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For molecular nanomagnets, excesses of magnetic entropy at finite low temperatures are induced by degrees of freedom resulting from macroscopic degeneracies of the molecular spin states. We experimentally demonstrate that this effect can be huge and efficiently exploited for enhancing the magnetic field dependence of the adiabatic temperature change. We specifically consider the ${\text{Mn}}_{32}$ molecular nanomagnet, proving it to be an excellent magnetic refrigerant below $T\ensuremath{\approx}2\text{ }\text{K}$, due almost entirely to its spin degeneracy.

The ageing effects due to the interaction with typical working fluids (water and air) of magnetic refrigerant materials have been analysed for up to one year. Among the many compounds reported to exhibit a sufficiently high magnetocaloric effect, we decided to artificially age Gd, which represents the first choice high magnetocaloric element, Gd5Si1.9Ge2.1, belonging to the series Gd5(SixGe4?x)4 which for x ~ 0.5 displays a giant magnetocaloric effect, and finally, the eutectic Gd75Cd25 alloy, presenting an almost constant adiabatic temperature rise in an interesting temperature span (260?280?K). Magnetothermal results give evidence that corrosion and corrosion/erosion processes take place with different results on the refrigerant properties of all the materials. The adiabatic temperature rise is strongly reduced due to surface oxidation which lowers thermal conduction while the effect on the refrigerant capacity Q is definitely smaller. The effects of corrosion/erosion processes are confirmed by quantitative chemical analysis performed on the refrigerant fluid before and after the ageing process. Employing working fluids with a reduced corrosive effect but with comparable thermal properties may be a viable way of avoiding corrosion damage; alternatively the use of additives to decrease the corrosive properties of the fluids is strongly suggested.

We report on the magnetocaloric properties of two molecule-based hexacyanochromate Prussian blue analogs, nominally CsNi[Cr(CN)_6](H_2O) and Cr_3[Cr(CN)_6]_2x12(H_2O). The former orders ferromagnetically below Tc=90 K, whereas the latter is a ferrimagnet below Tc=230 K. For both, we find significantly large magnetic entropy changes DSm associated to the magnetic phase transitions. Notably, our studies represent the first attempt to look at molecule-based materials in terms of the magnetocaloric effect for temperatures well above the liquid helium range., 4 pages, 6 figures