Synopsis: Putting a damper on the spin-torque effect

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Illustration: Alan Stonebraker

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 [1]).

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

[1] J. Sun, Physics 1, 33 (2008).

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