Magnetic refrigeration, leveraging the magnetocaloric effect (MCE), presents a promising eco-friendly alternative to vapor-compression and Joule-Thompson expansion refrigeration. Current vapor-compression systems suffer from low thermodynamic efficiency and reliance on high global warming potential refrigerants. Similarly, cryogenic cooling (like hydrogen liquefaction, crucial for large-scale hydrogen energy), which employs Joule-Thompson expansion, also benefits from improved efficiency. MCE-based refrigeration offers this improvement by utilizing the temperature change of a material upon application of a magnetic field. The development of high-performance, rare-earth-free magnetocaloric materials is particularly vital due to the cost and geopolitical issues surrounding rare-earth elements. The current research focuses on identifying such materials effective under moderate magnetic fields attainable with permanent magnets, reducing reliance on expensive superconducting magnets. This study aims to explore the magnetocaloric properties of a novel layered organic-inorganic hybrid coordination polymer, Co₄(OH)₆(SO₄)₂[enH₂], investigating its potential for hydrogen liquefaction applications around 20 K.

The search for effective magnetocaloric materials at cryogenic temperatures has predominantly centered on rare-earth elements, due to their large magnetic moments maximizing magnetic entropy change (Δ*S*_M). Materials like Gd₅Si₂Ge₂ exemplify this. However, the scarcity of rare earths drives the exploration of alternatives. Organic-inorganic hybrid materials offer tunable structure-property relationships through the selection of organic and inorganic building blocks. This approach has yielded success at ultra-low temperatures (T < 2 K) using Gd-based compounds. Examples include Gd(HCOO)₃ and Gd(OH)CO₃, showcasing comparable or superior magnetocaloric properties to GdGa₅O₁₂. However, the focus has shifted towards transition metal-based alternatives due to the sustainability concerns related to rare earths. Layered materials like β-Co(OH)₂ and CrCl₃ have demonstrated competitive performance with rare-earth based magnetocaloric compounds in the hydrogen liquefaction temperature regime, with significant Δ*S*_M. Co₃V₂O₈ and Co₂(OH)₄−ₓClₓ also show promise in this temperature range. These studies highlight the potential of layered, anisotropic, rare-earth-free materials for low-temperature applications. This research investigates Co₄(OH)₆(SO₄)₂[enH₂], a novel layered brucite-type hybrid material, in this context.

Single crystals of Co₄(OH)₆(SO₄)₂[enH₂] were synthesized via solvothermal methods. Structural characterization involved single-crystal X-ray diffraction (SCXRD) at 107 K, revealing a brucite-type structure with layers of Co(OH)₄(SO₄)₂ and Co(OH)₅(SO₄) octahedra separated by hydrogen-bonded ethylenediammonium cations. Fourier transform infrared (FTIR) spectroscopy confirmed the presence of hydroxyl groups. Sonication-assisted liquid-phase exfoliation in ethanol produced nanosheets, characterized by atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM). Magnetometry employed a Quantum Design MPMS XL-7T SQUID magnetometer to measure temperature-dependent magnetization (M(T)) parallel and perpendicular to the ab-plane, along with isothermal magnetization (M(μ₀H)) curves. AC magnetic susceptibility measurements investigated the frequency and field dependence of magnetic ordering. Heat capacity measurements (Quantum Design PPMS) under various magnetic fields provided data for indirect determination of adiabatic temperature change (Δ*T*_ad). Direct Δ*T*_ad measurements were made using pulsed magnetic fields at the Dresden High Magnetic Field Laboratory. Powder X-ray diffraction (PXRD) investigated the structural response under varying temperatures and magnetic fields. The Curie-Weiss law was employed to analyze the high-temperature magnetic susceptibility data. Critical exponent analysis was performed using Arrott plots and scaling analysis to determine the critical exponents (η, δ, β, γ) and classify the phase transition order.

