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Adiabatic demagnetization refrigeration to millikelvin temperatures with the distorted square lattice magnet NaYbGeO4

U. Arjun, K. M. Ranjith, A. Jesche, F. Hirschberger, D. D. Sarma, and P. Gegenwart
Phys. Rev. B 108, 224415 – Published 11 December 2023
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  • INTRODUCTION
  • METHODS
  • RESULTS
  • DISCUSSION
  • CONCLUSION
  • ACKNOWLEDGMENTS
    • Supplemental Material
    References

    Abstract

    We report the synthesis, characterization, low-temperature magnetic, and thermodynamic measurements of the millikelvin adiabatic demagnetization refrigeration (mK-ADR) candidate material NaYbGeO4 which exhibits a distorted square lattice arrangement of YbO6 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 ∼2K 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 SGS≃101 mJK−1cm−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.

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    • Received 28 September 2023
    • Accepted 13 November 2023

    DOI:https://doi.org/10.1103/PhysRevB.108.224415

    ©2023 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Magnetism
    1. Physical Systems
    AntiferromagnetsRare-earth magnetic materials
    1. Techniques
    Adiabatic demagnetizationSpecific heat measurementsSusceptibility measurements
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    U. Arjun1,2,*, K. M. Ranjith3, A. Jesche2, F. Hirschberger2, D. D. Sarma1, and P. Gegenwart2,†

    • 1Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru-560012, India
    • 2Experimental Physics VI, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, 86135 Augsburg, Germany
    • 3Laboratoire National des Champs Magnétiques Intenses-EMFL, CNRS, Université Grenoble Alpes, 38042 Grenoble, France

    • *arjunu@iisc.ac.in
    • †philipp.gegenwart@physik.uni-augsburg.de

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    Issue

    Vol. 108, Iss. 22 — 1 December 2023

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    Images

    • Figure 1
      Figure 1

      Crystal structure of NaYbGeO4. The YbO6 octahedra are linked via GeO4 tetrahedra and span a distorted square lattice grid (left panel). Right panel shows the possible connections between the square lattice grids. Different Yb-Yb distances are color coded.

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    • Figure 2
      Figure 2

      Powder x-ray diffraction pattern (open red circles) for NaYbGeO4 at room temperatures. The solid line represents the Rietveld refinement, with the vertical bars showing the expected Bragg peak positions and the lower solid blue line representing the difference between observed and calculated intensities.

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    • Figure 3
      Figure 3

      Inverse magnetic susceptibility 1/χ vs temperature of NaYbGeO4 at a field of 1 T. Dashed and solid lines represent the fits by Eq. (1) and Eq. (2), respectively.

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    • Figure 4
      Figure 4

      Isothermal magnetization M(H) of NaYbGeO4 at a temperature of 0.4K. Dashed line represents linear contribution (see text). The inset shows the low-temperature inverse magnetic susceptibility after subtracting χ0 along with the CW fit.

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    • Figure 5
      Figure 5

      Low-temperature magnetic specific heat (Cm) of NaYbGeO4 at several external magnetic fields (upper panel). Phonon contribution was subtracted from the raw data using a polynomial fit [53]. The lower panel displays the magnetic entropy Sm(T) obtained by integrating Cm/T over temperature. For the 0 and 1 T data the integration constant was adjusted such that a value of Rln2 (thin black line) is reached above 10 K. Two arrows show the ADR process.

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    • Figure 6
      Figure 6

      Cooling of NaYbGeO4 by ADR. The red and blue lines display the temporal evolution of temperature (left y axis) and magnetic field (right y axis). After precooling in a field of 5 T, the sample space was evacuated and subsequently the magnetic field was swept from 5 to 0 T at a rate of 0.15 T min−1. The inset enlarges the regime close to zero field, where the temperature reaches a minimum of 135 mK at 0.1 T and then saturates at 150 mK in zero field. The warm-up time amounts to 220 min.

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    • Figure 7
      Figure 7

      Temperature dependence of the total (magnetic) heat capacity of the NaYbGeO4-based ADR pellet determined from the warming curve shown in Fig. 6 by calculating CADR=Q̇/Ṫ (red points). The constant heat input Q̇=0.58μW was used to obtain agreement with the measured heat capacity (black points) above 0.4 K.

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