Using disorder to find new magnetic transition pathways

Friday, November 18, 2022

Portrait of Melissa Henderson

A team of physicists have found a new tunable pathway to manipulate nanoscale magnetic structures known as skyrmions. Their results advance efforts to access different configurations of these structures and increase their stability for magnetic memory applications, such as using these structures as new types of bits in classical or quantum computers.

Skyrmions are tiny structures present in some magnetic crystals, formed by the collective alignment of magnetic moments from individual atoms into a twisting vortex significantly larger than atomic scales. These vortices create 3-dimensional tube magnetic structures, similar to nanoscale tornadoes, that gather together into triangular or square lattice patterns of multiple vortices. The skyrmions and lattice patterns can be measured and controlled by external magnetic fields or currents.

“Previously, a lot of research has been devoted towards skyrmions in really pure, ideal single crystal systems,” says Melissa Henderson, a PhD student in the Department of Physics and Astronomy and at the Institute for Quantum Computing (IQC). “It’s been a commonly held belief that the total number of skyrmions in a sample, also known as the topological charge, is conserved during rearrangements. We discovered that this conservation of skyrmion number is not the case in crystals with substantial crystalline and chemical disorder, which leads to some really interesting properties and phenomena.”

The team, which includes collaborators from McMaster University and the National Institute of Standards and Technology (NIST), introduced disorder into their crystalline material by growing a site-disordered material which exhibits variations in atomic site occupancies, randomly incorporating cobalt manganese or zinc atoms into the repeating crystal pattern. This disorder interrupts the usually straight skyrmions to create a labyrinth of zig-zag patterns that merge, end, or separate in the crystal.

Using heat and rotating the magnetic field, alongside small angle neutron scattering measurements, the researchers heated up the sample to create the disordered skyrmions. In some experiments, the researchers then took the samples and rotated them in the magnetic field to reorient the skyrmions into an ordered triangle lattice pattern. Starting from either these ordered or disordered states, the researchers began to cool the material.

“As you cool it past a certain point, you'll exit the thermal equilibrium phase and go into a metastable phase, with the degree of skyrmion order in this phase dependent on the amount of order in the initial thermal equilibrium phase,” says Henderson. “Then as you keep cooling further, the exchange parameters will change more substantially, altering the ratios, magnitudes, and directions of the interactions. This will mediate a transition to a square lattice arrangement, so you go from a triangular to a square pattern.

“It was previously thought that in disordered samples, the disorder may inhibit the transition from triangular to square. That is why what we observed is surprising. We observed a disordered-to-ordered transition where we actually gain order when coming from the disordered triangular state to the square state. This is only possible by changing the topology of the system.”

To date, this skyrmion lattice transition has only been observed for conversions between ordered triangular to ordered square lattices in bulk systems. The researchers discovered an additional transition pathway between disordered triangular states to ordered square states that must undergo a change in the number of skyrmions present, upturning the previous belief that the number of skyrmions should stay constant during phase transitions.

Throughout the metastable phase, the researchers have shown additional disorder-induced effects where the skyrmions persist in a memory effect. The skyrmions are annihilated and then recovered in the metastable phase, thought to change between structures known as magnetic torons – tiny skyrmion filaments that maintain the topological charge of the skyrmion.

“Transition between ordered and disordered states has always fascinated people in general and physicists in particular. What are the pathways that lead one into another?” says Dr. Dmitry Pushin, faculty member in Waterloo’s Department of Physics and Astronomy and IQC. “Now we can study in situ such transitions where a quantum phase topology plays an important role and might help to advance spintronic devices.”

The paper Skyrmion Alignment and Pinning Effects in a Disordered Multi-Phase Skyrmion Material Co8Zn8Mn4 was published in the journal Physical Review B on September 29th.

Learn more about this research from Melissa Henderson