Calculated ro-vibrational fine-structure spectrum and weak-field zeeman splittings of the O2Ar van der Waals molecule

Ro-vibrational calculations are performed on O₂Ar explicitly including coupling to the O₂ electronic spin. Two different empirical potentials are used and give similar fine-structure spectra. The anisotropy in the potential is found to strongly perturb the O₂ fine-structure spectrum suggesting that the O₂Ar fine-structure spectrum can give detailed information about the anisotropy of the van der Waals interaction potential. Transition strengths for the complex fine-structure transitions are calculated and found to vary by two orders of magnitude. The Zeeman splitting of the levels by interaction with a weak magnetic field is also calculated.     

Giant isotope effect in the incoherent tunneling specific heat of the molecular nanomagnet Fe8

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 (MQT) in the ground doublet. Since gamma is found comparable to the expected tunneling rate, the MQT process has to be inelastic. A model for the coupling of the tunneling spins to the lattice is presented. Under transverse field, a crossover from nuclear-spin-mediated to phonon-induced tunneling is observed.     

Observation of quantum coherence in mesoscopic molecular magnets

By applying a transverse magnetic field B( perpendicular) of sufficient strength to the uniaxial molecular magnets Fe8 and Mn12, the tunneling splitting Delta(t) of their S = +/-10 magnetic ground states can be made large compared to perturbations such as hyperfine and dipolar interactions. We present evidence for such a Delta(t) from magnetic specific heat data below 1 K that is consistent with coherent quantum mechanical tunneling in a "mesoscopic" system under such conditions.     

Approach of single-molecule magnets to thermal equilibrium

We study the spin-lattice relaxation of single-molecule-magnets (SMM) using time-dependent specific heat C_m measurements. These molecular clusters, intermediate between paramagnetic atoms and ferromagnetic nanoparticles, are ideal systems to investigate if quantum phenomena contribute to relaxation at the mesoscopic scale. Experiments show indeed that relaxation to equilibrium proceeds by quantum tunnelling through the magnetic anisotropy energy barrier. For sufficiently high temperatures tunnelling takes place between excited magnetic states. Tunnelling via lower lying states can be promoted by applying a magnetic field B_⊥ perpendicular to the anisotropy axis. For sufficiently large B_⊥, the lowest energy states become quantum coherent superpositions. The equilibrium C_m is dominated, for T<1 K, by dipolar interactions between the molecular spins. A nearly isotropic Mn₆ cluster compound shows a transition to a ferromagnetic phase at For Ising-like SMM's, such as Mn₄, relaxation takes place by incoherent tunnelling between the lowest lying ±S states, assisted by interactions with phonons and nuclear spins. Tunnelling can then be promoted by lowering the symmetry of the molecule. In this case too, the molecular spins order if tunnelling remains sufficiently fast down to      

Magnetic spectra of the dimer O2Ar

With the magnetic-beam-resonance technique, magnetic rf transitions of expansion-cooled O₂Ar dimers have been measured in weak fields (B = 2.81 G). To measure the spectrum, particle-counting techniques have been applied to render beam signals as low as 1 count/s significant. Two empirical potentials have been used to calculate these Zeeman spectra. A perturbational model is developed by which the spectrum can be assigned; O₂Ar is found to have a T-shaped structure. Furthermore, best-fit values for the end-over-end rotational constant and the effective rotational barrier are obtained and suggestions for improvement of the interaction potential are made.     

Magnetic long-range order induced by quantum relaxation in single-molecule magnets

Can magnetic interactions between single-molecule magnets (SMMs) in a crystal establish long-range magnetic order at low temperatures deep in the quantum regime, where the only electron spin fluctuations are due to incoherent magnetic quantum tunneling (MQT)? Put inversely: can MQT provide the temperature dependent fluctuations needed to destroy the ordered state above some finite T(c), although it should basically itself be a T-independent process? Our experiments on two novel Mn4 SMMs provide a positive answer to the above, showing at the same time that MQT in the SMMs has to involve spin-lattice coupling at a relaxation rate equaling that predicted and observed recently for nuclear-spin-mediated quantum relaxation.     

Long-range ferromagnetic dipolar ordering of high-spin molecular clusters

We report the first example of a transition to long-range magnetic order in a purely dipolarly interacting molecular magnet. For the magnetic cluster compound Mn6O4Br4(Et2dbm)6, the anisotropy experienced by the total spin S = 12 of each cluster is so small that spin-lattice relaxation remains fast down to the lowest temperatures, thus enabling dipolar order to occur within experimental times at T(c) = 0.16 K. In high magnetic fields, the relaxation rate becomes drastically reduced and the interplay between nuclear- and electron-spin lattice relaxation is revealed.