Adiabatic demagnetization refrigeration with antiferromagnetically ordered NaGdP$_2$O$_7$

TL;DR

This study explores NaGdP$_2$O$_7$'s magnetic and thermodynamic properties, demonstrating its effectiveness as a precooling material in ADR systems with exceptionally long warm-up times and potential for efficient refrigeration at millikelvin temperatures.

Contribution

It provides the first detailed analysis of NaGdP$_2$O$_7$'s structure, magnetic behavior, and ADR performance, highlighting its advantages over Yb-based materials for low-temperature cooling.

Findings

Achieved a minimum temperature of 220 mK in ADR experiments.

Warm-up time exceeds 60 hours, much longer than Yb-based analogues.

NaGdP$_2$O$_7$ acts as a network of ferromagnetic chains with antiferromagnetic interchain coupling.

Abstract

We present a comprehensive study of the structural, magnetic, and thermodynamic properties, as well as the adiabatic demagnetization refrigeration (ADR) performance of NaGdP$_2$O$_7$. Although NaGdP$_2$O$_7$ exhibits antiferromagnetic ordering at a N\'eel temperature of $T_{\rm N} = 570$ mK in zero field, ADR experiments achieved a minimum temperature of 220 mK starting from $T = 2$ K under an applied magnetic field of $\mu_0H = 5$ T. The warm-up time back to $T = 2$ K exceeds 60 hours, which is roughly 50 times longer than that of its Yb-based analogue, underscoring the potential of NaGdP$_2$O$_7$ as an efficient precooling stage in double-stage ADR systems. We show that NaGdP$_2$O$_7$ can be seen as a network of ferromagnetic spin chains with antiferromagnetic interchain couplings and also investigate the influence of antiferromagnetic ordering on the magnetic entropy. We find that the temperature dependence of the entropy plays a more dominant role than its magnetic field dependence in the magnetically ordered state.