Applications of the magnetocaloric effect in single-stage, multi-stage and continuous adiabatic demagnetization refrigerators

•ADRs use the magnetocaloric effect to achieve efficient cooling over a wide temperature range.•Temperature of an ADR can be controlled by the magnetic field without dissipation.•Single-stage ADRs can operate over about a factor of 30 in temperature.•Multi-stage ADR configurations can operate over wide temperature ranges with improved efficiency and higher cooling power.•Multi-staging also enables continuous cooling with ADRs, with even higher cooling power and lower mass.

Adiabatic demagnetization refrigerators (ADR), based on the magnetocaloric effect, are solid-state coolers that were the first to achieve cooling well into the sub-kelvin regime. Although supplanted by more powerful dilution refrigerators in the 1960s, ADRs have experienced a revival due to the needs of the space community for cooling astronomical instruments and detectors to temperatures below 100 mK. The earliest of these were single-stage refrigerators using superfluid helium as a heat sink. Their modest cooling power (<1 μW at 60 mK [1]) was sufficient for the small (6 × 6) detector arrays [2], but recent advances in arraying and multiplexing technologies [3] are generating a need for higher cooling power (5–10 μW), and lower temperature (<30 mK). Single-stage ADRs have both practical and fundamental limits to their operating range, as mass grows very rapidly as the operating range is expanded. This has led to the development of new architectures that introduce multi-staging as a way to improve operating range, efficiency and cooling power. Multi-staging also enables ADRs to be configured for continuous operation, which greatly improves cooling power per unit mass. This paper reviews the current field of adiabatic demagnetization refrigeration, beginning with a description of the magnetocaloric effect and its application in single-stage systems, and then describing the challenges and capabilities of multi-stage and continuous ADRs.