Exact Activation Energy of Magnetic Single Domain Particles

I present the exact analytical expression for the activation energy as a function of externally applied magnetic fields for a single--domain magnetic particle with uniaxial anisotropy (Stoner--Wohlfahrt model), and investigate the scaling behavior of the activation energy close to the switching boundary.Comment: published in 2004, posted here for general accessabilit

Quantum tunnelling of magnetization in Mn12-ac studied by 55Mn NMR

We present an ultra-low temperature study (down to T = 20 mK) of the nuclear spin-lattice relaxation (SLR) in the 55Mn nuclei of the molecular magnet Mn12-ac. The nuclear spins act as local probes for the electronic spin fluctuations, due to thermal excitations and to tunnelling events. In the quantum regime (below T = 0.75 K), the nuclear SLR becomes temperature-independent and is driven by fluctuations of the cluster's electronic spin due to the quantum tunnelling of magnetization in the ground doublet. The quantitative analysis of the nuclear SLR shows that the presence of fast-tunnelling molecules, combined with nuclear intercluster spin diffusion, plays an important role in the relaxation process.Comment: RevTex, 5 pages, 3 eps figures; presented at the Internation Conference on Molecular Magnets (Valencia, 5 - 10 Oct. 2002); to be published in Polyhedro

Segmentation of 4D MR Renography Images Using Temporal Dynamics in a Level Set Framework

A novel 4D level set framework was developed to segment dynamic MR images into the cortex, medulla and collecting system. The novelty of the method is that it combines information from spatial anatomical structures and temporal dynamics. The accuracy of the fully automatic 4D level set algorithm was found to be comparable to manual segmentation performed by experts on renal anatomy. The algorithm requires less than one minute to automatically segment a single kidney 4D patient data set with more than 40 time points

Density Matrix Equation for a Bathed Small System and its Application to Molecular Magnets

The technique of density matrix equation (DME) for a small system interacting with a bath is explained in detail. Special attention is given to the nonsecular DME that is needed in the vicinity of overdamped tunnelling resonances in molecular magnets (MM). The relaxation terms of the DME for MM are represented in the universal form that does not employ any unknown spin-lattice coupling constants and absorbs the information about the spin Hamiltonian in the exact basis states and transition frequencies. This makes adding new types of anisotropy easy and error-free. The Mathematica code is available from the author.Comment: 47 one-column PRE-style pages, 7 figure

Low temperature magnetic hysteresis in Mn12_{12} acetate single crystals

Precise magnetic hysteresis measurements of small single crystals of Mn$_{12}$ acetate of spin 10 have been conducted down to 0.4 K using a high sensitivity Hall magnetometer. At higher temperature (>1.6K) step-like changes in magnetization are observed at regularly spaced magnetic field intervals, as previously reported. However, on lowering the temperature the steps in magnetization shift to higher magnetic fields, initially gradually. These results are consistent with the presence of a second order uniaxial magnetic anisotropy, first observed by EPR spectroscopy, and thermally assisted tunnelling with tunnelling relaxation occurring from levels of progressively lower energy as the temperature is reduced. At lower temperature an abrupt shift in step positions is found. We suggest that this shift may be the first evidence of an abrupt, or first-order, transition between thermally assisted and pure quantum tunnelling, suggested by recent theory.Comment: 8 pages, 4 figures, submitted to Europhys. Let

Variation of the density of states in amorphous GdSi at the metal-insulator transition

We performed detailed conductivity and tunneling mesurements on the amorphous, magnetically doped material $\alpha$-Gd$_x$Si$_{1-x}$ (GdSi), which can be driven through the metal-insulator transition by the application of an external magnetic field. Conductivity increases linearly with field near the transition and slightly slower on the metallic side. The tunneling conductance, proportional to the density of states $N(E)$, undergoes a gradual change with increasing field, from insulating, showing a soft gap at low bias, with a slightly weaker than parabolic energy dependence, i.e. $N(E) \sim E^c$, $c \lesssim 2$, towards metallic behavior, with $E^d$, $0.5 \lt d \lt 1$ energy dependence. The density of states at the Fermi level appears to be zero at low fields, as in an insulator, while the sample shows already small, but metal-like conductivity. We suggest a possible explanation to the observed effect.Comment: 6 pages, 6 figure

'Hole-digging' in ensembles of tunneling Molecular Magnets

The nuclear spin-mediated quantum relaxation of ensembles of tunneling magnetic molecules causes a 'hole' to appear in the distribution of internal fields in the system. The form of this hole, and its time evolution, are studied using Monte Carlo simulations. It is shown that the line-shape of the tunneling hole in a weakly polarised sample must have a Lorentzian lineshape- the short-time half-width $\xi_o$ in all experiments done so far should be $\sim E_0$, the half-width of the nuclear spin multiplet. After a time $\tau_o$, the single molecule tunneling relaxation time, the hole width begins to increase rapidly. In initially polarised samples the disintegration of resonant tunneling surfaces is found to be very fast.Comment: 4 pages, 5 figure

Observation of a Distribution of Internal Transverse Magnetic Fields in a Mn12-Based Single Molecule Magnet

A distribution of internal transverse magnetic fields has been observed in single molecule magnet (SMM) Mn12-BrAc in the pure magnetic quantum tunneling (MQT) regime. Magnetic relaxation experiments at 0.4 K are used to produce a hole in the distribution of transverse fields whose angle and depth depend on the orientation and amplitude of an applied transverse ``digging field.'' The presence of such transverse magnetic fields can explain the main features of resonant MQT in this material, including the tunneling rates, the form of the relaxation and the absence of tunneling selection rules. We propose a model in which the transverse fields originate from a distribution of tilts of the molecular magnetic easy axes.Comment: 4 page

Field-tuned quantum tunneling in a supramolecule dimer [Mn4]2[Mn_4]_2

Field-tuned quantum tunneling in two single-molecule magnets coupled antiferromagnetically and formed a supramolecule dimer is studied. We obtain step-like magnetization curves by means of the numerically exact solution of the time-dependent Schr\H{o}dinger equation. The steps in magnetization curves show the phenomenon of quantum resonant tunneling quantitatively. The effects of the sweeping rate of applied field is discussed. These results obtained from quantum dynamical evolution well agree with the recent experiment[W.Wernsdorfer et al. Nature 416(2002)406].Comment: 11 pages, 4 figures, 2 tables. Submited to Phys. Rev.

Photon-assisted tunneling in a Fe8 Single-Molecule Magnet

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.Comment: 4 pages, 4 figure