160 research outputs found
Quantum dynamics of the Neel vector in the antiferromagnetic molecular wheel CsFe8
The inelastic neutron scattering (INS) spectrum is studied for the
antiferromagnetic molecular wheel CsFe8, in the temperature range 2 - 60 K, and
for transfer energies up 3.6 meV. A qualitative analysis shows that the
observed peaks correspond to the transitions between the L-band states, from
the ground state up to the S = 5 multiplet. For a quantitative analysis, the
wheel is described by a microscopic spin Hamiltonian (SH), which includes the
nearest-neighbor Heisenberg exchange interactions and uniaxial easy-axis
single-ion anisotropy, characterized by the constants J and D, respectively.
For a best-fit determination of J and D, the L band is modeled by an effective
SH, and the effective SH concept extended such as to facilitate an accurate
calculation of INS scattering intensities, overcoming difficulties with the
dimension of the Hilbert space. The low-energy magnetism in CsFe8 is
excellently described by the generic SH used. The two lowest states are
characterized by a tunneling of the Neel vector, as found previously, while the
higher-lying states are well described as rotational modes of the Neel vector.Comment: 12 pages, 10 figures, REVTEX4, to appear in PR
Many-spin effects in inelastic neutron scattering and electron paramagnetic resonance of molecular nanomagnets
Many molecular magnetic clusters, such as single-molecule magnets, are
characterized by spin ground states with defined total spin S exhibiting
zero-field-splittings. In this work, the spectroscopic intensities of the
transitions within the ground-state multiplet are analyzed. In particular, the
effects of a mixing with higher-lying spin multiplets, which is produced by
anisotropic interactions and is neglected in the standard single-spin
description, are investigated systematically for the two experimental
techniques of inelastic neutron scattering (INS) and electron paramagnetic
resonance (EPR), with emphasis on the former technique. The spectroscopic
transition intensities are calculated analytically by constructing
corresponding effective spin operators perturbationally up to second order and
consequently using irreducible tensor operator techniques. Three main effects
of spin mixing are observed. Firstly, a pronounced dependence of the INS
intensities on the momentum transfer Q, with a typical oscillatory behavior,
emerges in first order, signaling the many-spin nature of the wave functions in
exchange-coupled clusters. Secondly, as compared to the results of a
first-order calculation, the intensities of the transitions within the spin
multiplet are affected differently by spin mixing. This allows one, thirdly, to
differentiate the higher-order contributions to the cluster magnetic anisotropy
which come from the single-ion ligand-field terms and spin mixing,
respectively. The analytical results are illustrated by means of the three
examples of an antiferromagnetic heteronuclear dimer, the Mn-[3 x 3] grid
molecule, and the single-molecule magnet Mn12.Comment: 18 pages, 3 figures, REVTEX4, to appear in PR
Exchange-coupling constants, spin density map, and Q dependence of the inelastic neutron scattering intensity in single-molecule magnets
The Q dependence of the inelastic neutron scattering (INS) intensity of
transitions within the ground-state spin multiplet of single-molecule magnets
(SMMs) is considered. For these transitions, the Q dependence is related to the
spin density map in the ground state, which in turn is governed by the
Heisenberg exchange interactions in the cluster. This provides the possibility
to infer the exchange-coupling constants from the Q dependence of the INS
transitions within the spin ground state. The potential of this strategy is
explored for the M = +-10 -> +- 9 transition within the S = 10 multiplet of the
molecule Mn12 as an example. The Q dependence is calculated for powder as well
as single-crystal Mn12 samples for various exchange-coupling situations
discussed in the literature. The results are compared to literature data on a
powder sample of Mn12 and to measurements on an oriented array of about 500
single-crystals of Mn12. The calculated Q dependence exhibits significant
variation with the exchange-coupling constants, in particular for a
single-crystal sample, but the experimental findings did not permit an
unambiguous determination. However, although challenging, suitable experiments
