Monte Carlo Simulations.

Standard Monte Carlo (MC) simulations of Glauber spin dynamics were performed using 32 × 32 vertex tetris lattices with periodic boundary conditions. For each MC time step, N single islands were chosen randomly (N is the number of islands in the lattice; cluster and loop flips were not considered), and the spin flip acceptance probability was [1+exp(Δ/kT)]−1, where Δ is the energy difference associated with the spin flip and k is Boltzmann’s constant. Only nearest- and next-nearest neighbor interactions were considered (J1 and J2), with J1=1.7J2, and temperature kT=0.7J2 that corresponds to room temperature in our structures; both values are consistent with micromagnetic MuMax3 simulations (36) also validated by directly comparing measured and simulated noise maps. At each value of (Bx, By), 106 MC annealing steps were performed to ensure thermal equilibrium, and then the magnetization was recorded for several million additional MC time steps to determine the average magnetization M and the thermodynamic fluctuations ⟨[δM(t)]2⟩ about this mean value.