In certain magnetic materials, atomic spins can collectively order into vortex-like patterns known as magnetic skyrmions. Such skyrmions are highly mobile and have a very small (typically below 100 nm) spatial extension — properties that have caught the attention of scientists developing a new data storage concept known as racetrack memory. In this type of non-volatile memory, skyrmions act as data carriers, capable of rapidly moving along a ferromagnetic strip for read/write operations. Clearly, a high degree of control of magnetic skyrmions is then required, and in this context, Chiming Jun and colleagues have now investigated the conditions needed for creating and manipulating skyrmions in nanoscale confinement (Nature Communications 8, 15569; 2017).

By means of ion-beam lithography, the authors first fabricated a nanosized wedge from a crystal of iron germanide (FeGe), with a width varying between about 10 and 180 nm. Combining electron holography and image-reconstruction techniques, they were then able to obtain direct-space magnetic induction maps of the sample for different temperatures and external magnetic fields — providing the data for a phase diagram with magnetic field, wedge width and temperature as the phase variables. After cooling from room temperature to 220 K, for example, an external field of about 150 mT resulted in a single linear skyrmion chain (pictured).

The magnetic phase diagram also featured states like distorted helical spirals, pure edge twists and zigzag skyrmion chains. The variety of skyrmion phases seen in the FeGe nanowedge is absent in bulk and thin-film settings, as is the ability of the skyrmions to take on different sizes and ellipticities.

Jun et al. also presented a theoretical framework — based on evaluations of the energies of the skyrmion states — for numerical simulations of the observed phases. After checking that the calculated phases matched the experimentally observed situations, they numerically explored phase variable ranges not covered by the experimental phase diagram. Specifically, the authors looked at how elliptical skyrmions and their nucleation process develop, corroborating a conservation law for skyrmion numbers.