Triangular rare-earth borates for milli-Kelvin adiabatic demagnetization refrigeration

绝热退磁制冷(ADR)是近年来备受关注的一种经典制冷技术，可替代昂贵的3He/4He稀释式制冷。已建立的含有ADR盐类的水存在化学不稳定性，需要进行精细处理以避免降解并确保良好的热接触。我们最近已经证明，不含水的KBaYb(BO3)2是一种很好的替代品，具有更高的熵密度，允许ADR低于20MK。含有银粉混合物以确保良好热耦合的烧结球团易于制造，价格低廉，即使加热到700°C也可以长期稳定，从而实现超高真空应用。KBaYb(BO_3)_2属于稀土基硼酸盐A+xBa2+yR3+(BO_3)_3-z(A=Na，K；R=稀土)，具有完美的三角形磁矩排列、可调节的R原子间距和非磁性层间原子的有序性/无序性。选择R允许人们从量子(R=Yb)到经典(Gd)自旋行为，并将Kramers(Dy)或非Kramers(Tb或Ho)情况下的低位晶体电场激发结合到ADR中。我们的探索性合成和低温磁性研究旨在解决几何受挫、量子涨落、海森堡偶极耦合和结构无序对ADR的影响。最终，我们确定了这类材料中最好的ADR物质，根据量子技术中多种应用所需的不同目标温度在10到200 MK之间的最佳熵密度。

Adiabatic demagnetization refrigeration (ADR) is a classical cooling technique with renewed recent attention as alternative to costly and elaborate 3He/4He dilution refrigeration. Established water containing ADR salts suffer from chemical instability which requires delicate treatment to avoid degradation and ensure good thermal contact. We have shown recently, that H2O-free KBaYb(BO3)2 is an excellent alternative with higher entropy density that allows ADR to below 20 mK. Sintered pellets with silver powder admixture to ensure good thermal coupling are easy to manufacture, inexpensive and long-term stable even upon heating up to 700°C, enabling also ultra-high vacuum applications. KBaYb(BO3)2 belongs to a family of rare-earth-based borates A+xBa2+yR3+(BO3)3-z (A=Na, K; R=rare earth) with a perfect triangular arrangement of magnetic moments, adjustable R-atom distances and order/randomness of non-magnetic inter-layer atoms. The choice of R allows one to tune from quantum (R=Yb) to classical (Gd) spin behavior and to incorporate low-lying crystal electric field excitations in the Kramers (Dy) or non-Kramers (Tb or Ho) case to ADR. Our explorative synthesis and low-temperature magnetic investigation aims to disentangle the influence of geometrical frustration, quantum fluctuations, Heisenberg versus dipolar couplings and structural disorder on ADR. Eventually we identify the best ADR substances of this material class with respect to an optimized entropy density for different target temperatures between 10 and 200 mK, as required for multiple applications in quantum technology.