The random-field Ising model (Sethna et al. 1993) exhibits scale-free avalanche dynamics near the disorder critical point R_c. As an external field sweeps through, clusters of spins flip collectively. Below R_c, system-spanning avalanches occur; above R_c, only microscopic flips. At criticality, avalanche sizes follow a power law P(S) ~ S^(-τ), τ ≈ 1.6, explaining Barkhausen noise in magnets.