# Magnetic cooling for microkelvin nanoelectronics on a cryofree platform

## TL;DR

This paper demonstrates a cryofree platform with 16 parallel demagnetization refrigerators achieving sub-millikelvin temperatures for nanoelectronics, with detailed thermal characterization and a thermal model explaining system performance.

## Contribution

It introduces a scalable cryofree cooling system with multiple nuclear refrigerators reaching 150 microkelvin, advancing microkelvin nanoelectronics research.

## Key findings

- Achieved temperatures down to 150 microkelvin.
- Maintained cold times of several days below 1 millikelvin.
- System exhibited heat leaks of a few nanowatts per mole.

## Abstract

We present a parallel network of 16 demagnetization refrigerators mounted on a cryofree dilution refrigerator aimed to cool nanoelectronic devices to sub-millikelvin temperatures. To measure the refrigerator temperature, the thermal motion of electrons in a Ag wire -- thermalized by a spot-weld to one of the Cu nuclear refrigerators -- is inductively picked-up by a superconducting gradiometer and amplified by a SQUID mounted at 4 K. The noise thermometer as well as other thermometers are used to characterize the performance of the system, finding magnetic field independent heat-leaks of a few nW/mol, cold times of several days below 1 mK, and a lowest temperature of 150 microK of one of the nuclear stages in a final field of 80 mT, close to the intrinsic SQUID noise of about 100 microK. A simple thermal model of the system capturing the nuclear refrigerator, heat leaks, as well as thermal and Korringa links describes the main features very well, including rather high refrigerator efficiencies typically above 80%.