Spin-torque—the torque induced on a magnetic layer due to a spin-polarized electric current passing through it—may lead to ways to control the operation of nanoscale magnetic devices. However, it is also a source of noise and instability in the future designs of magnetic recording read-heads that are only several tens of nm in size (see ).
Writing in Physical Review Letters, Neil Smith, Stefan Maat, Mathew Carey, and Jeffrey Childress of Hitachi Global Storage Technologies in San Jose, California, have found a way to increase the critical current for the onset of spin-torque induced oscillations, which could reduce this problem.
Smith and colleagues study a variant of a spin-valve stack: essentially a ferromagnetic layer with a fixed magnetization, a nonmagnetic spacer, and a second ferromagnetic layer in which the magnetization is free to rotate. An electric current passing perpendicular to the stack is polarized by one magnetic layer and this polarized current generates a spin-torque on the other layer. Beyond a critical current in either current direction, the spin-torque can induce instability oscillations in the free layer.
The Hitachi group modifies the free layer by adding a second, thinner ferromagnetic layer, separated from the first by a ruthenium spacer that mediates a strong antiparallel coupling between them. With this “synthetic-ferrimagnet,” the magnitude of the critical current increases by several fold, but only for one direction of electric current. This asymmetry is attributed to a spin-torque-induced coresonance of the two natural modes of oscillation of the synthetic ferrimagnet, which efficiently transfers energy out of the destabilized mode into the stable one. – Jessica Thomas
 J. Sun, Physics 1, 33 (2008).