Physicists in Europe and the US have developed a new technique for reversing the "core polarization" of the tiny magnetic whirlpools that exist in certain materials. The reversal, which can be achieved with much smaller magnetic fields than previous brute-force methods, could allow vortices to one day be used as a high-density medium for information storage (Nature 444 461).
Magnetic vortices occur in very small disks of magnetic material that are typically less than a few micrometres in diameter. Boundary effects at the edges and surfaces of the disk conspire to cause the magnetic field to “curl” around in a whirlpool-like structure, before popping out of the disk at the centre of the vortex. Vortices exist in one of two curl states – clockwise and anti-clockwise.
For each curl state, the magnetic field at the centre of the vortex can also point either up or down. These “core polarization” states can be flipped by applying a magnetic field. Such states are therefore a promising candidate for the high-density storage of digital information with a “0” corresponding to “up” and a “1” to “down”, for example.
Until recently, polarization states were flipped by applying a very strong magnetic field of about 0.5 T in the direction perpendicular to the surface of the disk. While this illustrates how stable a vortex-based computer memory would be to stray magnetic fields, 0.5 T is about 500 times too strong to be deployed in a practical data-storage device.
But now a completely new technique for vortex flipping has been devised by Bartel Van Waeyenberge of Ghent University in Belgium along with colleagues in Germany, the US and Austria. Instead of delivering a strong perpendicular field, they applied an oscillating (250 MHz) and very weak (0.1 mT) magnetic field along the surface of a disk of a soft magnetic material called Permalloy. This does not flip the vortex, but rather causes it to gyrate back and forth on the disk (see figure “When vortices collide”). By deliberately boosting the field to 1.5 mT for one period of oscillation (about 4 ns) an “anti-vortex” structure was made to appear along with a new vortex with opposite core polarization than the original vortex. Then, the original vortex and the anti-vortex annihilate each other leaving only the new vortex – thereby completing the flipping process. The vortex could then be flipped back by applying an identical pulse.
According to Van Waeyenberge, this flipping process could be adapted to write data in a high-density data storage device. He told Physics Web that the group are now trying to work out a practical way of reading the state of a vortex bit – but he would not elaborate on specific avenues of investigation. In this current work the researchers determined the state by measuring the direction of the magnetic field that pops out of the centre of the vortex. This core region of the vortex only measures about 10 nm across and in this experiment it was probed using a scanning transmission x-ray microscope connected to the Advanced Light Source accelerator at the Lawrence Berkeley National Laboratory in California – something that would not easily fit within a memory chip.
