Credit: ©2007 APS

In a ferromagnetic nanodisk, the magnetization tends to swirl around in the plane of the disk and can point either up or down at the centre of this 'magnetic vortex'. This binary state could be useful for information storage. However, forcing the vortex core to flip from up to down can require a fairly large magnetic field or long switching times. Now, two separate teams look at different approaches to overcome these technological hurdles.

In a combination of experimental and theoretical work, Teruo Ono of Kyoto University and colleagues in Japan and France, show that an oscillating current in which the electron spins are polarized in a certain direction can switch the vortex core in a permalloy nanodisk (Nature Mater. 6, 269–273; 2007). The spin current exerts a torque on the vortex core, which, according to simulation, causes it to rotate and ultimately reverse in about 20 nanoseconds.

Meanwhile, physicists at the Research Centre in Jülich and the Max Planck Institute in Stuttgart, both in Germany have simulated what happens when a short magnetic field pulse is applied in the plane of the disk (Phys. Rev. Lett. 98, 117201; 2007). They find that a short pulse of about 60 picoseconds and 800 Gauss in strength (the earth's magnetic field is 0.5 Gauss) is enough to switch the vortex direction. Vortices disappear and are reformed where the direction of the in-plane magnetization changes sharply. The image is a simulation by the German group, which shows, starting at the top left, how a vortex (the point at the centre where the different colours meet) can be erased by a magnetic pulse and then reform elsewhere.

Taken together, the studies show that short magnetic field pulses provide faster switching, but electrical vortex switching may be more technologically viable. Bit by bit, however, magnetic vortices are becoming more promising candidates for information storage.