Sub-1 K Adiabatic Demagnetization Refrigeration with Rare-Earth Borates Ba₃XB₉O₁₈ and Ba₃XB₃O₉, X = (Yb, Gd)

Adiabatic demagnetization refrigeration (ADR)is regaining relevance for refrigeration to temperatures below 1 K as global helium-3 supply is increasingly strained. While ADR at these temperatures is long established with paramagnetic hydrated salts, more recently, frustrated rare-earth oxides were found to offer higher entropy densities and practical advantages, since they do not degrade under heating or evacuation. We report structural, magnetic, and thermodynamic properties of the rare-earth borates Ba${}_3$XB${}_9$O${}_{18}$ and Ba${}_3$XB${}_3$O${}_9$ with X = (Yb, Gd). Except for Ba${}_3$GdB${}_9$O${}_{18}$, which orders at 108 mK, the three other materials remain paramagnetic down to their lowest measured temperatures. ADR performance starting at 2 K in a field of 5 T is analyzed and compared to literature.