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Register a new userGiant isotope effect in the incoherent tunneling specific heat of the molecular nanomagnet Fe8
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Time-dependent specific heat experiments on the molecular nanomagnet Fe8 and the isotopic enriched analogue 57Fe8 are presented. The inclusion of the 57Fe nuclear spins leads to a huge enhancement of the specific heat below 1 K, ascribed to a strong increase in the spin-lattice relaxation rate Gamma arising from incoherent, nuclear-spin-mediated magnetic quantum tunneling in the ground-doublet. Since Gamma is found comparable to the expected tunneling rate, the latter process has to be inelastic. A model for the coupling of the tunneling levels to the lattice is presented. Under transverse field, a crossover from nuclear-spin-mediated to phonon-induced tunneling is observed.
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We show that the dynamic magnetic susceptibility and the superparamagnetic blocking temperature of an Fe8 single molecule magnet oscillate as a function of the magnetic field Hx applied along its hard magnetic axis. These oscillations are associated with quantum interferences, tuned by Hx, between different spin tunneling paths linking two excited magnetic states. The oscillation period is determined by the quantum mixing between the ground S=10 and excited multiplets. These experiments enable us to quantify such mixing. We find that the weight of excited multiplets in the magnetic ground state of Fe8 amounts to approximately 11.6%.
A Mn30 molecular cluster is established to be the largest single-molecule magnet (SMM) discovered to date. Magnetization versus field measurements show coercive fields of about 0.5 T at low temperatures. Magnetization decay experiments reveal an Arrhenius behavior and temperature-independent relaxation below 0.2 K diagnostic of quantum tunneling of magnetization through the anisotropy barrier.The quantum hole digging method is used to establish resonant quantum tunneling. These results demonstrate that large molecular nanomagnets,having a volume of 15 nm^3, with dimensions approaching the mesoscale can still exhibit the quantum physics of the microscale.
We measure magnetization changes in a single crystal of the single-molecule magnet Fe8 when exposed to intense, short (<20 $mu$s) pulses of microwave radiation resonant with the m = 10 to 9 transition. We find that radiation induces a phonon bottleneck in the system with a time scale of ~5 $mu$s. The phonon bottleneck, in turn, drives the spin dynamics, allowing observation of thermally assisted resonant tunneling between spin states at the 100-ns time scale. Detailed numerical simulations quantitatively reproduce the data and yield a spin-phonon relaxation time of T1 ~ 40 ns.
The low temperature spin dynamics of a Fe8 Single-Molecule Magnet was studied under circularly polarized electromagnetic radiation allowing us to establish clearly photon-assisted tunneling. This effect, while linear at low power, becomes highly non-linear above a relatively low power threshold. This non-linearity is attributed to the nature of the coupling of the sample to the thermostat.These results are of great importance if such systems are to be used as quantum computers.
The synthesis, crystal structure and magnetic characterization of a high spin cluster comprising eight iron ions, are presented in this contribution. The cluster has formula [(tacn)6Fe8O2(OH)12Br4.3(ClO4)3.7]6H2O, (Fe8PCL) where tacn is the organic ligand 1,4,7-triazacyclononane. It can be considered a derivative of Fe8Br8, a cluster whose low temperature dynamics of the magnetization has been deeply investigated, where four of the bromide ions have been replaced by perchlorate anions. The structure of the central core of the two molecules, [Fe8O(OH)12(tacn)6]8+, is essentially the same, but Fe8PCL has a higher symmetry (Fe8Br8 crystallizes in the acentric P1 space group, while Fe8PCL crystallizes in the P21/c space group, monoclinic). The magnetic properties of Fe8PCL suggest it is very similar to Fe8Br having a S=10 ground state as well. The zero field splitting parameters were accurately determined by HF-EPR measurements. The two clusters have similar axial anisotropy but Fe8PCL has a larger transverse anisotropy. Ac susceptibility measurements revealed the cluster behaves like a superparamagnetic particle. However, due to the occurrence of large terms in the transverse anisotropy, the temperature dependence of the relaxation time can not be reproduced by a simple Arrhenius law. As observed in Fe8Br8, below 350 mK the relaxation time becomes temperature independent, indicating that a pure tunneling regime is attained. The comparison of the tunneling rate in the two clusters shows that in the perchlorate derivative the relaxation process is 35 times faster. The observed ratio of the tunneling rates is in reasonable agreement with that calculated from the tunneling splitting, i.e. the energy difference between the two almost degenerate lowest levels Ms = +/-10, in the two clusters.
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