A 3He-independent cryogen-free adiabatic demagnetization refrigerator for temperatures below 1 K.

Millikelvin environments are crucial for condensed matter experiments, yet the limited availability and rising cost of 3He have imposed a bottleneck on the application of mainstream millikelvin refrigerators: 3He refrigerators and dilution refrigerators. Recently, the adiabatic demagnetization refrigerator (ADR), which has been previously overshadowed by its 3He-based counterparts, is experiencing a revival as a 3He-independent alternative. Here, we present the design, construction, and performance of a cryogen-free ADR that strictly avoids the use of 3He. By incorporating a closed-cycle 1 K pot, the ADR realizes a novel architecture comprising a 1.5 K precooling system and a single demagnetization stage. The results show that the ADR can reach a base temperature of 113 mK and maintain temperatures below 300 mK for over 11 h, which approaches the performance of 3He refrigerators. The demagnetization stage features simply fabricated components, including a custom-designed 2 T superconducting magnet, a homemade chromium potassium alum salt pill, and a homemade mechanical heat switch. The components are arranged into a spacious and compatible layout, enabling future experimental integration. Overall, this work offers an alternative design route and a practical reference for developing 3He-independent ultra-low temperature systems for research purposes.