Adiabatic demagnetization refrigeration to millikelvin temperatures with the distorted square lattice magnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>NaYbGeO</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:math>
U. Arjun, K. M. Ranjith, Anton Jesche, F. Hirschberger, D. D. Sarma, P. Gegenwart
Abstract
We report the synthesis, characterization, low-temperature magnetic, and thermodynamic measurements of the millikelvin adiabatic demagnetization refrigeration (mK-ADR) candidate material ${\mathrm{NaYbGeO}}_{4}$ which exhibits a distorted square lattice arrangement of ${\mathrm{YbO}}_{6}$ 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 $\ensuremath{\sim}2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ 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 ${S}_{\mathrm{GS}}\ensuremath{\simeq}101 \mathrm{mJ}{\mathrm{K}}^{\ensuremath{-}1}{\mathrm{cm}}^{\ensuremath{-}3}$ 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.