1,083 research outputs found
Magnetic properties and spin dynamics in single molecule paramagnets Cu6Fe and Cu6Co
The magnetic properties and the spin dynamics of two molecular magnets have
been investigated by magnetization and d.c. susceptibility measurements,
Electron Paramagnetic Resonance (EPR) and proton Nuclear Magnetic Resonance
(NMR) over a wide range of temperature (1.6-300K) at applied magnetic fields,
H=0.5 and 1.5 Tesla. The two molecular magnets consist of
CuII(saldmen)(H2O)}6{FeIII(CN)6}](ClO4)38H2O in short Cu6Fe and the analog
compound with cobalt, Cu6Co. It is found that in Cu6Fe whose magnetic core is
constituted by six Cu2+ ions and one Fe3+ ion all with s=1/2, a weak
ferromagnetic interaction between Cu2+ moments through the central Fe3+ ion
with J = 0.14 K is present, while in Cu6Co the Co3+ ion is diamagnetic and the
weak interaction is antiferromagnetic with J = -1.12 K. The NMR spectra show
the presence of non equivalent groups of protons with a measurable contact
hyperfine interaction consistent with a small admixture of s-wave function with
the d-function of the magnetic ion. The NMR relaxation results are explained in
terms of a single ion (Cu2+, Fe3+, Co3+) uncorrelated spin dynamics with an
almost temperature independent correlation time due to the weak magnetic
exchange interaction. We conclude that the two molecular magnets studied here
behave as single molecule paramagnets with a very weak intramolecular
interaction, almost of the order of the dipolar intermolecular interaction.
Thus they represent a new class of molecular magnets which differ from the
single molecule magnets investigated up to now, where the intramolecular
interaction is much larger than the intermolecular one
Probing spin dynamics and quantum relaxation in LiY0.998Ho0.002F4 via 19F NMR
We report measurements of 19F nuclear spin-lattice relaxation 1/T1 as a
function of temperature and external magnetic field in LiY0.998Ho0.002F4 single
crystal, a single-ion magnet exhibiting interesting quantum effects. The 19F
1/T1 is found to depend on the coupling with the diluted rare-earth (RE)
moments. Depending on the temperature range, a fast spin diffusion regime or a
diffusion limited regime is encountered. In both cases we find it possible to
use the 19F nucleus as a probe of the rare-earth spin dynamics. The results for
1/T1 show a behavior similar to that observed in molecular nanomagnets, a
result which we attribute to the discreteness of the energy levels in both
cases. At intermediate temperatures the lifetime broadening of the crystal
field split RE magnetic levels follows a T3 power law. At low temperature the
field dependence of 1/T1 shows peaks in correspondence to the critical magnetic
fields for energy level crossings (LC). The results can be explained by
inelastic scattering between the fluorine nuclear spins and the RE magnetic
levels. A key result of this study is that the broadening of the levels at LC
is found to be become extremely small at low temperatures, about 1.7 mT, a
value which is comparable to the weak dipolar fields at the RE lattice
positions. Thus, unlike the molecular magnets, decoherence effects are strongly
suppressed, and it may be possible to measure directly the level repulsions at
avoided level crossings.Comment: 21 pages, 5 figure
Classical Heisenberg model of magnetic molecular ring clusters: Accurate approximants for correlation functions and susceptibility
The article of record as published may be found at https://doi.org/10.1063/1.476144We show that the measured magnetic susceptibility of molecular ring clusters can be accurately reproduced, for all but low temperatures T, by a classical Heisenberg model of N identical spins S on a ring that interact with isotropic nearest-neighbor interactions. While exact expressions for the two-spin correlation function, C{sub N}(n,T), and the zero-field magnetic susceptibility, {chi}{sub N}(T), are known for the classical Heisenberg ring, their evaluation involves summing infinite series of modified spherical Bessel functions. By contrast, the formula C{sub N}(n,T)=(u{sup n}+u{sup N{minus}n})/(1+u{sup N}), where u(K)=cothK{minus}K{sup {minus}1} is the Langevin function and K=JS(S+1)/(k{sub B}T) is the nearest-neighbor dimensionless coupling constant, provides an excellent approximation if N{ge}6 for the regime {vert_bar}K{vert_bar}{lt}3. This choice of approximant combines the expected exponential decay of correlations for increasing yet small values of n, with the cyclic boundary condition for a finite ring, C{sub N}(n,T)=C{sub N}(N{minus}n,T). By way of illustration, we show that, for T{gt}50K, the associated approximant for the susceptibility derived from the approximate correlation function is virtually indistinguishable from both the exact theoretical susceptibility and the experimental data for the {open_quotes}ferric wheel{close_quotes} molecular cluster ([Fe(OCH{sub 3}){sub 2}(O{sub 2}CCH{sub 2}Cl)]{sub 10}), which contains N=10 interacting Fe{sup 3+} ions, each of spin S=5/2, that are symmetrically positioned in a nearly planar ring. {copyright} {ital 1998 American Institute of Physics.
