Magnetic pins beat somersaults

Researchers are discovering new possibilities for magnetic storage

Dynamic switching of the vortex core: In the upper part, the "magnetic needles" of the vortex core are shown schematically, left with orientation down, right up. The lower part shows these two magnetization directions of the vortex core in two images taken with a magnetic scanning X-ray microscope at the Advanced Light Source in Berkeley, California, USA. In the middle of the bipolar magnetic field pulse is shown (250 MHz, in the peak 1.5 milli-Tesla), which causes the switching of the vortex core. © Max Planck Institute for Metals Research
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Scientists have discovered a new mechanism to switch small magnetic structures - so-called vortex nuclei - with weak magnetic fields quickly and without loss. So far you needed to very strong magnetic fields, which means a great technical effort. The new method, now published in Nature, may open up new opportunities in magnetic data storage.

The smallest magnetic structures, measuring a few millionths of a millimeter, have been seeing growing interest in science and technology for about ten years, mainly because of potential magnetic memory applications. In such structures, a fascinating quantum mechanical phenomenon occurs: The vortex core, which has been theoretically predicted for 40 years, but could only be detected in the experiment four years ago. In small magnetic platelets, the magnetized regions often join together to form planar closed magnetic circuits called vortices (singular: vortex).

Vortex core as magnetic storage?

If you imagine you're going to walk in a vortex with an atom-sized compass, the compass needle would always point to the plane - unless you're approaching the center of the vortex, its core: the atomic magnetic compass needles are rising there the surface and it arises in the smallest space (on a radius of about 20 atoms) the largest magnetic field possible in the material.

The magnetic needle can point either up or down in the vortex core. If one wants to use this orientation for magnetic data storage, one has to contend with the tremendous stability typical of vortex structures: So far, very high external magnetic fields of about half a tesla were needed to reverse the orientation of the vortex core. That's about one-third of the field that the strongest permanent magnet can deliver.

Vortex and anti-vortex combined

Researchers at the Max Planck Institute for Metals Research in Stuttgart have now found an elegant solution that makes it much easier to switch vortex cores. Using time-resolved magnetic scanning X-ray microscopy developed by the group of Hermann Stoll at the Institute, they discovered a previously unknown mechanism: the dynamic switching of the vortex core. By means of a short magnetic pulse, first of all a magnetic field is built up perpendicular to the vortex; this stimulates the whole structure into a collective movement of spins. display

(a) initial state: vortex core down; (b) first, an additional magnetization is visible upwards; (c) From this, a double peak forms: a vortex-antivortex pair; (d) Final state: After elimination of the original vortex by the antivortex, a vortex core remains uppermost. Max Planck Institute for Metals Research

Thus, as shown by micromagnetic simulations, magnetization is formed in the opposite direction at the edge of the original vortex - almost without energy expenditure. This creates a vortex-antivortex pair. The antivortex eliminates the original vortex, leaving only a vortex of opposite polarization.

Needle direction as a magnetic bit

Together with researchers from the University of Ghent, the Advanced Light Source in Berkeley, California, the Forschungszentrum J lich and the Universities of Regensburg and Bielefeld, the Max Planck scientists rounded off the vortex core 300 times weaker, but very short magnetic pulses effectively and targeted to switch.

It is possible that this switching mechanism, which was first observed, can be used for a completely new magnetic storage concept. The directions of the small, nanoscopic magnetic needles define a digital bit that is extremely stable against often unavoidable external influences such as heating or disturbing magnetic fields. With the newly discovered dynamic effect, the vortex core can be switched easily, almost without loss and, above all, extremely fast.

(MPG, 27.11.2006 - NPO)