Indirect imaging of electron spins

has been shown to be feasible. Spintronic devices seek to exploit both the charge and the spin of mobile electrons, but determining precisely where spin flips occur is exceedingly difficult because of short electron transit times. Spin carriers can, however, get trapped by defects and dynamically transfer their spin polarization to nearby nuclei. Now, researchers working at the US Army Research Laboratory in Adelphi, Maryland, have used magnetic resonance force microscopy (MRFM; see Physics Today, May 1997, page 9 ) to image three different spin-polarized nuclei in a single 3-µm-thick sample of gallium arsenide. The researchers first induced nonuniform spin polarization in a narrowly confined region of the sample. Then, while varying an applied magnetic field, they observed the spin contrast of gallium-69, gallium-71, and arsenic-75, revealed by 2-pm deflections of their cantilever at the appropriate resonance field strengths. Prior to this work, MRFM had detected a total of four nuclear spins: hydrogen, fluorine-19, sodium-23, and cobalt-59. The physicists say that their observations open up the possibility of three-dimensional imaging of spin-polarized electron currents in operating spintronic devices. (K. R. Thurber et al., Appl. Phys. Lett. 80 , in press.)