  Random fields disorder Ising ferromagnets by aligning single spins in the
direction of the random field in three space dimensions, or by flipping large
ferromagnetic domains at dimensions two and below. While the former requires
random fields of typical magnitude similar to the interaction strength, the
latter Imry-Ma mechanism only requires infinitesimal random fields. Recently,
it has been shown that for dilute anisotropic dipolar systems a third mechanism
exists, where the ferromagnetic phase is disordered by finite-size glassy
domains at a random field of finite magnitude that is considerably smaller than
the typical interaction strength. Using large-scale Monte Carlo simulations and
zero-temperature numerical approaches, we show that this mechanism applies to
disordered ferromagnets with competing short-range ferromagnetic and
antiferromagnetic interactions, suggesting its generality in ferromagnetic
systems with competing interactions and an underlying spin-glass phase. A
finite-size-scaling analysis of the magnetization distribution suggests that
the transition might be first order.
