Magnetic field relaxation in ferromagnetic Ising systems

We analyze the thermal magnetization reversal processes in magnetic grains. Two experiments are carried out: swtiching time and switching field experiments. In both cases, we find that the simulated behavior is coherent with existing experimental data (the streched exponent of the switching time experiment increases with the temperature and is superior to unity; there exists a master curve for the switching field experiment). Moreover, we simulated magnetic grains in a region of parameters where no experimental data are available. We find that the relaxation time distribution $P(\ln{\tau})$ is gaussian, and we find the existence of a strong field regime.Comment: 9 pages, 7 figures, J.M.M.

Magnetic energy-level diagrams of high-spin (Mn12_{12}-acetate) and low-spin (V15_{15}) molecules

The magnetic energy-level diagrams for models of the Mn12 and V15 molecule are calculated using the Lanczos method with full orthogonalization and a Chebyshev-polynomial-based projector method. The effect of the Dzyaloshinskii-Moriya interaction on the appearance of energy-level repulsions and its relevance to the observation of steps in the time-dependent magnetization data is studied. We assess the usefulness of simplified models for the description of the zero-temperature magnetization dynamics

Nonadiabatic Landau Zener tunneling in Fe_8 molecular nanomagnets

The Landau Zener method allows to measure very small tunnel splittings \Delta in molecular clusters Fe_8. The observed oscillations of \Delta as a function of the magnetic field applied along the hard anisotropy axis are explained in terms of topological quantum interference of two tunnel paths of opposite windings. Studies of the temperature dependence of the Landau Zener transition rate P gives access to the topological quantum interference between exited spin levels. The influence of nuclear spins is demonstrated by comparing P of the standard Fe_8 sample with two isotopically substituted samples. The need of a generalized Landau Zener transition rate theory is shown.Comment: 5 pages, 6 figure

Spin relaxation in Mn12-acetate

We present a comprehensive derivation of the magnetization relaxation in a Mn12-acetate crystal based on thermally assisted spin tunneling induced by quartic anisotropy and weak transverse magnetic fields. The overall relaxation rate as function of the magnetic field is calculated and shown to agree well with data including all resonance peaks. The Lorentzian shape of the resonances is also in good agreement with recent data. A generalized master equation including resonances is derived and solved exactly. It is shown that many transition paths with comparable weight exist that contribute to the relaxation process. Previously unknown spin-phonon coupling constants are calculated explicitly.Comment: 4 pages,4 EPS figures,LaTeX(europhys.sty);final version accepted for EP

Magnetic quantum coherence effect in Ni4 molecular transistors

We consider the electron transport in single molecule magnet transistors in the presence of Zeeman spin splitting and magnetic quantum coherence (MQC). The Zeeman interaction is extended along the leads, thereby producing gaps in the energy spectrum which allow electron transport with spin polarized along a certain direction. The MQC induces an effective coupling between localized spin states and continuum spin states in the single molecule magnet and leads, respectively. We investigate the conductance at zero temperature as a function of the applied bias and magnetic field, and show that the MQC is responsible for the appearence of resonances. Accordingly, we name them MQC resonances.Comment: 5 pages, Revtex

Feedback Effect on Landau-Zener-Stueckelberg Transitions in Magnetic Systems

We examine the effect of the dynamics of the internal magnetic field on the staircase magnetization curves observed in large-spin molecular magnets. We show that the size of the magnetization steps depends sensitively on the intermolecular interactions, even if these are very small compared to the intra-molecular couplings.Comment: 4 pages, 3 Postscript figures; paper reorganized, conclusions modifie

Level splittings in exchange-biased spin tunneling

The level splittings in a dimer with the antiferromagnetic coupling between two single-molecule magnets are calculated perturbatively for arbitrary spin. It is found that the exchange interaction between two single-molecule magnets plays an important role in the level splitting. The results are discussed in comparison with the recent experiment.Comment: 12 pages, to be published in Phys. Rev.

Nonexponential Relaxation of Magnetization at the Resonant Tunneling Point under a Fluctuating Random Noise

Nonexponential relaxation of magnetization at resonant tunneling points of nanoscale molecular magnets is interpreted to be an effect of fluctuating random field around the applied field. We demonstrate such relaxation in Langevin equation analysis and clarify how the initial relaxation (square-root time) changes to the exponential decay. The scaling properties of the relaxation are also discussed.Comment: 4 pages, 4 fgiure

Dislocation-induced spin tunneling in Mn-12 acetate

Comprehensive theory of quantum spin relaxation in Mn-12 acetate crystals is developed, that takes into account imperfections of the crystal structure and is based upon the generalization of the Landau-Zener effect for incoherent tunneling from excited energy levels. It is shown that linear dislocations at plausible concentrations provide the transverse anisotropy which is the main source of tunneling in Mn-12. Local rotations of the easy axis due to dislocations result in a transverse magnetic field generated by the field applied along the c-axis of the crystal, which explains the presence of odd tunneling resonances. Long-range deformations due to dislocations produce a broad distribution of tunnel splittings. The theory predicts that at subkelvin temperatures the relaxation curves for different tunneling resonances can be scaled onto a single master curve. The magnetic relaxation in the thermally activated regime follows the stretched-exponential law with the exponent depending on the field, temperature, and concentration of defects.Comment: 17 pages, 14 figures, 1 table, submitted to PR

Quantum dynamical calculations on the magnetization reversal in clusters of spin-1/2 particles:Resonant coherent quantum tunneling

In the present work the reversal of magnetization and the coherence of tunneling when an external magnetic field is rotated instantaneously are studied in systems of a few spin-1/2 particles described by an anisotropic Heisenberg Hamiltonian at T=0. Our calculations demonstrate that this model for small magnetic particles exhibits collective tunneling of the magnetization only for some specific resonant values of the applied magnetic field. These resonant effects occur at fields much lower than the values corresponding to the vanishing of the barrier in the Stoner-Wohlfarth model. The former model is at variance with the exact calculations presented in this paper.</p