Sub 1 K Adiabatic Demagnetization Refrigeration with Rare-Earth Borates Ba$_3$XB$_9$O$_{18}$ and Ba$_3$XB$_3$O$_9$, X = (Yb, Gd)

TL;DR

This paper investigates the potential of rare-earth borates Ba$_3$XB$_9$O$_{18}$ and Ba$_3$XB$_3$O$_9$ as refrigerants for sub-1 K adiabatic demagnetization refrigeration, highlighting their magnetic properties and thermodynamic performance.

Contribution

It provides new structural, magnetic, and thermodynamic data on these borates, demonstrating their suitability and performance for low-temperature ADR applications.

Findings

Most materials remain paramagnetic down to lowest temperatures.

Ba$_3$GdB$_9$O$_{18}$ orders at 108 mK.

ADR performance at 2 K in 5 T field is comparable to literature.

Abstract

Adiabatic demagnetization refrigeration (ADR) is regaining relevance for the 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 results.