Chiral magnets in the slow lane

A groundbreaking study led by Thorsten Hesjedal, Gerrit van der Laan, and Shilei Zhang from Oxford, Diamond, and ShanghaiTech University has uncovered unexpected slow relaxation processes in chiral magnets, a discovery that challenges the conventional understanding of magnetic dynamics. The study highlights the critical role of topological defects in slowing down the relaxation of non-collinear magnetic states considered for emerging skyrmionics applications. 

(a) The timescales for a noncollinear magnetic order, at which various magnetic dynamics take place after a pumping field is applied. (b) Sketch of the scattering geometry, where an additional in-plane field pulse 𝐵coil=7  mT is applied along the 𝑦 direction as the pump signal. (c) Time sequence of the measurement, illustrating the time delay 𝑡d. (d) Equilibrium phase diagram of the measured Cu2⁢OSeO3 single crystal, mapped via their signature REXS patterns (𝐵coil switched off).

Non-collinear magnetic orders, such as spin spirals and skyrmions, have become a central topic in modern magnetism research. These complex magnetic configurations, characterised by twisted spin textures, have topological properties that make them ideal candidates for next-generation spintronic devices. In particular, magnetic skyrmions are small, stable, and can be moved about at minimal energy cost, making them ideal for advanced information storage technologies. 

Typically, when these magnetic textures are disturbed, their relaxation back to equilibrium is believed to occur over a timescale of nanoseconds, as predicted by micromagnetic theories. However, the research team has discovered that under certain conditions, the relaxation processes can extend to hundreds of milliseconds or even seconds.

Pump-probe results for the three studied relaxation processes, showing the average over all magnetic peak intensities as a function of time delay. The green dots are measured data points with error bars, and the green lines are the exponential fitting curve.

Revealing the slow dynamics 

In their experiment, the researchers studied the archetypal chiral magnet Cu2OSeO3 using a novel time-resolved resonant elastic X-ray scattering (REXS) technique (Fig a). By applying a pulsed magnetic field and measuring the magnetic order's response, they were able to capture the entire relaxation process in real-time. Surprisingly, the team found that both the conical and skyrmion lattice phases took up to 0.2 seconds to decay to their equilibrium state – a timescale that is eight orders of magnitude longer than conventional predictions (Fig b). 

This extended relaxation is attributed to the formation of topological defects, such as dislocations and monopoles, located within the magnetic structure. These defects act as localised disturbances, slowing down the relaxation process as the system strives to unwind and return to its lowest energy state. This behaviour contrasts sharply with the rapid dynamics typically expected in magnetic systems and opens up new questions about the underlying physics of topological textures. 

Implications for future technologies 

The discovery of slow relaxation dynamics in non-collinear magnetic orders has far-reaching implications for both fundamental physics and applied technology. The ability to control and manipulate topological defects could lead to the development of more efficient, high-speed spintronic devices. Moreover, understanding these relaxation processes may pave the way for new approaches to information storage and processing. 

“We are excited to reveal these previously unexplored dynamics in chiral magnets,” says Professor van der Laan. “The slow dissipation of topological defects highlights an important mechanism that has been overlooked in the past. This understanding could play a crucial role in the future of spintronic technologies.” 

Professor Thorsten Hesjedal added: "Our novel pump-probe REXS technique reveals the experimental lifetime of emergent singularities in a non-collinear magnetic system and underscores an additional universal relaxation mechanism of the solitonic textures in the slow dynamics regime."

For more information on the subject matter, please contact:
Professor Thorsten Hesjedal (Oxford Physics): Thorsten.Hesjedal@physics.ox.ac.uk
Professor Gerrit van der Laan (Diamond Light Source): gerrit.vanderlaan@diamond.ac.uk
Professor Shilei Zhang (ShanghaiTech University): shilei.zhang@shanghaitech.edu.cn

Chenhao Zhang, Yang Wu, Jingyi Chen, Haonan Jin, Jinghui Wang, Raymond Fan, Paul Steadman, Gerrit van der Laan, Thorsten Hesjedal, and Shilei Zhang, Slow Equilibrium Relaxation in a Chiral Magnet Mediated by Topological Defects, Physical Review Letters (2024), DOI: https://doi.org/10.1103/PhysRevLett.133.166707