 micron thick silicon wafers; detecting direct and inverse spin Hall effect; and manipulating localized coherent spins as building blocks of futuristic solid-state-based quantum computers. A major teme of second-generation spintronics is exploration of various quantum phenomena that can be exploited to generate pure spin currents as a situation with no net charge current (illustrated by the top Figure on the right).
The Spintronics group at Delaware, which also operates DOE EPSCoR funded Center for Spintronics and Biodetection with intedisciplinary connections accross the campus and specially funded graduate student and postdoctoral positions, is involved in the following second-generation spintronics projects:
- fabrication of magnetic tunnel junctions (MTJ) with MgO or AlO insulating barriers (Xiao, Nowak)
- spin pumping and high frequency spin dynamics in MTJs (Xiao, Nikolic)
- ferromagnetic nanorings (Xiao)
- spin-dependent shot noise (Nowak, Nikolic)
- MTJ sensors for biological applications (Xiao, Nowak, Kolodzey)
- spin transfer torque driven by pure spin current (Ji)
- pure spin currents in non-local spin valves (Ji)
- spin Hall effect in multiterminal nanostructures (Nikolic)
- spins in quantum dots (Dotty)
- theory of spin-polarized transport and pure spin currents (Chui, Nikolic)
- topological insulator-based spintronic devices (Nikolic).
Campus-wide Interdisciplinary Collaboration:
- James Kolodzey (Dept. of Electrical and Computer Engineering)
- Matthew F. Doty (Dept. of Materials Science and Engineering)
F. Mahfouzi, J. Fabian. N. Nagaosa, and B. K. Nikolić, Charge pumping by magnetization dynamics in magnetic and semi-magnetic tunnel junctions with interfacial Rashba or bulk extrinsic spin-orbit couplings, Phys. Rev. B 85, 054406 (2012). [PDF]
B. K. Nikolic, L. P. Zarbo, and S. Souma, Spin currents in semiconductor nanostructures: A nonequilibrium Green function approach, Chapter 24, pages 814–866 in Volume I of "The Oxford Handbook on Nanoscience and Technology: Frontiers and Advances," Eds. A. V. Narlikar and Y. Y. Fu (Oxford University Press, Oxford, 2010). [PDF]
B. K. Nikolic and R. L. Dragomirova, What can we learn about the dynamics of transported spins by measuring shot noise in spin-orbit-coupled nanostructures?, Semicond. Sci. Tech. 24, 064006 (2009), review article for the special issue on "The effects of spin-orbit interaction on charge transport." [PDF]
S.-H. Chen, C.-R. Chang, J. Q. Xiao, and B. K. Nikolic, Spin and charge pumping in magnetic tunnel junctions with precessing magnetization: A nonequilibrium Green function approach, Phys. Rev. B 79, 054424 (2009). [PDF]
T. Moriyama, R. Cao, X. Fan, G. Xuan, B. K. Nikolic, Y. Tserkovnyak, J. Kolodzey, and John Q. Xiao, Tunnel barrier enhanced voltage signal generated by magnetization precession of a single ferromagnetic layer, Phys. Rev. Lett. 100, 067602 (2008). [PDF]
X. J. Wang, H. Zou, and Y. Ji, Spin transfer torque of cobalt nanoparticles, Appl. Phys. Lett. 93, 162501 (2008). [PDF]
A. S. Edelstein, G. A. Fischer, M. Pedersen, E. R. Nowak, Shu Fan Cheng, and C. A. Nordman, Progress toward a thousandfold reduction in 1/f noise in magnetic sensors using an ac microelectromechanical system flux concentrator (invited), J. Appl. Phys. 99, 08B317 (2006). [URL]