14 research outputs found
Quantum tunneling in a three dimensional network of exchange coupled single-molecule magnets
A Mn4 single-molecule magnet (SMM) is used to show that quantum tunneling of
magnetization (QTM) is not suppressed by moderate three dimensional exchange
coupling between molecules. Instead, it leads to an exchange bias of the
quantum resonances which allows precise measurements of the effective exchange
coupling that is mainly due to weak intermolecular hydrogen bounds. The
magnetization versus applied field was recorded on single crystals of [Mn4]2
using an array of micro-SQUIDs. The step fine structure was studied via minor
hysteresis loops.Comment: 4 pages, 4 figure
Asymmetric Lineshape due to Inhomogeneous Broadening of the Crystal-Field Transitions in Mn12ac Single Crystals
The lineshape of crystal-field transitions in single crystals of Mn12ac
molecular magnets is determined by the magnetic history. The absorption lines
are symmetric and Gaussian for the non-magnetized state obtained by zero-field
cooling (zfc). In the magnetized state which is reached when the sample is
cooled in a magnetic field (fc), however, they are asymmetric even in the
absence of an external magnetic field. These observations are quantitatively
explained by inhomogeneous symmetrical (Gaussian) broadening of the
crystal-field transitions combined with a contribution of off-diagonal
components of the magnetic susceptibility to the effective magnetic
permeability.Comment: 4 pages, 3 figure
Glauber dynamics in a single-chain magnet: From theory to real systems
The Glauber dynamics is studied in a single-chain magnet. As predicted, a
single relaxation mode of the magnetization is found. Above 2.7 K, the
thermally activated relaxation time is mainly governed by the effect of
magnetic correlations and the energy barrier experienced by each magnetic unit.
This result is in perfect agreement with independent thermodynamical
measurements. Below 2.7 K, a crossover towards a relaxation regime is observed
that is interpreted as the manifestation of finite-size effects. The
temperature dependences of the relaxation time and of the magnetic
susceptibility reveal the importance of the boundary conditions.Comment: Submitted to PRL 10 May 2003. Submitted to PRB 12 December 2003;
published 15 April 200
Linewidth of single photon transitions in Mn-acetate
We use time-domain terahertz spectroscopy to measure the position and
linewidth of single photon transitions in Mn-acetate. This linewidth is
compared to the linewidth measured in tunneling experiments. We conclude that
local magnetic fields (due to dipole or hyperfine interactions) cannot be
responsible for the observed linewidth, and suggest that the linewidth is due
to variations in the anisotropy constants for different clusters. We also
calculate a lower limit on the dipole field distribution that would be expected
due to random orientations of clusters and find that collective effects must
narrow this distribution in tunneling measurements.Comment: 5 pages, accepted to Physical Review
Field-tuned quantum tunneling in a supramolecule dimer
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
Spin-parity dependent tunneling of magnetization in single-molecule magnets
Single-molecule magnets facilitate the study of quantum tunneling of
magnetization at the mesoscopic level. The spin-parity effect is among the
fundamental predictions that have yet to be clearly observed. It is predicted
that quantum tunneling is suppressed at zero transverse field if the total spin
of the magnetic system is half-integer (Kramers degeneracy) but is allowed in
integer spin systems. The Landau-Zener method is used to measure the tunnel
splitting as a function of transverse field. Spin-parity dependent tunneling is
established by comparing the transverse field dependence of the tunnel
splitting of integer and half-integer spin systems.Comment: 4 pages, 6 figure
Semiconductive and Photoconductive Properties of the Single Molecule Magnets Mn-Acetate and FeBr
Resistivity measurements are reported for single crystals of
Mn-Acetate and FeBr. Both materials exhibit a
semiconductor-like, thermally activated behavior over the 200-300 K range. The
activation energy, , obtained for Mn-Acetate was 0.37 0.05
eV, which is to be contrasted with the value of 0.55 eV deduced from the
earlier reported absorption edge measurements and the range of 0.3-1 eV from
intramolecular density of states calculations, assuming = , the
optical band gap. For FeBr, was measured as 0.73 0.1 eV,
and is discussed in light of the available approximate band structure
calculations. Some plausible pathways are indicated based on the crystal
structures of both lattices. For Mn-Acetate, we also measured
photoconductivity in the visible range; the conductivity increased by a factor
of about eight on increasing the photon energy from 632.8 nm (red) to 488 nm
(blue). X-ray irradiation increased the resistivity, but was insensitive
to exposure.Comment: 7 pages, 8 figure