Spintronics: Taming spin currents.

T aking advantage of the fact that electrons, photons and some elementary excitations have an intrinsic angular momentum (spin) offers a fascinating opportunity to study materials properties. Introducing a net imbalance between spins pointing in different directions (say, 'up' and 'down') is one way to probe phenomena including spin–orbit and hyperfine coupling, pairing of unconventional superconductors, and the quantum Hall effect 1. This spin imbalance also naturally lends itself to a wide range of applications, from commercially available computer hard drives and magnetic random access memories, to more forward-looking spin transistors, magneto-logic gates, spin-lasers and even spin-based quantum computing 1–3. An elegant realization of such an imbalance is offered by spin currents, which flow without any charge transfer. These currents can circumvent the constraints of conventional electronics and enable low-power and high-bandwidth information transfer. So there are several good reasons for the quest to tame these spin currents effectively. Two related breakthroughs, in different materials systems, are now reported in Nature Materials. Ando et al. 4 report surprising results for a robust spin imbalance in low-resistance ferromagnetic metal/semiconductor junctions. Using a similar technique to generate and detect spin currents, Kurebayashi et al. 5 reveal an intriguing way to enhance spin currents in ferromagnetic insulators. How can spin currents be generated? Traditional approaches include optical orientation using circularly polarized light 1,6 and electrical spin injection from magnetic contacts 1,2,7 (Fig. 1a,b). But by using the well-established phenomenon of ferromagnetic resonance 8 , Ando et al. 4 and Kurebayashi et al. 5 chose spin pumping as an elegant alternative way to generate spin currents 4,5,9. To understand the spin pumping mechanism, we can view the preferred orientation of spins and their associated magnetic moments in ferromagnetic materials as a macroscopic spin or a compass needle carrying angular momentum characterized by a corresponding net magnetization M (Fig. 1c). On applying a static magnetic field not aligned with M, the resulting torque forces M to precess with a frequency proportional to the magnetic field. Eventually, just like the compass needle, damping effects will align M with the magnetic field. By adding a small oscillating magnetic field (for example using microwaves 4,5), the damping effects can be cancelled, leading to a resonant condition with a constant-angle precession of M. Effectively, this excited ferromagnet is a spin pump that transfers angular momentum between the ferromagnetic (F) and non-magnetic material (N). The transfer is mediated by magnons …