Adiabatic demagnetization refrigeration to millikelvin temperatures with the distorted square lattice magnet NaYbGeO4

We report the synthesis, characterization, low-temperature magnetic, and thermodynamic measurements of the millikelvin adiabatic demagnetization refrigeration (mK-ADR) candidate material NaYbGeO4 which exhibits a distorted square lattice arrangement of YbO6 magnetic units. Magnetization and specific heat indicate weakly interacting effective spin-1/2 moments below 10 K, with a Curie-Weiss temperature of only 15 mK, that can be polarized by magnetic fields of order 1 T. For the ADR performance test, we start the demagnetization from 5 T at a temperature of -2 K and reach a minimum temperature of 150 mK at zero field. The warming curve indicates a sharp magnetic transition in the heat capacity at 210 mK, implying only weak magnetic frustration. The entropy density of SGS <^> 101 mJ K-1cm-3 and hold time below 2 K of 220 min are competitive while the minimal temperature is higher compared to frustrated Ytterbium-oxide ADR materials studied under similar conditions.