are within the reach of today's instruments.Comment: 11 pages, 6 figures, REVTEX4, to appear in PR
A spatially resolved limb flare on Algol B observed with XMM-Newton
We report XMM-Newton observations of the eclipsing binary Algol A (B8V) and B
(K2III). The XMM-Newton data cover the phase interval 0.35 - 0.58, i.e.,
specifically the time of optical secondary minimum, when the X-ray dark B-type
star occults a major fraction of the X-ray bright K-type star. During the
eclipse a flare was observed with complete light curve coverage. The decay part
of the flare can be well described with an exponential decay law allowing a
rectification of the light curve and a reconstruction of the flaring plasma
region. The flare occurred near the limb of Algol B at a height of about 0.1R
with plasma densities of a few times 10^11 cm^-3 consistent with spectroscopic
density estimates. No eclipse of the quiescent X-ray emission is observed
leading us to the conclusion that the overall coronal filling factor of Algol B
is small.Comment: 8 pages, 7 figures, accepted by A&
Magnetic relaxation studies on a single-molecule magnet by time-resolved inelastic neutron scattering
Time-resolved inelastic neutron scattering measurements on an array of
single-crystals of the single-molecule magnet Mn12ac are presented. The data
facilitate a spectroscopic investigation of the slow relaxation of the
magnetization in this compound in the time domain.Comment: 3 pages, 4 figures, REVTEX4, to appear in Appl. Phys. Lett., for an
animation see also
http://www.dcb.unibe.ch/groups/guedel/members/ow2/trins.ht
Quantum Phase Interference and Neel-Vector Tunneling in Antiferromagnetic Molecular Wheels
The antiferromagnetic molecular wheel Fe18 of eighteen exchange-coupled
Fe(III) ions has been studied by measurements of the magnetic torque, the
magnetization, and the inelastic neutron scattering spectra. The combined data
show that the low-temperature magnetism of Fe18 is very accurately described by
the Neel-vector tunneling (NVT) scenario, as unfolded by semiclassical theory.
In addition, the magnetic torque as a function of applied field exhibits
oscillations that reflect the oscillations in the NVT tunnel splitting with
field due to quantum phase interference.Comment: 5 pages, 4 figures, REVTEX4, to appear in PR
Spin anisotropy effects in dimer single molecule magnets
We present a model of equal spin dimer single molecule magnets. The
spins within each dimer interact via the Heisenberg and the most general set of
four quadratic anisotropic spin interactions with respective strengths and
, and with the magnetic induction . We solve the model
exactly for , and for antiferromagnetic Heisenberg couplings
(), present curves at low for these cases. Low-
curves for and electron paramagnetic susceptibility
for are also provided. For weak anisotropy
interactions, we employ a perturbative treatment, and show that the Hartree and
extended Hartree approximations lead to reliable analytic results at low
and large for these quantities and for the inelastic neutron scattering
cross-section . Our results are discussed with
regard to existing experiments on Fe dimer
single molecule magnets, and suggest that one of them contains a substantial
amount of single-ion anisotropy, without any sizeable global spin anisotropy.
We urge further experiments of the above types on single crystals of Fe and
on some [Mn] dimers, in order to elucidate the precise values
of the various microscopic interactions.Comment: 30 pages, 25 figures, submitted to Phys. Rev.
Relationship between X-ray and ultraviolet emission of flares from dMe stars observed by XMM-Newton
We present simultaneous ultraviolet and X-ray observations of the dMe-type
flaring stars AT Mic, AU Mic, EV Lac, UV Cet and YZ CMi obtained with the
XMM-Newton observatory. During 40 hours of simultaneous observation we identify
13 flares which occurred in both wave bands. For the first time, a correlation
between X-ray and ultraviolet flux for stellar flares has been observed. We
find power-law relationships between these two wavelength bands for the flare
luminosity increase, as well as for flare energies, with power-law exponents
between 1 and 2. We also observe a correlation between the ultraviolet flare
energy and the X-ray luminosity increase, which is in agreement with the
Neupert effect and demonstrates that chromospheric evaporation is taking place.Comment: 8 pages, 4 figures, 3 tables, accepted by A&A (30 Sept. 2004
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