Adiabatic demagnetization refrigeration to mK temperatures with the distorted square lattice magnet NaYbGeO$_{4}$

TL;DR

This study introduces NaYbGeO$_{4}$ as a promising material for milli-Kelvin adiabatic demagnetization refrigeration, demonstrating effective cooling from 2 K to 150 mK with weak magnetic frustration and competitive entropy density.

Contribution

The paper reports the synthesis and characterization of NaYbGeO$_{4}$ as a new candidate for mK-ADR, showing its magnetic properties and cooling performance under practical conditions.

Findings

Achieved minimum temperature of 150 mK from 2 K starting point.

Demonstrated weak magnetic frustration with a sharp transition at 210 mK.

Entropy density and hold time are competitive with existing Ytterbium-oxide ADR materials.

Abstract

We report the synthesis, characterization, low-temperature magnetic, and thermodynamic measurements of the novel milli-Kelvin adiabatic demagnetization refrigeration (mK-ADR) candidate material NaYbGeO$_4$ which exhibits a distorted square lattice arrangement of YbO$_{6}$ magnetic units. Magnetization and specific heat indicate weakly interacting effective spin-1/2 moments below 10~K, with a Curie-Weiss temperature of only 15~mK, that can be polarized by magnetic fields of order 1~T. For the ADR performance test, we start the demagnetization from 5~T at a temperature of $\sim 2$~K and reach a minimum temperature of 150~mK at zero field. The warming curve indicates a sharp magnetic transition in the heat capacity at 210~mK, implying only weak magnetic frustration. The entropy density of $S_{\rm GS}\simeq 101$ mJ K$ ^{-1}$cm$^{-3}$ and hold time below 2~K of 220~min are competitive while the minimal temperature is higher compared to frustrated Ytterbium-oxide ADR materials studied under similar conditions.