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

• Applicant: Professor Dr. Philipp Gegenwart
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 514162746

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.

• DFG Programme: Research Grants
• Co-Investigator: Dr. Anton Jesche