Switching a nanomagnet is all in the timing

Abstract

Switching a nanomagnet is all in the timing Jonathan Sun IBM Research Division, Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, USA Published November 3, 2008 If a magnet is small enough, an electric current carrying polarized spins can flip it around. Scientists are finding clever ways to control this spin-torque effect precisely, both for when it is wanted and when it is not. A Viewpoint on: Coherent control of nanomagnet dynamics via ultrafast spin torque pulses Samir Garzon, Longfei Ye, Richard A. Webb, Thomas M. Crawford, Mark Covington, and Shehzaad Kaka Phys. Rev. B 78, 180401 (2008) - Published November 03, 2008 Download PDF (free) Illustration: Alan Stonebraker/stonebrakerdesignworks.com Separate Window | Enlarge Figure 1: The spin-torque effect in a nanomagnet. (top left) If the polarization of the spins in the current (ns, green arrow) is parallel to the easy axis (black arrow), the spin torque opposes the natural damping and opens up the angle of the precession cone at all points along the cone. (top right) If ns makes a finite angle with the easy axis, the spin torque opens the precession cone angle around a half circle centered at point Q, but closes the cone angle in the other half circle centered at point P. Applying two successive spin-torque pulses either both at point Q, or at points P and Q, increases or decreases the net effectiveness of the spin torque in inducing a dynamic switch. (bottom) Schematic of the nanopillar structure. The free layer is the right layer and the current is applied along the horizontal axis of the pillar. The polarization of the current (green arrows) also rotates as a result of the spin-torque effect. When a spin-polarized current passes through a ferromagnet, it induces a torque on the ferromagnetic moment, an effect referred to as spin-transfer torque, or spin torque for short [ ] (see Fig. 1). This ability to flip a magnet with a current directly - rather than indirectly with a magnetic field - opens up a number of technological possibilities for magnetic memory and compact microwave oscillators [, ]. The spin-torque effect can also be a bit of a nuisance in some devices because it amplifies the thermal noise in a nanomagnet [,, ]-a problem for magnetic read heads. At present, many groups are exploring how to precisely control nanomagnets with a current pulse. Samir Garzon and colleagues at the University of South Carolina, in collaboration with Seagate Research, report [ ] they can control the switching of a nanomagnet with a pair of ultrafast, optically generated current pulses better than with the single long current pulses that have been tried in the past. The results, reported in a Physical Review B Rapid Communication, open a new route to nanomagnetic switching.