• We applied rare-earth Yb-based compounds as magnetic refrigeration materials capable of cooling down below 1 K. 　
• We performed adiabatic demagnetization cooling for the enlarged Yb-based compounds in order to demonstrate the utilities as magnetic refrigeration materials.　
• The performance of Yb-based magnetic refrigerants can be explained by the knowledge about the strongly correlated f-electron systems. This study is interdisciplinary research connecting basic condensed-matter physics to applied research.

　When a magnetic field is applied to a magnetic substance holding giant entropy below 1 K, magnetic moments possessed by magnetic atoms are aligned for the direction of the magnetic field. When the magnetic field is removed under the adiabatic condition, temperature of the magnetic material decreases to around absolute zero. This process is called as magnetic refrigeration. Since magnetic refrigeration needs to only suitable materials and magnet, it is convenient to cool down. Paramagnetic salts which are not user-friendly because of the low thermal conductivity and chemical instability have been used as cryogenic magnetic refrigeration materials above 30 years. 

  We have focused on Ytterbium(Yb)-based compounds to develop new magnetic refrigeration materials without the above problems. As a result, we found that cooling performance is deeply involved with the knowledge about basic research in condensed-matter physics, such as competing of the magnetic interaction between the magnetic moments of Yb, and the strong correlation between the localized f-electrons in Yb and the conduction electrons. In addition, we performed adiabatic demagnetization cooling by using a commercial refrigerator in which these Yb-based compounds over 10 g were mounted. Consequently, we demonstrated that these materials are usable as magnetic refrigeration materials capable of cooling down to below 1 K. 

  At present, generating cryogenic temperatures is not easy. In contrast, cryogenic temperatures are needed by various research areas like condensed-matter physics, astrophysics, and quantum computer. In the future, our research will develop more and more as rare interdisciplinary research connecting the basic condensed-matter physics to applied material research.

Reference
  Yasuyuki Shimura and Yoshifumi Tokiwa, Butsuri (in Japanese) 78, 461 (2023).