Nuclear spin driven resonant tunnelling of magnetisation in Mn12 acetate

Current theories still fail to give a satisfactory explanation of the observed quantum phenomena in the relaxation of the magnetisation of the molecular cluster Mn12 acetate. In the very low temperature regime, Prokof'ev and Stamp recently proposed that slowly changing dipolar fields and rapidly fluctuating hyperfine fields play a major role in the tunnelling process. By means of a faster relaxing minor species of Mn12ac and a new experimental 'hole digging' method, we measured the intrinsic line width broadening due to local fluctuating fields, and found strong evidence for the influence of nuclear spins on resonance tunnelling at very low temperatures (0.04 - 0.3K). At higher temperature (1.5 - 4K), we observed a homogeneous line width broadening of the resonance transitions being in agreement with a recent calculation of Leuenberger and Loss.Comment: 7 pages, 6 figures, submitted to Europhys. Let

Entanglement in finite spin rings with noncollinear Ising interaction

We investigate the entanglement properties of finite spin rings, with noncollinear Ising interaction between nearest neighbours. The orientations of the Ising axes are determined either by the spin position within the ring (model A) or by the direction of the bond (model B). In both cases, the considered spin Hamiltonians have a point group symmetry, rather than a translation invariance, as in spin rings with collinear Ising interaction. The ground state of these models exhibit remarkable entanglement properties, resembling GHZ-like states in the absence of an applied magnetic field (model B). Besides, the application of an homogeneous magnetic field allows to modify qualitatively the character of the ground state entanglement, switching from multipartite to pairwise quantum correlations (both models A and B)

Dynamical Monte Carlo investigation of spin reversals and nonequilibrium magnetization of single-molecule magnets

In this paper, we combine thermal effects with Landau-Zener (LZ) quantum tunneling effects in a dynamical Monte Carlo (DMC) framework to produce satisfactory magnetization curves of single-molecule magnet (SMM) systems. We use the giant spin approximation for SMM spins and consider regular lattices of SMMs with magnetic dipolar interactions (MDI). We calculate spin reversal probabilities from thermal-activated barrier hurdling, direct LZ tunneling, and thermal-assisted LZ tunnelings in the presence of sweeping magnetic fields. We do systematical DMC simulations for Mn$_{12}$ systems with various temperatures and sweeping rates. Our simulations produce clear step structures in low-temperature magnetization curves, and our results show that the thermally activated barrier hurdling becomes dominating at high temperature near 3K and the thermal-assisted tunnelings play important roles at intermediate temperature. These are consistent with corresponding experimental results on good Mn$_{12}$ samples (with less disorders) in the presence of little misalignments between the easy axis and applied magnetic fields, and therefore our magnetization curves are satisfactory. Furthermore, our DMC results show that the MDI, with the thermal effects, have important effects on the LZ tunneling processes, but both the MDI and the LZ tunneling give place to the thermal-activated barrier hurdling effect in determining the magnetization curves when the temperature is near 3K. This DMC approach can be applicable to other SMM systems, and could be used to study other properties of SMM systems.Comment: Phys Rev B, accepted; 10 pages, 6 figure

Towards the chemical tuning of entanglement in molecular nanomagnets

Antiferromagnetic spin rings represent prototypical realizations of highly correlated, low-dimensional systems. Here we theoretically show how the introduction of magnetic defects by controlled chemical substitutions results in a strong spatial modulation of spin-pair entanglement within each ring. Entanglement between local degrees of freedom (individual spins) and collective ones (total ring spins) are shown to coexist in exchange-coupled ring dimers, as can be deduced from general symmetry arguments. We verify the persistence of these features at finite temperatures, and discuss them in terms of experimentally accessible observables.Comment: 5 pages, 4 figure

Interplay of the Kondo Effect and Spin-Polarized Transport in Magnetic Molecules, Adatoms and Quantum Dots

We study the interplay of the Kondo effect and spin-polarized tunneling in a class of systems exhibiting uniaxial magnetic anisotropy, such as magnetic molecules, magnetic adatoms, or quantum dots coupled to a single localized magnetic moment. Using the numerical renormalization group method we calculate the spectral functions and linear conductance in the Kondo regime. We show that the exchange coupling between conducting electrons and localized magnetic core generally leads to suppression of the Kondo effect. We also predict a nontrivial dependence of the tunnel magnetoresistance on the strength of exchange coupling and on the anisotropy constant.Comment: 4 pages with 4 EPS figures (version as accepted for publication in Physical Review Letters

Multiple nearest-neighbor exchange model for the frustrated magnetic molecules Mo72Fe30 and Mo72Cr30

Our measurements of the differential susceptibility dM/dH of the frustrated magnetic molecules Mo72Fe30 and Mo72Cr30 reveal a pronounced dependence on magnetic field (H) and temperature (T) in the low H - low T regime, contrary to the predictions of existing models. Excellent agreement with experiment is achieved upon formulating a nearest-neighbor classical Heisenberg model where the 60 nearest-neighbor exchange interactions in each molecule, rather than being identical as has been assumed heretofore, are described by a two-parameter probability distribution of values of the exchange constant. We suggest that the probability distribution provides a convenient phenomenological platform for summarizing the combined effects of multiple microscopic mechanisms that disrupt the idealized picture of a Heisenberg model based on a single value of the nearest-neighbor exchange constant.Comment: 8 pages, 5 figure

Phases of anisotropic dipolar antiferromagnets

We study systems of classical magnetic dipoles on simple cubic lattices with dipolar and antiferromagnetic exchange interactions. By analysis and Monte Carlo (MC) simulations, we find how the antiferromagnetic phases vary with uniaxial and fourfold anisotropy constants, C and D, as well as with exchange strength J. We pay special attention to the spin reorientation (SR) phase, and exhibit in detail the nature of its broken symmetries. By mean field theory and by MC, we also obtain the ratio of the higher ordering temperature to the SR transition temperature, and show that it depends mainly on D/C, and rather weakly on J. We find a reverse SR transition.Comment: 10 LaTeX pages, 14 eps figures. Submitted to PRB on 03 October 2005. Accepted on 13 December 200

Orbital Kondo effect in Cobalt-Benzene sandwich molecules

We study a Co-benzene sandwich molecule bridging the tips of a Cu nanocontact as a realistic model of correlated molecular transport. To this end we employ a recently developed method for calculating the correlated electronic structure and transport properties of nanoscopic conductors. When the molecule is slightly compressed by the tips of the nanocontact the dynamic correlations originating from the strongly interacting Co 3d shell give rise to an orbital Kondo effect while the usual spin Kondo effect is suppressed due to Hund's rule coupling. This non-trivial Kondo effect produces a sharp and temperature-dependent Abrikosov-Suhl resonance in the spectral function at the Fermi level and a corresponding Fano line shape in the low bias conductance

Mechanisms of decoherence in weakly anisotropic molecular magnets

Decoherence mechanisms in crystals of weakly anisotropic magnetic molecules, such as V15, are studied. We show that an important decohering factor is the rapid thermal fluctuation of dipolar interactions between magnetic molecules. A model is proposed to describe the influence of this source of decoherence. Based on the exact solution of this model, we show that at relatively high temperatures, about 0.5 K, the quantum coherence in a V15 molecule is not suppressed, and, in principle, can be detected experimentally. Therefore, these molecules may be suitable prototype systems for study of physical processes taking place in quantum computers.Comment: 4 pages RevTeX, 1 figure (PostScript