Adiabatic Demagnetization — Magnetic Cooling

Reaching millikelvin temperatures through entropy manipulation in a spin system

Step 1: Apply magnetic field isothermally

How it works

A paramagnetic salt has two entropy contributions: spin entropy S_spin (disordered spins) and lattice entropy S_lat.

S_spin = Nk_B ln(2) at B=0
S_spin → 0 as B/T → ∞

Step 1 — Isothermal magnetization: Apply field B at constant T. Spins align, S_spin drops. Heat flows to reservoir.

Step 2 — Adiabatic demagnetization: Isolate sample, remove field. Total entropy constant. Spin entropy rises → T drops dramatically.

T_f = T_i · (B_f/B_i)

Modern experiments reach ~1 mK. Nuclear demagnetization goes to μK.