Image: B.C.Choi/M.R.Freeman/University of Alberta
Despite the fact that a great many storage devices--among them computer hard drives and cassette tapes--rely on magnetic materials, scientists don't really know how they operate on a microscopic scale. During writing and re-writing, for instance, individual regions within the materials called domains reverse the directions in which they are magnetized. And many factors--including temperature and defects--can influence which way the domains flip. Now scientists have found through close examination that external magnetic fields can be used to coordinate and accelerate some of these flips--perhaps clearing the way to new and improved read/write heads and greater storage density.
Mark Freeman and colleagues at the University of Alberta used a kind of high-tech, superfast strobe light to watch the magnetization reversals in action. The instrument--a scanning Kerr microscope--bounced femtosecond pulses of laser light off the surface of rectangular Ni80Fe20 samples, measuring 15 nanometers thick and grown on small strips of gold. The surface magnetism of the samples altered the polarization of the light pulses, providing the information needed to create images of their states.
The scientists first applied a magnetic field running parallel to the length of a sample--a so-called longitudinal bias field--to align all of its atomic spins, and then sent a quick flash of current through the gold strip. This switching pulse temporarily created a second magnetic field in the opposite, tranverse direction from the first. What they found was that, exposed to the longitudinal bias field, the samples' magnetization responded to the current after a 3.5 nanosecond lag (top image). But when it flipped back, under the influence of the fleeting transverse field, the magnetization settled within one nanosecond (bottom image). Without the transverse field, the researchers explain, the magnet switches only as tiny areas flip and eventually join together--hence the delay; with it, though, they all seem to turn at once.