Domain walls on the fast track

Magnetic moments don’t necessarily point in the same direction everywhere in a ferromagnet. More often, domains of different orientations coexist, separated by thin domain walls. Moving those walls with spin-polarized current is potentially a convenient way to write bits to magnetic random-access memory or to shuttle sequences of bits to and fro in three-dimensional memory devices. But such applications require that domain walls be moved quickly and with minimal current. Unfortunately, the materials best suited to yield such highly mobile domain walls are also the most susceptible to Walker breakdown, a turbulence-triggering instability that slows domain-wall speeds to a crawl. Now, researchers led by Gilles Gaudin and Ioan Mihai Miron of Spintec laboratory in Grenoble, France, have figured out a way around that problem. They crafted 500-nm-wide nanowires consisting of cobalt, the active ferromagnetic layer, sandwiched between platinum and aluminum oxide, as shown here. The resulting inversion asymmetry produces an out-of-plane electric field that gives rise to a fortuitous spin–orbit coupling: As electrons pass along a nanowire, their spins tend to tilt to one side, producing a magnetic torque that stabilizes the domain wall even at large current densities. Unconstrained by Walker breakdown, the domain walls reached speeds of up to 400 m/s, more than fast enough for memory applications. The researchers say they’ll now work toward achieving comparable speeds with less current. (I. M. Miron et al., Nat. Mater. 10, 419, 2011.)—Ashley G. Smart