Persistent spin dynamics in the $S=1/2$ ${\text{V}}_{15}$ molecular nanomagnet

We present muon spin-lattice relaxation measurements in the ${\text{V}}_{15}$ spin 1/2 molecular nanomagnet. We find that the relaxation rate in low magnetic fields $\left(<5\text{kG}\right)$ is temperature independent below $\sim 10\text{K}$, implying that the molecular spin is dynamically fluctuating down to 12 mK. These measurements show that the fluctuation time increases as the temperature is decreased and saturates at a value of $\sim 6\text{ns}$ at low temperatures. The fluctuations are attributed to ${\text{V}}_{15}$ molecular spin dynamics perpendicular to the applied magnetic-field direction, induced by coupling between the molecular spin and nuclear spin bath in the system.

Figure 1

(Color online) The core of the ${\text{V}}_{15}$ molecule. Only ${\text{V}}^{\text{IV}}$ ions are shown here. The arrows indicate the ${\text{V}}^{\text{IV}}$ spins.

Figure 2

(Color online) Example muon spin-relaxation curves at (a) $T=15\text{mK}$ and different magnetic fields and (b) $H=50\text{G}$ and different temperatures. The solid lines are fits to Eq. (3).

Figure 3

(Color online) The muon spin-lattice relaxation rate as a function of temperature for different values of external magnetic field.

Figure 4

(Color online) The spin-lattice relaxation rate as a function of the applied magnetic field for $T=15\text{mK}$ (squares) and $T=2.7\text{K}$ (circles). The solid lines are fits to Eq. (8).

Figure 5

(Color online) (a) The correlation time $\tau$, (b) the size of the local field experienced by muons $\Delta$, and (c) the value of $f$ as a function of temperature.