Aluminum nuclear-demagnetization refrigerator for powerful continuous cooling

Many laboratories routinely cool samples to 10 mK, but relatively few can cool condensed matter below 1 mK. Easy access to the microkelvin range would prove highly desirable in fields such as quantum sensors and quantum materials. Such temperatures are achieved with adiabatic nuclear demagnetization. Existing nuclear-demagnetization refrigerators (NDRs) are “single-shot,” and the recycling time is incompatible with some submillikelvin experiments. Furthermore, a high cooling power is required to overcome the excess heat load of nanowatt order on NDRs precooled by cryogen-free dilution refrigerators. We report the performance of an aluminum NDR designed for powerful cooling when part of a dual-stage continuous NDR (CNDR). Its thermal resistance is minimized to maximize the cycling rate of the CNDR and consequently its cooling power. At the same time, its susceptibility to eddy current heating is minimized. A CNDR based on two of the aluminum NDRs presented here would achieve a cooling power of approximately 40 nW at 560 $\text{μ}\mathrm{K}$ less than $6$ days after cooling from room temperature, with a small offset in electronic temperature that decreases as the time-dependent heat load decays.