In previous work , which we recently reviewed in [2,3,4], we discovered a critical point in the behavior of hysteretic systems. Adding disorder to the system, we found a second order transition from hysteresis loops with a macroscopic jump or burst (roughly as seen in the supercooling of liquids) to smoothly varying hysteresis loops (as seen in most magnets). We have studied the critical point in the nonequilibrium zero temperature random field Ising model (RFIM) (which is a simple model for magnets, that has aplications far beyond magnetic hysteresis and associated Barkhausen Noise), using mean field theory, renormalization group techniques, and numerical simulations in 2,3,4, and 5 dimensions. In a large region near the critical disorder the model exhibits power law distributions of noise (avalanches), universal behavior, and a diverging length scale [5,6,7].
We review the two main theoretical frameworks for understanding Barkhausen noise and other avalanche-like phenomena. We show that while the theories predict a response which is symmetric in time, various measurements show a persistent time-asymmetry. In the ABBM model, assuming Gaussian pinning field statistics guarantees time-symmetric Barkhausen noise. On the other hand, our recent experiments show non-Gaussianity in the effective pinning field of an amorphous soft metallic ferromagnet. We suggest a possible connection between the non-Gaussianity of the pinning field and the observed time asymmetries.