49 research outputs found
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 -GdSi (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 , 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. , , towards metallic behavior, with , 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
Low temperature magnetic hysteresis in Mn acetate single crystals
Precise magnetic hysteresis measurements of small single crystals of
Mn 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
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
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
'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 in all experiments done so far should be
, the half-width of the nuclear spin multiplet. After a time
, 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
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.
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
Crossover between Thermally Assisted and Pure Quantum Tunneling in Molecular Magnet Mn12-Acetate
The crossover between thermally assisted and pure quantum tunneling has been
studied in single crystals of high spin (S=10) uniaxial molecular magnet Mn12
using micro-Hall-effect magnetometry. Magnetic hysteresis and relaxation
experiments have been used to investigate the energy levels that determine the
magnetization reversal as a function of magnetic field and temperature. These
experiments demonstrate that the crossover occurs in a narrow (0.1 K) or broad
(1 K) temperature interval depending on the magnitude of the field transverse
to the anisotropy axis.Comment: 5 pages, 4 figure
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.
Quantum Step Heights in Hysteresis Loops of Molecular Magnets
We present an analytical theory on the heights of the quantum steps observed
in the hysteresis loops of molecular magnets. By considering the dipolar
interaction between molecular spins, our theory successfully yields the step
heights measured in experiments, and reveals a scaling law for the dependence
of the heights on the sweeping rates hidden in the experiment data on Fe
and Mn. With this theory, we show how to accurately determine the tunnel
splitting of a single molecular spin from the step heights.Comment: 4 pages, 5 figure