What do earthquakes and exotic magnetic phases have in common? They exhibit complex behaviors emerging from countless random events that can’t be tracked individually. Nevertheless, their behaviors can be analyzed statistically, in ways that don’t require knowledge of every fault slip or spin flip.
One such approach is to measure “crackling.” The concept originated in the observation of the crackling sounds made by ferromagnets as their domain walls move jerkily in response to external magnetic fields. In general, crackling systems respond to evolving conditions through many small twitches and a few large jolts. A characteristic mathematical relationship between the twitches and jolts allows phenomena to be grouped into meaningful classes based on the parameters of this relationship.
At the Advanced Light Source (ALS), researchers applied a crackling analysis to an iron/gadolinium heterostructure exhibiting ordered stripes, disordered stripes, and skyrmion phases. Using coherent soft x-ray scattering at Beamline 12.0.2, the researchers measured crackling behaviors in the phases’ speckle fluctuations as the magnetic field and temperature were varied.
In both stripe phases, the fluctuations were abrupt, whereas changes in the skyrmion phase were smooth—a fundamental difference that comports with high skyrmion mobility, likely due to topological protection. And because fluctuations tend to increase when phase changes occur, the data helped pinpoint the temperatures and magnetic fields that define these critical points.
With the improved coherence anticipated with the ALS Upgrade Project (ALS-U), the technique can be productively applied to many complex materials of interest for emerging information technologies, where device stability depends on minimizing the effects of nanoscale fluctuations.
A. Singh, J.C.T Lee, K.E. Avila, Y. Chen, S.A. Montoya, E.E. Fullerton, P. Fischer, K.A. Dahmen, S.D. Kevan, M.K. Sanyal, and S. Roy, “Scaling of domain cascades in stripe and skyrmion phases,” Nat. Commun. 10, 1988 (2019), doi:10.1038/s41467-019-09934-z.