The strong influence of nuclear spins on resonant quantum tunneling in the molecular cluster ${\mathrm{Fe}}_8$ is demonstrated for the first time by comparing the relaxation rate of the standard ${\mathrm{Fe}}_8$ sample with two isotopic modified samples: (i) ${}^{56}\mathrm{Fe}$ is replaced by ${}^{57}\mathrm{Fe}$, and (ii) a fraction of ${}^1\mathrm{H}$ is replaced by ${}^2\mathrm{H}$. By using a recently developed “hole digging” method, we measured an intrinsic broadening which is driven by the hyperfine fields. Our measurements are in good agreement with numerical hyperfine calculations. For $T>\mathrm{}1.5\mathrm{}$ K, the influence of nuclear spins on the relaxation rate is less important, suggesting that spin-phonon coupling dominates the relaxation rate.

• Received 4 November 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.2965