19F nuclear spin relaxation and spin diffusion effects in the single ion magnet LiYF4:Ho3+
Temperature and magnetic field dependences of the 19F nuclear spin-lattice
relaxation in a single crystal of LiYF4 doped with holmium are described by an
approach based on a detailed consideration of the magnetic dipole-dipole
interactions between nuclei and impurity paramagnetic ions and nuclear spin
diffusion processes. The observed non-exponential long time recovery of the
nuclear magnetization after saturation at intermediate temperatures is in
agreement with predictions of the spin-diffusion theory in a case of the
diffusion limited relaxation. At avoided level crossings in the spectrum of
electron-nuclear states of the Ho3+ ion, rates of nuclear spin-lattice
relaxation increase due to quasi-resonant energy exchange between nuclei and
paramagnetic ions, in contrast to the predominant role played by electronic
cross-relaxation processes in the low-frequency ac-susceptibility.Comment: 27 pages total, 5 figures, accepted for publication, Eur. Phys. J.
Observation of the cluster spin-glass phase in La_{2-x}Sr_{x}CuO_{4} by anelastic spectroscopy
An increase of the acoustic absorption is found in La_{2-x}Sr_{x}CuO_{4} (x =
0.019, 0.03 and 0.06) close to the temperatures at which freezing of the spin
fluctuations in antiferromagnetic-correlated clusters is expected to occur. The
acoustic absorption is attributed to changes of the sizes of the quasi-frozen
clusters induced by the vibration stress through magnetoelastic coupling.Comment: LaTeX, 2 PostScript figures, submitted to Phys. Rev.
NMR and SR detection of unconventional spin dynamics in Er(trensal) and Dy(trensal) molecular magnets
Measurements of proton Nuclear Magnetic Resonance (1H NMR) spectra and
relaxation and of Muon Spin Relaxation (SR) have been performed as a
function of temperature and external magnetic field on two isostructural
lanthanide complexes, Er(trensal) and Dy(trensal) featuring
crystallographically imposed trigonal symmetry. Both the nuclear 1/T1 and muon
longitudinal relaxation rates, LRR, exhibit a peak for temperatures T
lower than 30K, associated to the slowing down of the spin dynamics, and the
width of the NMR absorption spectra starts to increase significantly at T ca.
