Breakdown of Barkhausen Criticality in an Ultrathin Ferromagnetic Film

Analysis of Barkhausen avalanches in thin films of magnetic material has been an active area of research due to the proximity of the dimensionality crossover. However, the measurement of avalanches in ultrathin films (thickness <~ 20 nm) by conventional inductive pick-up methods is challenging due to the small mass of the material involved. Here, we describe an optical method to analyze Barkhausen avalanches in ultrathin magnetic films with a temporal resolution of ~ 10 μs. We measure the avalanche statistics of 15-nm thick film of NiFe grown by pulsed laser ablation, where we observe a breakdown of the power-law probability distributions of s and T as well as a breakdown of the power-law relation between <s(T)> and <T>. The observations could not be explained within the framework of the established front propagation or nucleation models under adiabatic evolution. We attempt to explain our findings by performing simulations of the 2D random field Ising model under non-adiabatic evolution with a constant field step, and obtain good qualitative agreement between experimental and simulated probability distributions for high disorder levels. We conclude that the observed peculiarities are a consequence of strong pinning and the departure from the adiabatic condition during magnetic field sweep.