2D Ising model near Tc — β=1/8, γ=7/4, ν=1 (exact Onsager 1944)
The 2D Ising model (Lenz-Ising) has an exact critical temperature kTc/J = 2/ln(1+√2) ≈ 2.269 (Onsager 1944). Critical exponents: magnetization β=1/8 (⟨m⟩~|t|^β), susceptibility γ=7/4 (χ~|t|^-γ), correlation length ν=1 (ξ~|t|^-ν). At Tc, clusters of all sizes appear — scale-invariant fractal structure.