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Not All Vortex States are Created Equal

A team of researchers, headed by Peter Fischer and Mi-Young Im of MSD’s Center for X-Ray Optics, in collaboration with colleagues in Japan, have discovered that magnetic-vortex formation in ferromagnetic nanodisks is asymmetric, contrary to common assumption. Their results are relevant to implementing nanodisks in data storage devices as the asymmetry could lead to failure during initialization.

When ferromagnetic materials are formed into nanodisks, the electron spins curl into vortices with needle-like cores. A given vortex state is defined by chirality, clockwise or anticlockwise, and core polarization, up or down: a total of four possible configurations, which were previously assumed to be equally probable.

Employing the unique X-ray beams at Berkeley Lab’s Advanced Light Source, the researchers simultaneously imaged the polarity and chirality of arrays of permalloy nanodisks as they initialized the vortex states. By collecting statistics from more than 1500 measurements, they determined that the generation probability for a given state is not symmetric and that some states are 1–2% more likely than others.

The researchers attribute the asymmetry to a combination of spin–orbit coupling at the disk surface and extrinsic factors like surface roughness.

For more information check out the article "New Phenomenon in Nanodisk Magnetic Vortices".

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