Viewpoint Make your Spins Spin

Many devices used for computing, information storage, and other electronic applications rely on controlled manipulation of the spin angular momentum carried by electrons. Recent advances in spin-dependent phenomena hold promise for future technologies, but often these effects are hard to detect. In Physical Review Letters, Ronghua Liu and co-workers from Emory University in Atlanta, Georgia, combine two spin effects in a design that takes us closer to further insight and more applications of spin-dependent electronic (spintronic) phenomena [1]. One of the spin effects the authors use is spin transfer torque, which occurs when the spins of an electronic charge current rotate the magnetization orientation of a magnetically ordered material. The other effect used is the spin Hall effect, which occurs when electrons move through a conductor under the influence of an applied electric field. This, in turn, splits the electrons into spinup and spin-down populations that move towards opposite sides of the sample. With these effects, Liu et al. were able to detect magnetization dynamics using electrical transport measurements with devices (Fig. 1) that contained just one single ferromagnetic component—Permalloy (a magnetic nickel-iron alloy)—instead of the commonly used twocomponent systems. One of the main advances of this work is that compared to earlier experiments, this detection does not require sophisticated optical spectroscopy or complex device structures, therefore this experimental approach will be accessible to a wide range of research groups. This result should thus provide both a different approach for spintronic devices as well as a better understanding of magnetic dynamics. So, how does one achieve these spin effects? One common way of creating spin transfer torque is by passing a charge current through a ferromagnetic conductor, which induces a net spin polarization. When such a spinpolarized charge current subsequently enters another ferFIG. 1: In the device investigated by Liu et al.[1], a magnetic field H defines the stable equilibrium position of the magnetization M in the ferromagnetic Permalloy layer. The gold contacts enable a high charge-current density I in a small area of the platinum/Permalloy bilayer. The part of the current that flows through the platinum generates a spin current in the platinum (Pt) layer between the gold (Au) and Permalloy (Py) interfaces, with a net spin accumulation developing at the platinum/Permalloy interface. This spin accumulation reduces the magnetic damping in the adjacent ferromagnetic Permalloy layer, which in turn gives rise to spontaneous excitation of magnetization precession (wider cones). The resistivity changes associated with the time varying magnetization subsequently result in voltage changes and concomitant microwave generation at the rf frequencies characteristic for the magnetization precession. (APS/A. Hoffmann)