Skyrmions
Skyrmions are highly localised nanoscale regions in a magnetic material with a swirling magnetisation texture. The magnetisation points upward at the outer edge and downward at the centre. They can be thought of as small rolled-up domain structures. Skyrmions are stable, small, and highly tunable, making them a promising building block for the memories and computers of the future.
When a laser pulse hits a magnetic material, the material heats up extremely rapidly. On a timescale of picoseconds — a trillionth of a second — skyrmions can suddenly nucleate or decay. This phenomenon has already been experimentally demonstrated in several materials, but a solid theoretical understanding was still lacking.
Until now, scientists used atomistic spin dynamics simulations to study this: simulations in which the behaviour of every individual atom in the material is computed separately. This works, but it is enormously computationally expensive and does not provide a simple intuitive understanding of the underlying physics.
The new theory
Rein Liefferink, a PhD candidate at Radboud University, developed a new, coarse-grained effective theory. Rather than computing every atom separately, the theory describes the magnetic skyrmions directly. A skyrmion is still extremely small — typically a few to tens of nanometers across, roughly a thousand times thinner than a human hair — but that is far larger than a single atom. By describing what happens at this level, the computational complexity is drastically reduced.
The key idea is that a skyrmion can only nucleate or decay when the system has enough energy to overcome an energy barrier. Imagine a ball sitting in a potential well. The ball stays put unless it receives enough energy to climb over the rim. Only then does it transition to a different state. For a skyrmion, the same principle applies: the heat from the laser pulse provides the system with energy, and there is a certain probability that this energy is sufficient to overcome the barrier – causing a skyrmion to nucleate or decay.
Because everything happens so rapidly, all these events can be treated independently of one another. Liefferink: 'This means you only need to look locally at what happens to each individual skyrmion and then sum over the entire system.' This yields a single transparent formula that describes the dynamics of all skyrmions together. This formula shows good agreement with both existing atomistic spin dynamics simulations and experiments, while drastically reducing computational complexity.'
