A quick switch for magnetic needles

Magnetic vortex cores, which can be used as particularly stable storage points for data bits, can now be switched much faster

April 12, 2011

Microscopically tiny ferromagnetic platelets exhibit a phenomenon which could be exploited in the future for particularly stable magnetic data storage: so-called magnetic vortex cores. These are needle-shaped magnetic structures measuring 20 nanometres (millionths of a millimetre) in diameter. Five years ago, researchers at the Max Planck Institute for Intelligent Systems (formerly the Max Planck Institute for Metals Research) in Stuttgart found a way to reverse the magnetic field needles despite their stability using only a tiny amount of energy so that their tips pointed in the opposite direction. Such a switching process is necessary to enable the vortex cores to be used in data processing. The Stuttgart scientists have now discovered a new mechanism which makes this switching process at least 20 times faster and confines it to a far smaller region than before. Magnetic vortex cores could thus provide a means of data storage which is stable, fast and greatly miniaturized.

The new switching mechanism allows more compact storage

There are ingenious mechanisms, however, which make it possible to switch the vortex core with smaller magnetic fields without losing the magnetic stability. Five years ago, the Stuttgart researchers working with colleagues at the University of Gent, the Advanced Light Source in Berkeley, California, the Jülich Research Center and the Universities of Regensburg and Bielefeld, found a way of dynamically switching the vortex core with a magnetic field pulse which is 300 times weaker. The scientists observed this previously unknown mechanism using so-called time-resolved magnetic scanning X-ray microscopy, which was developed at the Stuttgart Max Planck Institute and carried out at the Advanced Light Source in Berkeley.

A quick switch for bits: The new switching mechanism works at 5.0 gigahertz and makes it possible to reverse the polarity of a magnetic vortex core shown in an inner sample detail within 0.2 nanoseconds. The new mechanism is thus twenty times faster than the old one at 0.24 gigahertz.

Since then, the department Schütz at the Max Planck Institute for Intelligent Systems has worked in close collaboration with the Helmholtz Zentrum at BESSY II in Berlin on developing a new type of scanning X-ray microscope. The Stuttgart physicists, together with researchers from the Universities of Gent and Regensburg, have now discovered a further mechanism which can be used to switch a vortex core at least 20 times faster than was previously the case, i.e. within around 200 picoseconds; it involves the researchers applying a magnetic field pulse in the gigahertz range. Five years ago they used a pulse in the megahertz range and the switching took four nanoseconds.

The newly discovered mechanism generates so-called spin waves, i.e. fluctuations in the magnetization of the material which propagate like waves. As the team of scientists found out, these excitations are able to switch the vortex core. The group of researchers also succeeded in describing the theory of this phenomenon. “We can predict that it is possible to decrease the switch-over time by a further factor of 10,” says Hermann Stoll.

If the aim is to use a vortex core as a storage bit, the effect now observed has a further advantage in addition to its higher switching speed. The vortex core remains almost stationary when the switching is done with spin waves. When switched more slowly with frequencies in the megahertz range, as was the case with the mechanism discovered five years ago, on the other hand, it had to be moved far from its equilibrium position; this meant that a storage bit required more space. The new mechanism thus makes further miniaturisation possible if data storage media which operate according to this principle are to be developed in the future.

CM/PH

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