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Not All Vortex States are Created Equal

Below: Images of magnetic vortex states from full-field magnetic transmission soft X-ray microscopy, identifying chirality (left) and polarity (right), for combined states as illustrated in color.

A team 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—for 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 much more likely than others.

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

 
“Symmetry breaking in the formation of magnetic vortex states in a permalloy nanodisk.” M.-Y. Im, P. Fischer, K. Yamada, T. Sato, S. Kasai, Y. Nakatani and T. Ono; Nature Communications 3:983 (2012). DOI: 10.1038/ncomms1978