Large sub-Kelvin magnetocaloric effect and phase behavior of gadolinium nitrate hexahydrate

We report a comprehensive magnetothermal study of the classical gadolinium salt [Gd(NO3)3] · 6H2O at temperatures approaching absolute zero. Despite its long-standing availability as a commercial compound, its magnetocaloric properties have remained unexplored. Using electron paramagnetic resonance, magnetic susceptibility, heat capacity, and direct measurements of the magnetocaloric effect, we reveal that weak dipolar interactions combined with axial crystal-field anisotropy drive antiferromagnetic ordering at TN = 0.21 K. Remarkably, [Gd(NO3)3] · 6H2O exhibits an isothermal magnetic entropy change of −ΔSm = 26.1 J K−1 kg−1 at T = 0.6 K for a modest field variation of ΔB = 1 T and an adiabatic temperature change of ΔTad = 5.0 K at T = 1.6 K for the same ΔB, placing its performance among the strongest reported for Gd-based refrigerants under comparable low-field and sub-Kelvin conditions. These results provide a reference for understanding magnetocaloric behavior in hydrated salts and highlight the interplay between dipolar coupling and crystal-field anisotropy.