The Co₄(OH)₆(SO₄)₂[enH₂] crystal structure features a layered brucite-type arrangement, with Co(OH)₄(SO₄)₂ and Co(OH)₅(SO₄) octahedra forming layers separated by ethylenediammonium cations. Magnetization measurements revealed a sharp increase below 20 K, indicative of ferromagnetic ordering, with negligible hysteresis. The saturation magnetization (M_sat) was approximately 0.13 μB/Co atom at low temperatures, suggesting a high-spin Co²⁺ ground state. AC susceptibility measurements revealed a dynamic character of the magnetic order, with frequency-dependent peaks in both the real (χ′) and imaginary (χ″) components. The temperature dependence of the inverse field-normalized magnetization (μ₀H/M) indicated deviations from Curie-Weiss behavior above T_c. Curie-Weiss fitting of the high-temperature data revealed ferromagnetic interactions within the ab-plane (θ_ab = 21.2 K) and antiferromagnetic interactions along the c-axis (θ_c = -50.1 K). Isothermal magnetization curves showed nearly saturated magnetization in low fields (μ₀H < 1 T), reaching M_sat ≈ 2.80 μB/Co atom below T_c. The coercive field was exceptionally low (μ₀H_coercive ≈ 5 mT). Analysis of the magnetic entropy change (Δ*S*_M) and adiabatic temperature change (Δ*T*_ad) indicated a peak Δ*S*_M = -6.3 J kg⁻¹K⁻¹ and Δ*T*_ad = 1.98 K for a 1 T field change near 13 K, values competitive with rare-earth containing alloys. Heat capacity measurements confirmed the second-order nature of the phase transition. The critical exponents obtained from scaling analysis were consistent with a 2D XY model, supporting the layered magnetic nature. Direct Δ*T*_ad measurements corroborated the indirect values obtained from heat capacity, except near T_c, where heat loss to the sample holder was suggested to be responsible for the discrepancy.

The excellent magnetocaloric properties of Co₄(OH)₆(SO₄)₂[enH₂] are attributed to its layered structure and the resulting easy-plane magnetic anisotropy. The minimal hysteresis observed is crucial for cyclic operation, unlike many first-order materials. The small coercive field is likely due to the strong easy-plane anisotropy, the competition between in-plane ferromagnetic (FM) and out-of-plane antiferromagnetic (AFM) interactions, and the dynamic nature of the magnetic ground state as evidenced by AC susceptibility. The observed frequency-dependent peaks in AC susceptibility, while typical of glassy magnetic systems, are attributed to the layered structure rather than spin-glass behavior. The behavior is similar to that observed in other ferromagnetic materials such as ErAl₂, (Dy₀.₅Er₀.₅)Al₂, ErTi₂Ga, and DyTi₂Ga, suggesting this dynamic character is a general feature of layered magnetic structures. The high performance of Co₄(OH)₆(SO₄)₂[enH₂] compared to other rare-earth free compounds such as Co₃V₂O₈ demonstrates its potential.

This research successfully synthesized and characterized a novel rare-earth-free layered coordination polymer, Co₄(OH)₆(SO₄)₂[enH₂], exhibiting exceptional magnetocaloric properties near the hydrogen liquefaction temperature. The material's layered structure, easy-plane anisotropy, and minimal hysteresis make it a promising candidate for cryogenic magnetic refrigeration. The structural flexibility of this material suggests potential for optimization via substitutions of transition metals like Fe or Mn, and tuning of the interlayer spacing. Further studies are needed to explore the thermal conductivity of this material, and the potential for improvement through such substitutions, paving the way for sustainable, efficient cryogenic refrigeration.

The heat loss in direct adiabatic temperature change (Δ*T*_ad) measurements near the critical temperature, due to the experimental setup (silver epoxy and GE varnish acting as heat sinks), likely resulted in an underestimation of the actual Δ*T*_ad. The polycrystalline nature of the sample used for heat capacity measurements, resulting from imperfect ab-plane alignment, also might have led to slightly lower Δ*S*_M values compared to single crystal measurements. These limitations highlight the need for further studies using optimized experimental configurations for accurate measurements, particularly near the phase transition.