Correlations and reproducibility in magnetic avalanches

When a slowly varying magnetic field is applied to a ferromagnet, the response is comprised of a complex series of abrupt jumps, or avalanches, in magnetization. These rapid changes in magnetization appear as voltage spikes in a pickup coil wrapped around the sample:

For more detail on magnetic avalanches and hysteresis, visit Jim Sethna and collaborators at Cornell

Under normal circumstances there is no correlation between the avalanches observed at a given field on successive magnetic field cycles, but we have found that certain ferromagnetic alloys can show a very different behavior. Some of the observed avalanches occur at precisely the same field on each cycle, while others show considerable variability. When the signal from many cycles is averaged, peaks occur at the locations of the reproducible avalanches (see figure below), resulting in a fingerprint of the fixed disorder that underlies the magnetic noise. We have found that the fingerprint is independent of temperature and of the rate at which the external field is changing, which is inconsistent with simple models of domain wall motion typically employed to describe the noise. Our results suggest that the interplay between dynamic complexity and fixed disorder is an essential feature of the observed signal.

For more details, here is a postscript copy (500K) of a paper entitled "Reproducibility of magnetic avalanches in an Fe-Ni-Co alloy" (Phys. Rev. Lett. 75, p. 4694 (1995)).

In addition to looking at the reproducibility of the noise signal on successive magnetic field cycles, we have been investigating the correlations in the avalanche sequence within a field cycle. We showed that a negative correlation exists within the signal: large avalanches tend to be well separated. This correlation is a consequence of the long-range magnetic interactions in ferromagnets. We showed that including a global interaction into a simple model of domain wall motion results in self-organized criticality. ("Interface Depinning, Self-Organized Criticality, and the Barkhausen Effect", Phys. Rev. Lett. 75, p. 276 (1995))