Copper Powder Demagnetization Cell for Electron Transport Measurements Compatible with Cryogen-free Cryostats

To meet the user demand for ultralow electron temperatures in low-dimensional devices, ³He immersion cells are employed in at the MagLab High B/T Facility [1]. Inspired by previous modeling [2] and experimental results [3] on cooling liquid ³He below 1 mK, the immersion cell was modified to include a special copper powder demagnetization refrigerant for use on the Bluefors LD250 "dry" cryogenic system equipped with a 14 T magnet. By incorporating copper in a form of powder into liquid ³He immersion cell, the design significantly reduces eddy current heating caused by vibrations from the pulse tube cryocoolers, which is a known issue with "dry" cryogenic systems.

The first series of experiments demonstrated that the refrigerant cooled the liquid ³He in the cell down to below 2 mK and in a final magnetic field up to 1 T. This result significantly expands "the temperature - magnetic field" phase space accessible for user experiments using the cryogen-free dilutrion refrigerator in the MagLab High B/T Facility.

In the future designs, improvements aim to mitigate heat leaks and consequently, greatly improve precooling time after magnetization and hold times after demagnetization below 10 mK. 

[1] J.S. Xia et al., Physica B 280 (2000) 491.

[2] R.C.M. Dow, A.M. Guenault, and G.R. Pickett, J. Low Temp. Phys. 47 (1982) 477.

[3] D.I. Bradle et al., J. Low Temp. Phys. 57 (1984) 359.