Magnetic Vortices Normally Veer 30 Degrees Off Course. Mainz Filmed a Kind That Goes Straight.

Physicists at Johannes Gutenberg University Mainz have watched magnetic skyrmions travel in a straight line along an applied current, confirming a prediction that has underpinned a decade of proposals for a new kind of computer memory. The results were published in Nature Physics.

A skyrmion is a nanometer-scale knot in the magnetization of a thin film — a vortex-like texture that behaves as a discrete, movable object and is topologically protected, meaning it cannot be smoothed out without a large energy cost. Because a skyrmion is stable, tiny and can be pushed around with modest electrical currents, it has been an obvious candidate for a storage bit: encode data in the presence or absence of a skyrmion, then shuttle the whole train past a read head along a nanowire.

The problem has always been that skyrmions do not go where they are pushed. In ordinary ferromagnetic films, a current-driven skyrmion drifts sideways, deflected by as much as 30 degrees from the direction of current flow — the skyrmion Hall effect, a magnetic analogue of the sideways force that deflects charged particles in a magnetic field. In a racetrack memory device, a bit that veers off-axis eventually runs into the edge of the wire and annihilates. Theory has long predicted that antiferromagnets should not suffer from this, because their two opposing magnetic sublattices produce transverse forces that cancel.

Mona Bhukta, working in the group of Professor Mathias Kläui, tested that directly. "Skyrmions in antiferromagnetic systems move along the current direction, meaning the skyrmion Hall effect does not come into play," the team reported.

Seeing this required imaging objects tens of nanometers across while they move on nanosecond timescales. The group used time-resolved X-ray microscopy at BESSY II, the synchrotron light source in Berlin, effectively building a stroboscopic film of skyrmion motion. The measurements captured not only straight-line transport but also collisions: mobile skyrmions ran into skyrmions pinned at defects in the film and rebounded, rather than merging or annihilating.

That second observation is arguably the more useful one. A memory device does not contain one skyrmion; it contains many, packed close together, and their mutual interactions and their response to the inevitable defects in a real film determine whether a data train stays ordered. By measuring how skyrmions scatter off pinned neighbors, the Mainz group produced a quantitative framework for those interactions rather than a single-particle demonstration.

Antiferromagnetic devices carry other advantages that have kept interest alive despite being far harder to work with than ferromagnets: they emit no stray magnetic field, are insensitive to external fields, and have intrinsic dynamics in the terahertz range, orders of magnitude faster than ferromagnetic switching. The tradeoff is that they are correspondingly difficult to read, since there is little net magnetization to detect.