Magnetic quantum tunneling in Fe8 with excited nuclei

We investigate the effect of dynamic nuclear-spin fluctuation on quantum tunneling of the magnetization (QTM) in the molecular magnet Fe8 by increasing the nuclei temperature using radio frequency (rf) pulses before the tunneling measurements. Independently we show that the nuclear-spin-spin relaxation time T2 has strong temperature dependence. Hence, in principle, the rf pulses should modify the nuclear-spin dynamic. Due to very long spin-lattice relaxation time, the rf pulses do not change the electrons spin temperature. Nevertheless, we found no effect of the nuclear-spin temperature on the tunneling probability. This suggests that in our experimental conditions only the hyperfine-field strength is relevant for QTM. We demonstrate theoretically how this can occur.