Quantum physics of molecular magnets

In this thesis we focus on various aspects of quantum physics in molecular magnets, in particular, in Mn12-acetate. This thesis is divided into three parts.;In the first part, we present a review on molecular magnets. Since Mn 12-acetate has a large spin (equal to 10), the theory of tunneling of a large spin is discussed as well as the early experiments that were performed two decades ago and which has shown spin tunneling, in particular, the ones that were performed on gamma-Fe2O3 and on antiferromagnetic ferritin. Then, the first experiments that presented evidence on spin tunneling in Mn12-acetate are outlined in detail. Magnetic hysteresis curves are shown and Landau-Zener effect in molecular magnets is discussed. Quantum classical crossover between thermally assisted and pure quantum tunneling regimes is described.;Finally, magnetic avalanches are introduced: they are another feature of the magnetization curve in Mn12-Acetate where there is a sudden reversal in the magnetization. We exploit the first two experiments performed to elucidate the nature of magnetic avalanches in Mn12-acetate and the theory developed as a result of these experiments.;In the second part of this thesis, we focus on three of my publications on quantum magneto-mechanical effects. First, a recent experiment on Einstein-de Haas effect in a NiFe film deposited on a microcantilever is discussed. The cantilever was placed inside a coil that generated an ac magnetic field. Oscillation of the cantilever was measured by a fiber-optic interferometer positioned above the tip of the cantilever. When the frequency of the ac field matched the resonance frequency of the cantilever the amplitude of the oscillations was about 3 nm. The data were analyzed within a model that replaced the mechanical torque due to change in the magnetization with the effect of the periodic force acting on the fictitious point mass at the free end of the cantilever so this model did not account for the microscopic dynamics of the Einstein-de Haas effect. This motivated us to develop a more rigorous theoretical framework for the description of the dynamics of the Einstein-de Haas effect that we applied to the problem of the magnetic cantilever.;We then study the quantum dynamics of a magnetic molecule deposited on a microcantilever. Amplitude and frequencies of the coupled magneto-mechanical oscillations have been computed. We show that oscillations of the spin and the cantilever occur independently at frequencies Delta/h and o n respectively, unless these two frequencies come very close to each other.;The results show that the splitting delta has no free parameters and that for a given resonance, Delta = hon, the relative splitting delta depends only on the position of the molecule on the cantilever. We then show that existing experimental techniques permit observation of the driven coupled oscillations of the spin and the cantilever, as well as of the splitting of the mechanical modes of the cantilever caused by spin tunneling.;Finally, the dynamics of a magnetic molecule bridged between two conducting leads is investigated. We start by reviewing various experiments performed when there is a weak coupling between the molecule and the leads and when there is a strong coupling which results in the Kondo effect. Experimental efforts were mainly motivated to measure the electronic current through a single molecule. We study the dynamics of the total angular momentum that couples spin tunneling to the mechanical rotations. We show that the Landau-Zener spin transition produced by the time-dependent magnetic field generates a unique pattern of mechanical oscillations that can be detected by measuring the electronic tunneling current through the molecule.;In the last and final part, we present our numerical work to describe quantum magnetic deflagration in Mn12-acetate. This part is related to magnetic deflagration as discussed in part I of this thesis. The focus is on the quantum features of magnetic deflagration which are exhibited by the maxima in the speed of deflagration front as a function of the applied magnetic field. We review recent work on the effect of the dipolar field in forming self-organized fronts of spin tunneling, and present our enhanced computational work on the calculation of the relaxation rate. Previously, spin relaxation rates were calculated using a simple Arrhenius exponent. In this thesis we calculate the relaxation rate as a function of both the external field and temperature using the density matrix formalism and use them to study the effect of the transverse field on the front speed of deflagration.