= Adiabatic demagnetization
A cooling method that reduces an applied <magnetic field> while suppressing <heat transfer> to or from the material. For a suitable material exhibiting <paramagnetism>, an approximately reversible field reduction can lower its <temperature> at approximately constant <entropy>.
The preparation matters as much as the field reduction. First establish <thermal equilibrium> with a cold <heat bath>, then raise the <magnetic field> while maintaining the bath <temperature>: this is <magnetization> in an <isothermal process>. The bath accepts <heat> and the material loses <magnetic entropy>. Next open a <heat switch>, thermally isolating the working material, and lower the <magnetic field>. If the sweep maintains internal <thermal equilibrium> with negligible <entropy production>, this second leg is an <isentropic process>. An <adiabatic process> alone does not guarantee constant <entropy>.
The attraction is a controllable <energy> scale: changing the <magnetic field> changes the separation of magnetic <energy levels>, even across a thermally insulating gap. This lets an already cold solid become colder without needing a colder contacting <heat bath>. A practical <refrigerator> also needs a load, a period of absorbing its <heat>, and a reset that rejects accumulated <entropy> to the bath. A single field sweep has finite cooling capacity.
\Image[/adiabatic-demagnetization-cold-stage-sequence.png]
{title=Functional cold stage and <heat switch> sequence for <adiabatic demagnetization>}
{id=adiabatic-demagnetization-cold-stage-sequence}
{description=Original functional schematic. The controlled thermal link closes during <magnetization> in an <isothermal process> and opens before the <magnetic field> falls. The sensing coil is a readout, not a thermal connection. This is a functional diagram rather than a reconstruction of historical geometry.}
{source=adiabatic-demagnetization-cold-stage-sequence.py}
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