Just because a magnet has a net moment—a north pole and a south pole—that doesn’t mean that all of its unpaired spins point in the same direction. More likely, several magnetic domains having different orientations will coexist, each separated from its neighbors by thin transition regions called domain walls.
And those domain walls don’t always stay put. They shift, for example, when an external magnetic field causes some domains to expand and others to shrink. Domain walls can also be displaced by spin-polarized current. As an electron passes from one domain to the next, it exchanges a spin-transfer torque with the intervening domain wall; the electron’s spin tilts in one direction and the domain wall’s spins tilt, ever so slightly, in the other. A current of electrons acting in concert can cause the wall to propagate.
Using current to move domain walls is potentially a convenient way to toggle a...
