Ising playground

Square lattice of spins ±1 (shown light / dark). Energy uses nearest neighbors with periodic boundaries; units J = kB = 1. Metropolis flips: accept with probability min(1, e−ΔE/T). The 2D critical temperature is Tc ≈ 2.269: below it you tend to see large aligned domains; above it, disorder; near Tc, slow, patchy fluctuations.