Gadolinium oxyorthogermanate Gd₂GeO₅: An efficient solid refrigerant material for magnetic cryocoolers

Abstract

We report here the magnetic interactions, magnetocaloric effect and thermodynamic parameters of an efficient magnetic refrigerant material, Gd₂GeO₅, through magnetization and heat capacity measurements, as well as a mean-field and scaling analysis approach. An isotropic antiferromagnetic exchange coupling with energy scale ε_ex = J_exS² ≈ 2.6 K is estimated from analysing the magnetization dependences, in agreement with the couplings derived from the density functional theory (DFT). The Gd₂GeO₅ shows an exceptionally large isothermal magnetic entropy change of 0.36 J K⁻¹ cm⁻³ (50.3 J K⁻¹ kg⁻¹) and relative cooling power of 4.34 J cm⁻³ (611.3 J kg⁻¹) for a field change of 8.9 T, with the highest adiabatic temperature change being 22.2 K, which is quite impressive for use in helium-free magnetic cryocoolers.

Introduction

In recent years, the cooling requirements for T < 20 K temperatures are growing terrifically, such as in space detectors, scanners, and spintronic devices, as well as in creating cryogenic environments for quantum computing and liquifying helium and hydrogen [[1], [2], [3], [4]]. The adiabatic demagnetization refrigeration (ADR) technique that exploits the magnetocaloric effect is thus becoming increasingly popular, which describes the changes of adiabatic temperature and magnetic entropy following changes of magnetic fields [1,[5], [6], [7], [8]]. Magnetic refrigeration can offer an energy savings of up to 30% compared to traditional gas compressors by developing proper refrigeration procedures, and possesses characterizations of high efficiency and reliability, low microphonics and thermophonics [9]. Besides, the thermodynamic responses are highly reversible, thus suitable for a diverse array of large-scale utilization.

The research of magnetic refrigeration is an ongoing endeavour, which consists of designing prototypes of refrigerators, as well as searching for suitable refrigerants. The applicable magnetocaloric materials should be of good chemical stability, easy fabrication, high resistance to mechanical corrosion, high thermal conductivity, as well as good durability without fracturing or degradation in the thermal, pressure and magnetic cycles [7,8]. More specifically, the desirable characteristics of magnetocaloric materials should include low magnetic ordering temperature (typically 1–2 K below the lowest limit of the desired operating temperature window), large magnetic ions densities with weak crystal-field effect, as well as low electronic and lattice specific heat so that the internal heat load is minimized during the magnetic cycle [1]. In addition, the large magnetic entropy change shall be readily acquired within the lowest possible field changes, which can lower the economic cost significantly.

According to the above-mentioned criteria, the gadolinium compounds shall be prominent alternatives under consideration in the sense of magnetic entropy retaining [1,10,11]. The spin-orbit coupling in Gd³⁺ ions vanish up to the first order, and thus the splitting of the (2S + 1)-degenerate spin multiplets due to crystal-field interactions is relatively insignificant [12,13]. Consequently, the magnetic ordering transition is generally suppressed to a very low temperature, and the full reservation of magnetic entropy Rln8 per ion is available for temperatures of 2–20 K.

As a valid instance, the gadolinium gallium garnet Gd₃Ga₅O₁₂ (GGG) has long been the benchmark material for cryogenic magnetic refrigerant materials, which shows a high magnetic density and broad magnetic transition due to frustrated spin arrangements. The maximum isothermal magnetic entropy change for GGG is 0.30 J K⁻¹ cm⁻³ (42.4 J K⁻¹ kg⁻¹) under an applied field change of 9 T [1,[14], [15], [16]]. However, the magnetocaloric effect of GGG is far from satisfactory in terms of utilization efficiency of Gd³⁺ spin entropy. Further investigation of materials that shows a more pronounced magnetocaloric effect is appealing. A compromise is necessary to control the magnetic density and suppress the magnetic ordering simultaneously [17,18]. Therefore, we focus on the gadolinium oxyorthogermanate Gd₂GeO₅ with compacted spin centres and limited magnetic-passive elements Ge, which is clearly advantageous in regard to magnetic density.

Below, we investigate the magnetic and thermodynamic parameters of Gd₂GeO₅ through bulk magnetization and heat capacity measurements. Analysis of the magnetization isotherms reveals moderate antiferromagnetic interactions between isotropic Gd³⁺ spins, which endows the system with remarkably high magnetocaloric effects. The lack of magnetic ordering down to T < 2 K, and the sufficient low phonon contributions of heat capacity suggest that the Gd₂GeO₅ may be quite impressive for use in helium-free magnetic cryocoolers.