Adiabatic demagnetization refrigeration to mK temperatures in NaGdGeO₄ crystal

Adiabatic demagnetization refrigeration (ADR) technology is regarded as an effective, easy and sustainable method to replace evaporation or dilution cooling of ³He/⁴He. In practical applications, it is important for magnetic refrigeration materials to possess excellent magnetocaloric effect (MCE) under a small applied field, which is advantageous to further simplifying design and reducing costs. In this article, the centimeter-scaled crystal of NaGdGeO₄ is grown by top seed crystal method. The magnetic performance and MCE of NaGdGeO₄ compound are investigated. NaGdGeO₄ exhibits a low phase transition temperature of 0.7 K, indicating its potential as an ultra-low temperature magnetic refrigerant. Large magnetic entropy changes of 16.7 and 34.5 J·kg^-1·K^-1 are obtained under small field changes of 1 and 2 T at 0.8 and 1.2 K, respectively, which is almost 2 times larger than that of commercial magnetic refrigerant Gd₃Ga₅O₁₂ under the same conditions. For the measurement of quasi-ADR, when initial conditions are μ ₀ H =6 T, T = 2 K, the temperature trace of non-oriented crystal and powder samples passe a minimum value of 132.5 mK and 157.9 mK, respectively. These outstanding performances suggest that NaGdGeO₄ is a promising candidate for ultra-low temperature magnetic refrigeration applications.