  We show that both square and kagome artificial spin ice systems exhibit
disorder-induced nonequilibrium phase transitions, with power law avalanche
distributions at the critical disorder level. The different nature of
geometrical frustration in the two lattices produces distinct types of critical
avalanche behavior. For the square ice, the avalanches involve the propagation
of locally stable domain walls separating the two polarized ground states, and
the scaling collapse agrees with an interface depinning mechanism. In contrast,
avalanches in the fully frustrated kagome ice exhibit pronounced branching
behaviors that resemble those found in directed percolation. The kagome ice
also shows an interesting crossover in the power-law scaling of the avalanches
at low disorder. Our results show that artificial spin ices are ideal systems
in which to study nonequilibrium critical point phenomena.
