Magnets Fast and Small

When you place a magnetic dipole, such as an electron or a ferromagnetic domain, in a magnetic field, it will align with the field. It will, however, not be able to do so instantaneously: To reach the new magnetization direction, it will have to spin about the field axis, a process called precessional motion that is gradually opposed by damping. In 1956, Kikuchi (1) gave the first theoretical answer to the question of the minimum time needed to reverse a dipole owing to this damped precessional motion. But more than 40 years had to elapse before the minimal field strength required for precessional magnetization reversal could be evaluated experimentally. Last year, Back et al. showed (2) in a series of spectacular experiments using picosecond field pulses that when an impinging electron beam crosses a cobalt layer with in-plane magnetization, the magnetic field H created by the beam current leads to magnetization reversal of those regions whose magnetization is initially orthogonal to the field direction. This is a rather counterintuitive result, as common sense would suggest that a field applied antiparallel to the magnetization direction would be most effective in achieving reversal. These experiments thus provided a direct proof of precession as the primary source of magnetization rotation.