Magnetic swirls

The 2007 physics Nobel Prize awarded achievements in the field of magnetism. When they started their fundamental research, the laureates Albert Fert and Peter Grünberg did certainly not foresee in how little time their results would be used for everyday applications in computer hard disks’ drives.

Dr. Karsten Küpper and Dr. Jürgen Fassbender from the Forschungszentrum Dresden-Rossendorf (FZD) tackle similar fundamental questions concerning the physics of magnetism whose potential applications are unpredictable today. More precisely, they study magnetic vortices, which are like magnetic swirls on the nanoscale (one nanometer is the billionth part of a meter). These magnetic cores, located in the center of the magnetic swirl, have a size of only about 10 nanometers and a very stable magnetization. Hence, experts consider them as potential candidates for future non volatile magnetic memories.

Today researchers study the basic physical phenomena of magnetic vortices, observed experimentally for the first time only a few years ago. A vortex can be described as a round, thin ferromagnetic disc with a diameter of only a few micrometers showing a circular magnetization, to some extent similar to the wind in a tornado. In the center of the disk a very small core of about 20 atoms only exhibits a perpendicular magnetization (like the eye of a tornado storm points towards the earth). Applying a magnetic field to a magnetic vortex pushes the vortex away from the center of the disk towards the frame. If one then turns the field off abruptly, the vortex moves either clockwise or counter clockwise on a spiral like trajectory back into its initial position in the center of the disk. This special movement is called gyration. In principal, the perpendicular magnetization of the vortex core can point either upwards or downwards, and four different kinds of movement can be found: right- and left rotating magnetic swirls, combined either with an up- or downward directed perpendicular core magnetization.

Analogous to any other physical particle or particle like property one can find an anti-particle, i.e. an antivortex in the present case. The physicists of the FZD could now tackle the dynamic magnetic properties of two vortices and an antivortex, i.e. the movement of the three cores in response to a short magnetic field pulse. Usually a vortex and an antivortex annihilate immediately under emission of energy. However, two vortices located around an antivortex can built up a pretty stable micromagnetic unit, a so called single cross-tie wall. The experiments concerning the magnetization dynamics and the subsequent core movements were performed at the Swiss Light Source of the Paul Scherrer Institute in Switzerland. Fundamental questions were the driving force for these investigations: How do the two vortices and the antivortex influence the dynamic properties of the overall structure and the movement of the cores themselves? Do antivortex and vortices attract or repel each other in this specific arrangement? Are the subsequent spiral motions of the cores amplified or damped? Are other components of the overall cross-tie like the domain walls important for the overall dynamics?

Dr. Jürgen Fassbender sums up the outcome: “We could study some intriguing effects, in particular the gyrating movement of an antivortex has not been investigated experimentally so far. Due to comparison with complementary simulations we now understand details of the dynamic interaction between the three cores. Furthermore we could unravel the orientation of the three cores via analyzing their movements, although the lateral resolution of the used microscope is not high enough to extract the core orientation directly.”

What’s next? Dr. Jürgen Fassbender’s nanomagnetism team is now ready for its new challenge: to create a single antivortex and to experimentally investigate the magnetization dynamics of it for the first time. All this will certainly help in understanding the magnetization dynamics of even more complex micromagnetic structures, which might lay the basis for unforseen technological advances in the future.

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Christine Bohnet alfa

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