Adiabatic demagnetization cooling well below the magnetic ordering temperature in the triangular antiferromagnet KBaGd(BO3)2

Abstract

Crystal structure, thermodynamic properties, and adiabatic demagnetization refrigeration (ADR) effect in the spin-$\frac{7}{2}$ triangular antiferromagnet $\mathrm{KBaGd}{({\mathrm{BO}}_{3})}_{2}$ are reported. With the average nearest-neighbor exchange coupling of 44 mK, this compound shows magnetic order below ${T}_{N}=263$ mK in zero field. The ADR tests reach the temperature of ${T}_{min}=122\phantom{\rule{0.16em}{0ex}}\mathrm{mK}$, more than twice lower than ${T}_{N}$, along with the entropy storage capacity of 192 mJ ${\mathrm{K}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and the hold time of more than 8 h in the PPMS setup, both significantly improved compared to the spin-$\frac{1}{2}\phantom{\rule{4pt}{0ex}}{\mathrm{Yb}}^{3+}$ analog. We argue that $\mathrm{KBaGd}{({\mathrm{BO}}_{3})}_{2}$ shows a balanced interplay of exchange and dipolar couplings that together with structural randomness and geometrical frustration shift ${T}_{min}$ to well below the ordering temperature ${T}_{N}$, therefore facilitating the cooling.