The strong influence of nuclear spins on resonant quantum tunneling in the molecular cluster Fe8 is demonstrated for the first time by comparing the relaxation rate of the standard Fe8 sample with two isotopic modified samples: (i) 56 Fe is replaced by 57 Fe, and (ii) a fraction of 1 H is replaced by 2 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 > 1.5 K, the influence of nuclear spins on the relaxation rate is less important, suggesting that spin-phonon coupling dominates the relaxation rate.PACS numbers: 75.45.+j, 75.60Ej Mesoscopic quantum phenomena are actively investigated both for fundamental science and for future applications, for instance in quantum computing. Magnetic molecular clusters are among the most promising candidates to observe mesoscopic quantum phenomena [1,2]. One of the most prominent examples is an octanuclear iron(III) cluster, called Fe 8 (Fig. 1), with a spin ground state of S = 10 [3]. Below 360 mK, the magnetization relaxes through a pure tunneling process giving rise to a stepped hysteresis cycle [4]. Furthermore, the tunnel splitting ∆ of Fe 8 shows periodic oscillations when a transverse magnetic field is applied along the hard axis [5], a long searched phenomenon in magnetism associated with the Berry phase [6], and predicted several years before [7]. Since ∆ is extremely small for the ground state tunneling, ca. 10 −7 K at H = 0, the tunneling process should occur only in an extremely narrow magnetic field range, ca. 10 −8 T, and should be practically unobservable. However, a recent theory proposes that the tunneling is mediated by fluctuating hyperfine fields generated by magnetic nuclei [8], but direct experimental evidence is so far lacking.In order to study the influence of nuclear spins, we increased the hyperfine coupling by the substitution of 56 Fe with 57 Fe, and decreased it by the substitution of 1 H with 2 H. We found that the relaxation rate of magnetization in the tunneling regime shows a clear isotope effect which we attribute to the changed hyperfine coupling. et al. [9]. For the synthesis of the 57 Fe-enriched sample, 57 Fe 8 , a 13 mg foil of 95% inriched 57 Fe was dissolved in a few drops of HCl/HNO 3 (3 : 1) and the resulting solution was used as the iron source in the standard procedure.The 2 H-enriched Fe 8 sample, D Fe 8 , was crystallized from pyridine-d 5 and D 2 O (99%) under an inert atmosphere at 5 • C by using a non-deuterated Fe(tacn)Cl 3 precursor. The amount of isotope exchange was not quantitatively evaluated, but it can be reasonably assumed that the H atoms of H 2 O and of the bridging OH groups, as well as a part of those of the NH groups of the tacn ligands are replaced by deuterium while the aliphatic hydrogens are essentially not affected. The crystalline materials were carefully checked by elemental analysis and single-crystal X-ray diffraction.FIG. 1. Schematic vie...
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