50K, a temperature sizably higher than the one of the LRR peaks. The LRR peaks
have a field and temperature dependence different from those previously
reported for all Molecular Nanomagnets. They do not follow the
Bloembergen-Purcell-Pound scaling of the amplitude and position in temperature
and field and thus cannot be explained in terms of a single dominating
correlation time c determined by the spin slowing down at low
temperature. Further, for T lower than 50K the spectral width does not follow
the temperature behavior of the magnetic susceptibility chi. We suggest, using
simple qualitative considerations, that the observed behavior is due to a
combination of two different relaxation processes characterized by the
correlation times LT and HT, dominating for T lower than 30K and T
higher than 50K, respectively. Finally, the observed flattening of LRR for T
lower than 5K is suggested to have a quantum origin
Identification des deux espèces jumelles Beryx mollis Abe 1959 et B. splendens Lowe 1834, à l'aide de caractères morphologiques et méristiques simples : rapport d'opération ZoNéCo
N-TiO2 Photocatalysts highly active under visible irradiation for NOX abatement and 2-propanol oxidation
N-doped TiO2 powders were prepared by two different sol–gel methods. Samples were characterised
by X-ray diffraction (XRD), BET specific surface area measurements (SSA), scanning electron microscopy
(SEM), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and Electron Paramagnetic
Resonance (EPR). XPS measurements revealed a signal at 400 eV assignable to nitrogen in the
form of Ti N O. EPR signals are attributed to molecular NO trapped with cavities/defects possibly interacting
with oxygen vacancies. The photocatalytic activity under UV and visible light was determined
following the abatement of NOx and the photodegradation of 2-propanol in gas–solid systems. N-doped
TiO2 showed a higher activity compared with the pristine commercial and home prepared samples
under visible light irradiation. A good photoactivity in the abatement of both NOx and 2-propanol is
also observed for mechanical dispersions of N-TiO2 in CaCO3 serving as a model in view of perspective
application in photocatalytically active construction and architectural materials
new route for tizanidine administration: a pharmacokinetics and light microscope autoradiography study
Spasticity could represent a complication of several central nervous system (CNS)
disorders. Multiple sclerosis and painful paroxysmal syndromes (e.g. trigeminal
neuralgia) are pathologies in which anti-spastic drugs are used to a greater extent .
Baclofen, tizanidine (TIZ), benzodiazepines, dantrolene, and, more recently, gabapentin
are the pharmacological agents more widely used. Baclofen, TIZ, benzodiazepines,
gabapentin, clonidine but not dantrolene are active on CNS. The alpha2 adrenoceptor
agonist TIZ is one of the most eff ective and largely used anti-spastic drugs. Oral
treatment is the only route of anti-spastics administration, although it may cause
problems of bioavailability and/or compliance in spastic patients with impaired
deglutition. This study was designed to assess the possibility of develop a new route
of administration of TIZ and to identify its targets in the spinal cord.
New Zealand rabbits were treated with oral (OR , n=6), intramuscular (IM, n=6)
or intranasal (IN, n=6) TIZ (3.2 mg/kg/day). Plasma concentration was measured by
HPLC on samples collected at 0, 30, 60, 90, 120 and 480 min after treatment. Curves
of average concentrations of TIZ vs. time were constructed. In plasma, TIZ reached a
peak between 45-65 min after administration. Cmax was in the range of 268.33–1213.64
ng/ml for IM treatment, of 73.95–135.92 ng/ml for IN treatment and of 16.86–857.25
ng/ml for OR treatment. After pharmacokinetic studies, diff erent spinal cord tracts
were removed and used for radioligand binding assay and autoradiography. Using
[3H]-RX821002 ([3H]-RX) (0.1-14nM), the Kd and Bmax for every spinal cord segment
were calculated. The non-specifi c binding was obtained with 100μM of (-)-epinephrine.
[3H]-RX sites were accumulated in the superfi cial laminae of dorsal horn. Dense
[3H]-RX binding in control was seen over the superfi cial dorsal horn (laminae I-II)
and centrally located lamina X. The ventral horn showed moderate levels of binding.
[3H]-RX was displaced after TIZ treatment using diff erent administration routes. The
heaviest accumulation of silver grains (lowest displacement of TIZ) occurred after OR
administration of the compound, the lowest after IM administration. These results
suggest that both IM and IN administration of TIZ may represent routes of administration
of the drug alternative to the OR one.
Considering predictable adverse eff ects of IM treatment, the IN administration
could represent the elective route to administrate this kind of drugs
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