Frustrated magnet for adiabatic demagnetization cooling to milli-Kelvin temperatures

Tokiwa, Yoshifumi  ; Bachus, S.*; Kavita, K.*; Jesche, A.*; Tsirlin, A. A.*; Gegenwart, P.*

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/superconductivity. One of the major techniques to reach milli-Kelvin temperatures is adiabatic demagnetization refrigeration. This method uses almost non-interacting magnetic moments of paramagnetic salts where large distances suppress interactions between the moments. The large spatial separations are facilitated by water molecules, with a drawback of reduced stability of the material. Here, we show that the water-free frustrated magnet KBaYb(BO) can be ideal for refrigeration, achieving at least 22 mK. Compared to conventional refrigerants, KBaYb(BO) does not degrade even under high temperatures and ultra-high vacuum. Further, its magnetic frustration 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.