  Generation of very low temperatures has been crucially important for
applications and fundamental research, as low-temperature quantum coherence
enables operation of quantum computers and formation of exotic quantum states,
such as superfluidity and superconductivity. One of the major techniques to
reach milli-Kelvin temperatures is adiabatic demagnetization refrigeration
(ADR). This method uses almost non-interacting magnetic moments of paramagnetic
salts where large distances suppress interactions between the magnetic ions.
The large spatial separations are facilitated by water molecules, with a
drawback of reduced stability of the material. 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 upon demagnetization under adiabatic conditions.
Compared to conventional refrigerants, KBaYb(BO$_3)_2$ does not degrade even
under high temperatures and ultra-high vacuum conditions. Further, its
frustrated magnetic network and structural randomness enable cooling to
temperatures several times lower than the energy scale of magnetic
interactions, which is the main limiting factor for the base temperature of
conventional refrigerants.
