4,346 research outputs found
Relaxation in the 3D ordered CoTAC spin chain by quantum nucleation of 0D domain walls
We have shown that resonant quantum tunnelling of the magnetisation (QTM),
until now observed only in 0D cluster systems (SMMs), occurs in the molecular
Ising spin chain, CoTAC ([(CH_3)_3NH]CoCl_3 - 2H_2O) which orders as a canted
3D-antiferromagnet at T_C=4.15 K. This effect was observed around a resonant
like field value of 1025 Oe. We present here measurements of the relaxation of
the magnetisation as a function of time, from the zero field cooled (ZFC)
antiferromagnet state and from the saturated ferromagnet state. We show that,
at the resonant field, the relaxation from the saturated state occurs in a
complicated process, whereas, surprisingly, in the case of the ZFC state, the
relaxation is exponential.Comment: 4 pages, 5 figures, LT25 proceeding
DFT Calculations as a Tool to Analyse Quadrupole Splittings of Spin Crossover Fe(II) complexes
Density functional methods have been applied to calculate the quadrupole
splitting of a series of iron(II) spin crossover complexes. Experimental and
calculated values are in reasonable agreement. In one case spin-orbit coupling
is necessary to explain the very small quadrupole splitting value of 0.77 mm/s
at 293 K for a high-spin isomer
A Leadership Case Study: Application of Koestenhaum\u27s Diamond Leadership Theory
Peter Koestenbaum\u27s Diamond Leadership Theory illustrates how a leader\u27s greatness can be evaluated by looking at four different categories: ethics, courage, reality, and vision. According to his Diamond Leadership Theory, he stresses that all four dimensions must be equally developed and balanced, in both the personal and professional life of the leader. Koestenbaum contends that a balanced leader will show leadership skills in all aspects, not just in their professional lives.
The following research was conducted using the Diamond Leadership Theory as a foundation for studying a leader in the business world. By understanding these basic principles, the researcher was able to apply these characteristics to the leader and make conclusions regarding the validity of Koestenbaum\u27s theory to this particular individual
The effects of atomic oxygen on the thermal emittance of high temperature radiator surfaces
Radiator surfaces on high temperature space power systems such as SP-100 space nuclear power system must maintain a high emittance level in order to reject waste heat effectively. One of the primary materials under consideration for the radiators is carbon-carbon composite. Since carbon is susceptible to attack by atomic oxygen in the low earth orbital environment, it is important to determine the durability of carbon composites in this environment as well as the effect atomic oxygen has on the thermal emittance of the surface if it is to be considered for use as a radiator. Results indicate that the thermal emittance of carbon-carbon composite (as low as 0.42) can be enhanced by exposure to a directed beam of atomic oxygen to levels above 0.85 at 800 K. This emittance enhancement is due to a change in the surface morphology as a result of oxidation. High aspect ratio cones are formed on the surface which allow more efficient trapping of incident radiation. Erosion of the surface due to oxidation is similar to that for carbon, so that at altitudes less than approximately 600 km, thickness loss of the radiator could be significant (as much as 0.1 cm/year). A protective coating or oxidation barrier forming additive may be needed to prevent atomic oxygen attack after the initial high emittance surface is formed. Textured surfaces can be formed in ground based facilities or possibly in space if emittance is not sensitive to the orientation of the atomic oxygen arrival that forms the texture
Unconventional antiferromagnetic correlations of the doped Haldane gap system YBaNiZnO
We make a new proposal to describe the very low temperature susceptibility of
the doped Haldane gap compound YBaNiZnO. We propose a new
mean field model relevant for this compound. The ground state of this mean
field model is unconventional because antiferromagnetism coexists with random
dimers. We present new susceptibility experiments at very low temperature. We
obtain a Curie-Weiss susceptibility as expected
for antiferromagnetic correlations but we do not obtain a direct signature of
antiferromagnetic long range order. We explain how to obtain the ``impurity''
susceptibility by subtracting the Haldane gap contribution to
the total susceptibility. In the temperature range [1 K, 300 K] the
experimental data are well fitted by . In the temperature range [100 mK, 1 K] the experimental data are
well fitted by , where increases with
. This fit suggests the existence of a finite N\'eel temperature which is
however too small to be probed directly in our experiments. We also obtain a
maximum in the temperature dependence of the ac-susceptibility which
suggests the existence of antiferromagnetic correlations at very low
temperature.Comment: 19 pages, 17 figures, revised version (minor modifications
Phonon superradiance and phonon laser effect in nanomagnets
We show that the theory of spin-phonon processes in paramagnetic solids must
take into account the coherent generation of phonons by the magnetic centers.
This effect should drastically enhance spin-phonon rates in nanoscale
paramagnets and in crystals of molecular nanomagnets.Comment: 4 PR pages, 1 Figur
Low-Temperature Features of Nano-Particle Dynamics
In view of better characterizing possible quantum effects in the dynamics of
nanometric particles, we measure the effect on the relaxation of a slight
heating cycle. The effect of the field amplitude is studied; its magnitude is
chosen in order to induce the relaxation of large particles (~7nm), even at
very low temperatures (100mK). Below 1K, the results significantly depart from
a simple thermal dynamics scenario.Comment: 1 tex file, 4 PostScript figure
Intrinsic avalanches and collective phenomena in a Mn(II)-free radical ferrimagnetic chain
Magnetic hysteresis loops below 300 mK on single crystals of the Mn(II) -
nitronyl nitroxide free radical chain (Mn(hfac)_2({\it R})-3MLNN) present
abrupt reversals of the magnetization, or avalanches. We show that, below 200
mK, the avalanches occur at a constant field, independent of the sample and so
propose that this avalanche field is an intrinsic property. We compare this
field to the energy barrier existing in the sample and conclude that the
avalanches are provoked by multiple nucleation of domain-walls along the
chains. The different avalanche field observed in the zero field cooled
magnetization curves suggests that the avalanche mechanisms are related to the
competition between ferromagnetic and antiferromagnetic order in this compound.Comment: 9 pages, 7 fig, to be published in Phys. Rev.
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