Adiabatic Demagnetization Refrigeration
Adiabatic demagnetization cools by aligning and then slowly randomizing magnetic moments, reaching microkelvin temperatures below the range of dilution refrigerators.
The magnetic route to cold
Adiabatic demagnetization refrigeration, or ADR, cools a system by manipulating the entropy stored in magnetic moments. A paramagnetic material full of magnetic dipoles has high magnetic entropy when the dipoles point in random directions and low entropy when a strong field aligns them. Cycling between these states pumps heat out of the coldest stage.
The cooling cycle
First the paramagnet is magnetized while thermally connected to a heat sink; aligning the moments releases heat, which the sink absorbs, keeping temperature roughly constant. Then the magnet is thermally isolated and the field is slowly reduced. Because the process is adiabatic and reversible, total entropy is conserved: as magnetic entropy rises with the randomizing moments, the lattice and electron entropy must fall, and the temperature drops sharply.
Nuclear demagnetization
Using electronic magnetic moments, ADR reaches the millikelvin range. To go lower, nuclear demagnetization uses the far smaller nuclear moments, typically of copper or a copper alloy. Precooled by a dilution refrigerator to a few millikelvin in a large field, then demagnetized, a nuclear stage can reach microkelvin temperatures for the electrons and even nanokelvin for the nuclear spins themselves.
One-shot versus continuous
A single demagnetization stage is a one-shot cooler: it warms as it absorbs heat and must be remagnetized to reset. Multi-stage and continuous ADR designs stagger two or more paramagnetic pills with heat switches so that one cools the load while another recycles, giving uninterrupted operation. ADR is valued in space missions because it needs no pumped liquids and works in any orientation and in microgravity.
- Cools by manipulating magnetic entropy of aligned dipoles
- Magnetize with heat sink attached, then isolate and demagnetize
- Nuclear demagnetization reaches microkelvin temperatures
- Continuous ADR uses staggered stages and heat switches