Continuous-wave electron paramagnetic resonance (EPR) measurements on a 1% diluted polycrystalline sample of [Gd@Y(NO3)3]6H2O were recorded using a Bruker Elexys 580 spectrometer operating in X-band (9.715 GHz). The spectrometer was equipped with an Oxford CF900 continuous-flow cryostat, which employs liquid-helium to cool down the sample environment. The microwave power was adjusted to avoid saturation effects. The magnetic field was measured with a Bruker ER035 gaussmeter. Direct-current (DC) magnetic measurements were performed on a polycrystalline powder sample (ca. 5 mg), loaded into a plastic capsule and immobilized with eicosane wax to prevent grain movement. Isothermal magnetization curves were collected in magnetic fields between 0-7 T over the temperature range 2-10 K. DC magnetic susceptibility data were recorded over the same temperature range under a constant applied magnetic field of B=0.1 T. All data obtained were corrected for diamagnetic contributions coming from both the sample holder and eicosane wax. In addition, the complex alternating-current (AC) magnetic susceptibility was measured under zero applied magnetic field over the temperature range 0.05-4 K, using a frequency of 525 Hz and an AC field amplitude of 2·10^{-4} T. A thin pressed polycrystalline pellet (ca. 6 mg) was prepared for these experiments, and susceptibility data were initially recorded in arbitrary units. Absolute values of the real component were obtained by comparison with DC susceptibility data collected with the MPMS-3 in the overlapping temperature range (2-4 K), where dissipative processes are negligible. The imaginary component is reported in arbitrary units. Heat capacity measurements were carried out using the relaxation method in a Quantum Design PPMS equipped with a 3He cryostat, covering temperatures down to 0.4 K under constant magnetic fields from 0 to 7 T. Additional measurements extended to 0.05 K under fields up to 1 T using a Quantum Design PPMS DynaCool with DR module. Samples consisted of thin pressed polycrystalline pellets (ca. 1 mg for 3He and ca. 0.2 mg for DR). In both setups, Apiezon N grease (0.2 mg) was applied to ensure good thermal contact between the sample and the calorimeter platform. Its contribution to the total heat capacity was subtracted using a phenomenological correction. Direct quasi-adiabatic MCE measurements were performed for complete demagnetization processes. For these experiments, a pressed polycrystalline pellet of ca. 5 mg was mounted on a calorimeter platform coated with ca. 0.2 mg of Apiezon N grease to improve thermal coupling. In addition, the MCE was also evaluated indirectly by applying standard thermodynamic relations to the measured magnetization and heat capacity data.

We report magnetothermal measurements on the classical gadolinium salt [Gd(NO3)3]·6H2O down to temperatures approaching absolute zero. By combining electron paramagnetic resonance, magnetic susceptibility, and heat-capacity data, we show that the magnetic behavior of Gd(III) ions is dominated by crystal-field anisotropy, until magnetic dipolar interactions induce antiferromagnetic ordering at the Néel temperature T_N = 0.21 K. Estimates of the magnetocaloric effect reveal an isothermal magnetic entropy change of -Delta-Sm = 26.1 J K-1 kg-1 for a field variation of Delta-B = 1 T in the sub-Kelvin regime, highlighting the low operational temperature achievable with this material.