Imaging electrical spin injection/transport in spintronics devices: Scott Crooker (Los Alamos)
This was another cool (and very understandable) talk based on recent work on trying to understand the physical processes involved in spin injection and transport in lateral feromagnet/semiconductor structures. In a seminal paper in 1990, Datta (no relation to me) and Das proposed one of the earliest versions of a spin-FET: that is, a field-effect transistor made from doped silicon (versus the carbon nanotube FETs we make in our lab on a regular basis) with ferromagnetic contacts. The point, of course, is that the functionality of the device is to come not from coupling to the charge of the electron, but to its spin degree of freedom. It’s just a very cool idea, and people have taken it pretty far since then (although I’m not sure that industry will be ’switching’ to spin-based transistors anytime soon. Get it - switching to transistors? Hilarious.) Although many, many proposals currently exist, they all need certain things: a way to electrically inject spin-polarized electrons into the semiconducting channel, a way for these spins to be transported, a way to controllably manipulate the spins (i.e. with an external field), and a way to electrically detect this spin-polarized current. In particular, one of the key ways of confirming this electrical detection is using the ‘Hanle effect’ due to precession and dephasing of the spins in a transverse field.
Although this had been observed in all-metal devices (including the channel), a number of subtleties prevented a similar observation in semiconductor devices, until Crooker et al.’s work. What they did was use scanning Kerr rotation microscopy (using a continuous-wave (cw) probe laser and a sample resting on the cold finger of an optical cryostat, with an applied transverse field) to measure the out-of-plane component of the spin, and sure enough, they were able to obtain a Hanle signal. They extended (and continue to extend) this in a number of ways, comparing their data to a drift-diffusion model, injecting spins optically and seeing how the conductance changes, and even studying the effect of an applied strain (which interestingly leads to a term in the Hamiltonian that looks like a Rashba spin-orbit interaction with E replaced by the strain). A number of questions remain to be answered, but this work represents an interesting step forward.
Further reading…
- S. A. Crooker et al., “Imaging Spin Transport in Lateral Ferromagnet/Semiconductor Structures“, Science 309 2191 (2005), X. Lou et al., “Electrical Detection of Spin Accumulation at a Ferromagnet-Semiconductor Interface“, PRL 96, 176603 (2006), and X. Lou et al., “Electrical Detection of Spin Transport in Lateral Ferromagnet-Semiconductor Devices“, Nature Physics 3, 197 (2007).
- Some of the first work on strain-induced effects: G. L. Bir and G. E. Pikus, “Symmetry and strain-induced effects in semiconductors” (Wiley, 1974) - can be found on BorrowDirect.
- First spin-FET: S. Datta and B. Das, “Electronic analog of the electro-optic modulator“, APL 56, 665 (1990).
- Somewhat related: chapter 2 of D. D. Awschalom, D. Loss and N. Samarth eds., “Semiconductor Spintronics and Quantum Computation” (Springer 2002).
