Low-lying magnetic excitations and magnetocaloric effect of molecular magnet K₆[V₁₅As₆O₄₂(H₂O)] · 8H₂O

Low-temperature heat capacity measurements were performed on the molecular nanomagnet ${\text{K}}_6\left[{\text{V}}_{15}{\text{As}}_6{\text{O}}_{42}\right({\text{H}}_2\text{O}\left)\right]\cdot 8{\text{H}}_2\text{O}$ (V15). The low-lying magnetic excitations are clearly evidenced by the Schottky anomalies in the specific-heat data. The energy levels determined from the low-temperature observables agree well with the three-spin model for V15. The magnetocaloric effect of V15 is examined. The maximum entropy change of $5.31{\text{Jkg}}^{-1}{\text{K}}^{-1}$ is found for a field change of $\Delta H=(8-0.5)\text{T}$ at $\sim 1.5\text{K}$. In spite of the low ground-state spin of V15, a drastic entropy change of $4.12{\text{Jkg}}^{-1}{\text{K}}^{-1}$ is observed for a field change of $\Delta H=(8-0.05)\text{T}$ at 0.4 K, which is comparable to the entropy change of some high-spin sub-kelvin magnetic coolers at such low temperatures. Anisotropy and consequent zero-field splitting result in this characteristic of V15 and may open new possibilities in the design of ultra-low-temperature molecular coolers. (letter)