Phase transition in a 2D ferromagnetic spin lattice
The 2D Ising model undergoes a second-order phase transition at T_c ≈ 2.269 J/k_B (Onsager 1944). Below T_c spins align into magnetic domains (ordered phase); above T_c thermal fluctuations destroy order. Near T_c, domain walls become fractal and correlations extend to infinity.