2,052 research outputs found
Development of computer program NAS3D using Vector processing for geometric nonlinear analysis of structures
An algorithm for vectorized computation of stiffness matrices of an 8 noded isoparametric hexahedron element for geometric nonlinear analysis was developed. This was used in conjunction with the earlier 2-D program GAMNAS to develop the new program NAS3D for geometric nonlinear analysis. A conventional, modified Newton-Raphson process is used for the nonlinear analysis. New schemes for the computation of stiffness and strain energy release rates is presented. The organization the program is explained and some results on four sample problems are given. The study of CPU times showed that savings by a factor of 11 to 13 were achieved when vectorized computation was used for the stiffness instead of the conventional scalar one. Finally, the scheme of inputting data is explained
Magnetization of Charge-ordered la(2-x)sr(x)nio(4+delta)
We report magnetization measurements on La(2-x)Sr(x)NiO(4+ delta) single
crystals, with 0 < x < 0.5. Glassy behaviour associated with the formation of
spin-charge stripes, and a separate spin-glass phase at low temperatures were
observed. We have also found a `memory effect' in the magnetic field --
temperature history, which is found to be suppressed in the low temperature
spin state of the x = 0.33 crystal.Comment: 2 pages, 2 figures. Presented at ICM2003 to appear in J. Magn. Magn.
Mat
Magnetization of La(2-x)Sr(x)NiO(4+ delta) (0 < x < 0.5) and observation of novel memory effects
We have studied the magnetization of a series of spin-charge ordered
La(2-x)Sr(x)NiO(4+delta) single crystals with 0 < x < 0.5. For fields applied
parallel to the ab plane there is a large irreversibility below a temperature
T(F1) ~ 50 K and a smaller irreversibility that persists up to near the charge
ordering temperature. We observed a novel memory effect in the thermo-remnant
magnetization across the entire doping range. We found that these materials
retain a memory of the temperature at which an external field was removed, and
that there is a pronounced increase in the thermo-remnant magnetization when
the system is warmed through a spin reorientation transition.Comment: 11 pages, 12 figure
Spin reorientation transition in the incommensurate stripe-ordered phase of La3/2Sr1/2NiO4
The spin ordering of La3/2Sr1/2NiO4 was investigated by magnetization
measurements, and by unpolarized- and polarized-neutron diffraction. Spin
ordering with an incommensurability epsilon ~ 0.445 is observed below T_so ~ 80
K. On cooling, a spin reorientation is observed at 57 +/- 1 K, with the spin
axes rotating from 52 +/- 4 degrees to 78 +/- 3 degrees. This is the first time
a spin reorientation has been observed in a La2-xSrxNiO4+delta compound having
incommensurate stripe order.Comment: REVTex 4. 4 pages including 4 figures. Minor changes to text.
Accepted to be published in Physical Review
Magnetic spectrum of the two-dimensional antiferromagnet La2CoO4 studied by inelastic neutron scattering
We report measurements of the magnetic excitation spectrum of the layered
antiferromagnet La2CoO4 by time-of-flight neutron inelastic scattering. In the
energy range probed in our experiments (0-250 meV) the magnetic spectrum
consists of spin-wave modes with strong in-plane dispersion extending up to 60
meV, and a nearly dispersionless peak at 190 meV. The spin-wave modes exhibit a
small (~1 meV) dispersion along the magnetic zone boundary. We show that the
magnetic spectrum can be described very well by a model of a Heisenberg
antiferromagnet that includes the full spin and orbital degrees of freedom of
Co2+ in an axially-distorted crystal field. The collective magnetic dynamics
are found to be controlled by dominant nearest-neighbour exchange interactions,
strong XY-like single-ion anisotropy and a substantial unquenched orbital
angular momentum.Comment: 8 pages, 7 figure
Magnetic Order and Dynamics in Stripe-Ordered La2-xSrxNiO4
We have studied magnetic correlations in several compositions of
stripe-ordered La2-xSrxNiO4. In this paper we show how polarized-neutron
scattering has helped uncover important features of the magnetic ordering and
spin dynamics. In particular, polarization analysis has enabled us (1) to
characterize a spin reorientation transition, (2) to identify anisotropy gaps
in the spin excitation spectrum, and (3) to investigate an anomalous dip in the
spin-wave intensity suggestive of coupling between collective spin and charge
excitations.Comment: 4 pages, 3 figs. Presented at PNSXM, Venice. To appear in Physica
Quasiparticle Breakdown and Spin Hamiltonian of the Frustrated Quantum Pyrochlore YbTiO in Magnetic Field
The frustrated pyrochlore magnet YbTiO has the remarkable
property that it orders magnetically, but has no propagating magnons over wide
regions of the Brillouin zone. Here we use inelastic neutron scattering to
follow how the spectrum evolves in cubic-axis magnetic fields. At high fields
we observe in addition to dispersive magnons also a two-magnon continuum, which
grows in intensity upon reducing the field and overlaps with the one-magnon
states at intermediate fields leading to strong renormalization of the
dispersion relations, and magnon decays. Using heat capacity measurements we
find that the low and high field regions are smoothly connected with no sharp
phase transition, with the spin gap increasing monotonically in field. Through
fits to an extensive data set we re-evaluate the spin Hamiltonian finding
dominant quantum exchange terms, which we propose are responsible for the
anomalously strong fluctuations and quasiparticle breakdown effects observed at
low fields.Comment: 5 pages main text + 19 pages supplemental materia
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