198 research outputs found
Use of reversible hydrides for hydrogen storage
The addition of metals or alloys whose hydrides have a high dissociation pressure allows a considerable increase in the hydrogenation rate of magnesium. The influence of temperature and hydrogen pressure on the reaction rate were studied. Results concerning the hydriding of magnesium rich alloys such as Mg2Ca, La2Mg17 and CeMg12 are presented. The hydriding mechanism of La2Mg17 and CeMg12 alloys is given
Magnetic anomalies in the spin chain system, SrCuZnIrO
We report the results of ac and dc magnetization (M) and heat-capacity (C)
measurements on the solid solution, SrCuZnIrO. While the Zn
end member is known to form in a rhombohedral pseudo one-dimensional
KCdCl structure with an antiferromagnetic ordering temperature of
(T =) 19 K, the Cu end member has been reported to form in a monoclinically
distorted form with a Curie temperature of (T =) 19 K. The magnetism of the
Zn compound is found to be robust to synthetic conditions and is broadly
consistent with the behavior known in the literature. However, we find a lower
magnetic ordering temperature (T) for our Cu compound (~ 13 K), thereby
suggesting that T is sensitive to synthetic conditions. The Cu sample
appears to be in a spin-glass-like state at low temperatures, judged by a
frequency dependence of ac magnetic susceptibility and a broadening of the C
anomaly at the onset of magnetic ordering, in sharp contrast to earlier
proposals. Small applications of magnetic field, however, drive this system to
ferromagnetism as inferred from the M data. Small substitutions for Cu/Zn (x =
0.75 or 0.25) significantly depress magnetic ordering; in other words, T
varies non-monotonically with x (T ~ 6, 3 and 4 K for x = 0.25, 0.5, and
0.67 respectively). The plot of inverse susceptibility versus temperature is
non-linear in the paramagnetic state as if correlations within (or among) the
magnetic chains continuously vary with temperature. The results establishComment: 7 pages, 7 figures, Revte
Magnetic frustration in a stoichiometric spin-chain compound, CaCoIrO
The temperature dependent ac and dc magnetization and heat capacity data of
CaCoIrO, a spin-chain compound crystallizing in a KCdCl-derived
rhombohedral structure, show the features due to magnetic ordering of a
frustrated-type below about 30 K, however without exhibiting the signatures of
the so-called "partially disordered antiferromagnetic structure" encountered in
the isostructural compounds, CaCoO and CaCoRhO. This class
of compounds thus provides a variety for probing the consequences of magnetic
frustration due to topological reasons in stoichiometric spin-chain materials,
presumably arising from subtle differences in the interchain and intrachain
magnetic coupling strengths. This compound presents additional interesting
situations in the sense that, ac susceptibility exhibits a large frequency
dependence in the vicinity of 30 K uncharacteristic of conventional
spin-glasses, with this frustrated magnetic state being robust to the
application of external magnetic fields.Comment: Physical Review (Rapid Communications), in pres
A microscopic model for a class of mixed-spin quantum antiferromagnets
We propose a microscopic model that describes the magnetic behavior of the
mixed-spin quantum systems RBaNiO (R= magnetic rare earth). An
evaluation of the properties of this model by Quantum Monte Carlo simulations
shows remarkable good agreement with the experimental data and provides new
insight into the physics of mixed-spin quantum magnets.Comment: revised version to be published in Phys. Rev.
Random interactions and spin-glass thermodynamic transition in the hole-doped Haldane system YCaBaNiO
Magnetization, DC and AC bulk susceptibility of the =1 Haldane chain
system doped with electronic holes, YCaBaNiO
(0x0.20), have been measured and analyzed. The most striking
results are (i) a sub-Curie power law behavior of the linear susceptibility,
, for temperature lower than the Haldane gap
of the undoped compound (x=0) (ii) the existence of a spin-glass thermodynamic
transition at = 2-3 K. These findings are consistent with (i) random
couplings within the chains between the spin degrees of freedom induced by hole
doping, (ii) the existence of ferromagnetic bonds that induce magnetic
frustration when interchain interactions come into play at low temperature.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
Comparison of S=0 and S=1/2 Impurities in Haldane Chain Compound,
We present the effect of Zn (S=0) and Cu (S=1/2) substitution at the Ni site
of S=1 Haldane chain compound . Y NMR allows us to
measure the local magnetic susceptibility at different distances from the
defects. The Y NMR spectrum consists of one central peak and several
less intense satellite peaks. The shift of the central peak measures the
uniform susceptibility, which displays a Haldane gap 100 K and it
corresponds to an AF coupling J260 K between the near-neighbor Ni spins.
Zn or Cu substitution does not affect the Haldane gap. The satellites, which
are evenly distributed on the two sides of the central peak, probe the
antiferromagnetic staggered magnetization near the substituted site, which
decays exponentially. Its extension is found identical for both impurities and
corresponds accurately to the correlation length (T) determined by Monte
Carlo (QMC) simulations for the pure compound. In the case of non-magnetic Zn,
the temperature dependence of the induced magnetization is consistent with a
Curie law with an "effective" spin S=0.4 on each side of Zn, which is well
accounted by Quantum Monte Carlo computations of the spinless-defect-induced
magnetism. In the case of magnetic Cu, the similarity of the induced magnetism
to the Zn case implies a weak coupling of the Cu spin to the nearest- neighbor
Ni spins. The slight reductionin the induced polarization with respect to Zn is
reproduced by QMC computations by considering an antiferromagnetic coupling of
strength J'=0.1-0.2 J between the S=1/2 Cu-spin and nearest-neighbor Ni-spin.Comment: 15 pages, 18 figures, submitted to Physical Review
Impurities in Heisenberg Antiferromagnets
The Heisenberg Antiferromagnet is studied in the presence of two kinds
of local impurities. First, a perturbed antiferromagnetic bond with
at the center of an even-length open chain is considered. Using the density
matrix renormalization group method we find that, for sufficiently strong or
weak , a bound state is localized at the impurity site, giving rise to an
energy level in the Haldane gap. The energy of the bound state is in agreement
with perturbative results, based on chain-end excitations, both in the
weak and strong coupling limit. In a region around the uniform limit, ,
no states are found with energy below the Haldane gap. Secondly, a
impurity at the center of an otherwise even-length open chain is considered.
The coupling to the impurity is varied. Bound states in the Haldane gap
are found {\it only} for sufficiently weak (antiferromagnetic) coupling. For a
impurity coupled with a strong (antiferromagnetic) bond, {\it no}
states are found in the Haldane. Our results are in good qualitative agreement
with recent experiments on doped NENP and YBaNiO.Comment: 29 pages, RevTeX 3.0, 12 uuencoded postscript figures include
Chain-Boundary Excitations in the Haldane Phase of 1D Systems
The chain-boundary excitations occurring in the Haldane phaseof
antiferromagnetic spin chains are investigated. The bilinear-biquadratic
hamiltonian is used to study these excitations as a function of the strength of
the biquadratic term, , between . At the AKLT point,
, we show explicitly that these excitations are localized at the
boundaries of the chain on a length scale equal to the correlation length
, and that the on-site magnetization for the first site is
. Applying the density matrixrenormalization group we show that
the chain-boundaryexcitations remain localized at the boundaries for
. As the two critical points are approached the
size of the objects diverges and their amplitude vanishes.Comment: 4 Pages, 4 eps figures. Uses RevTeX 3.0. Submitted to PR
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