Spin Glasses (Edwards & Anderson 1975) are disordered magnets where couplings Jij are random — some ferromagnetic (+), some antiferromagnetic (−). Competing interactions cause frustration: not all bonds can be simultaneously satisfied. The system freezes into a complex energy landscape with exponentially many local minima.
The Edwards-Anderson order parameter q = [⟨s_i⟩²]_av measures spin freezing without long-range order. Below T_g, q > 0 (frozen), but |m| ≈ 0 (no net magnetization). The spin glass phase is distinct from both ferromagnets and paramagnets. The replica symmetry breaking solution (Parisi 1979, Nobel 2021) revealed an ultrametric organization of pure states.