Two-step magnetic cooling cycle reaching millikelvin temperatures
Adiabatic demagnetization (Debye/Giauque 1926) cools by: (1) isothermally magnetizing a paramagnetic salt (entropy drops as spins align), (2) thermally isolating and slowly removing B (entropy conserved → T drops). The entropy S(T,B) = R·ln(2J+1) - f(B/T). Each cycle reaches T_final = T_i · (B_final/B_max). Nuclear demagnetization (μN vs μB) reaches microkelvin!