Frustrated magnet for adiabatic dimagnetization cooling to milli-Kelvin temperatures

Due to the supply insecurity of Helium-3, adiabatic demagnetization refrigeration (ADR) for cooling to milli-Kelvin temperatures has been attracting much attention. Conventional refrigerants, paramagnetic salts, contain water molecules. The presence of H$_2$O causes lack of stability in the air and special care has to be taken for the handling. Here, we show that an H$_2$O-free frustrated magnet KBaYb(BO$_3$)$_2$ can be ideal refrigerant for ADR, achieving at least 22\,mK. KBaYb(BO$_3)_2$ does not degrade under severe conditions like high temperatures and ultra-high vacuum. Further, its magnetic frustration enables refrigeration to temperatures several times lower than the energy scale of magnetic interactions, which is the limit for conventional refrigerants.