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

TL;DR

This study demonstrates that KBaGd(BO3)2, a triangular antiferromagnet, can achieve adiabatic demagnetization cooling below its magnetic ordering temperature, with improved entropy capacity and hold time, due to its unique interplay of interactions.

Contribution

The paper reports the first demonstration of ADR in KBaGd(BO3)2 achieving temperatures below its magnetic ordering point, highlighting its potential for low-temperature cooling applications.

Findings

Achieved ADR cooling down to 122 mK, below the ordering temperature of 263 mK.

Significantly improved entropy storage capacity and hold time compared to similar compounds.

Balanced exchange and dipolar interactions facilitate cooling below magnetic ordering temperature.

Abstract

Crystal structure, thermodynamic properties, and adiabatic demagnetization refrigeration (ADR) effect in the spin-7/2 triangular antiferromagnet KBaGd(BO3)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 $T_{min} = 122$ mK, more than twice lower than $T_N$, along with the entropy storage capacity of 192 mJK$^{-1}$cm$^{-3}$ and the hold time of more than 8 hours in the PPMS setup, both significantly improved compared to the spin-1/2 Yb$^{3+}$ analog. We argue that KBaGd(BO3)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.