New publication in Nature Communications

January 08, 2015
Mi-Young Im, Ki-Suk Lee, Andreas Vogel, Jung-Il Hong, Guido Meier and Peter Fischer
Stochastic formation of magnetic vortex structures in asymmetric disks triggered by chaotic dynamics
Nature Communications 5, 5620 (2014)

Fluctuations like Barkhausen noise in ferro- and antiferromagnetic materials are driven by non-deterministic phenomena. As the length scales in nanomagnetism approach fundamental limits such as the exchange length, the question arises, whether there is an intrinsic stochastic nature, which would limit the performance of high-density magnetic storage devices aiming at faster processing speed. Understanding and controlling stochastic behavior in nanoscale magnetic systems are thus of great interest, for example, on magnetization reversal of nano-size domains in ultrathin magnetic films or domain wall motion in magnetic nanowires.

In this work the non-trivial spin configuration in a magnetic vortex is used as a prototype for fundamental studies of nanoscale spin behavior with potential applications in magnetic information technologies. An important milestone for achieving applications is a reliable control of vortex structures. However, controlling magnetic processes is hampered by stochastic behavior, which is associated with thermal fluctuations in general. Here we show that the dynamics in the initial stages of vortex formation on a sub-nanosecond timescale dominates the stochastic behavior observed at steady state. Our results show that the intrinsic stochastic nature of vortex creation can be controlled by adjusting the inter-disk distance in asymmetric disk arrays.

The origin of our experimental observation of stochastic nucleation of circularity in magnetic vortices is explained by micromagnetic simulations, which provide evidence that the chaotic dynamics in the initial stages of vortex formation on an ultrafast timescale can trigger the stochastic behavior at steady state in asymmetric disk arrays. The work not only demonstrates that dynamics itself can be a decisive factor causing the stochastic behavior but also it shows the possibility to control the ‘intrinsic’ stochastic nature in the vortex creation process.

This work is a collaboration of the Max-Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany, with the Hamburg Centre for Ultrafast Imaging (CUI), the Center for X-ray Optics, Lawrence Berkeley National Laboratory, California, USA, the Daegu Gyeongbuk Institute of Science and Technology, Daegu, Korea, and the Ulsan National Institute of Science and Technology, Ulsan, Korea.

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