Among the various strategies to achieve novel non-volatile memory applications with increased speed and storage capability, there is increasing interest in the possibilities offered by magnetic field- or electrical current-based control of magnetic domain walls — interfaces between regions of different magnetization — in ferromagnetic stripes. One example, the ‘racetrack’ memory concept, is based on recording bits as domain walls pinned by little indentations, or notches, in narrow stripes.

Sheng-Huang Huang and Chih-Huang Lai from the National Tsing Hua University in Taiwan1 now describe the fundamental details of the pinning and depinning mechanisms in ferromagnetic stripes less than 200 nm wide. “To increase memory density, the feature sizes of the magnetic nanostripe must to be reduced,” Lai explains. For narrow stripes, however, the pinning of the domain walls tends to be very stable. Finding ways to increase the depinning probability is therefore essential.

In their studies, the two scientists first used micromagnetic simulations to examine how the domain walls move when an electrical current flows through the stripe. The main result, which partly confirms previous knowledge, was that for currents below a certain threshold, the domain walls adopted a transverse orientation, perpendicular to the stripe, and moved with constant velocity. At higher currents however, the velocity oscillated and the walls alternated between transverse and anti-vortex types.

Fig. 1: Schematic diagram showing the domain wall configuration in the vicinity of a notch for different conditions of current and notch distance.

The second step was to study the motion for varying currents and distances from a notch, and to examine the probability for the wall to be pinned or depinned (Fig. 1). The picture emerging from the various simulations is that the wall is more likely to be depinned if, when it reaches the notch, it is in the anti-vortex configuration.

“We revealed that the anti-vortex wall stores more potential energy, and this energy helps the domain wall to be depinned from the notch,” says Lai. The results could prove essential in the design of high-density magnetic storage or memory devices.