41 research outputs found
Alternating commensurate-incommensurate structures in the magnetic phase diagram of CsNiF3
The magnetic phase diagram of the quasi one-dimensional spinchain system
CsNiF below the N\'eel temperature is determined. For magnetic fields
perpendicular to the spin chains incommensurate phases are predicted. From
linear spin-wave theory we obtain the instability line of the paramagnetic
phase as a function of the strength and the direction of the field. The system
undergoes a transition to a commensurate or an incommensurate phase depending
on the direction of the magnetic field. In the commensurate phase the
characterizing wave vector is locked to values describing a two-sublattice
structure, whereas in the incommensurate phase the wave vector changes
continuously between the corresponding two-sublattice wave vectors.Comment: 11 pages, LaTeX, 5 figures, sent to PRB Rapid Communicatio
Enhancement of Anisotropy due to Fluctuations in Quasi-One-Dimensional Antiferromagnets
It is shown that the observed anisotropy of magnetization at high magnetic
fields in RbMnBr3 , a quasi-one-dimensional antiferromagnet on a distorted
stacked triangular lattice, is due to quantum and thermal fluctuations. These
fluctuations are taken into account in the framework of linear spin-wave theory
in the region of strong magnetic fields. In this region the divergent
one-dimensional integrals are cut off by magnetic field and the bare easy-plane
anisotropy. Logarithmical dependence on the cutoff leads to the "enhancement"
of the anisotropy in magnetization. Comparison between magnetization data and
our theory with parameters obtained from neutron scattering experiments has
been done.Comment: 15 pages + 5 postscript figures available upon request, RevTex
Ordered phase and scaling in models and the three-state antiferromagnetic Potts model in three dimensions
Based on a Renormalization-Group picture of symmetric models in three
dimensions, we derive a scaling law for the order parameter in the
ordered phase. An existing Monte Carlo calculation on the three-state
antiferromagnetic Potts model, which has the effective symmetry, is shown
to be consistent with the proposed scaling law. It strongly supports the
Renormalization-Group picture that there is a single massive ordered phase,
although an apparently rotationally symmetric region in the intermediate
temperature was observed numerically.Comment: 5 pages in REVTEX, 2 PostScript figure
Small-angle neutron scattering and magnetization study of HoNi₂B₂C
The superconducting and magnetic properties of HoNi₂B₂C single crystals are investigated through transport, magnetometry and small-angle neutron scattering (SANS) measurements. In the magnetic phases that enter below the superconducting critical temperature, the small-angle neutron scattering data uncover networks of magnetic
surfaces. These likely originate from uncompensated moments, e.g., at domain walls pinned to crystallographic grain boundaries. The field and temperature dependent behavior of SANS appears consistent with the metamagnetic transitions reported in earlier works
Aging in a topological spin glass
We have examined the nonconventional spin glass phase of the 2-dimensional
kagome antiferromagnet (H_3 O) Fe_3 (SO_4)_2 (OH)_6 by means of ac and dc
magnetic measurements. The frequency dependence of the ac susceptibility peak
is characteristic of a critical slowing down at Tg ~ 18K. At fixed temperature
below Tg, aging effects are found which obey the same scaling law as in spin
glasses or polymers. However, in clear contrast with conventional spin glasses,
aging is remarkably insensitive to temperature changes. This particular type of
dynamics is discussed in relation with theoretical predictions for highly
frustrated non-disordered systems.Comment: 4 pages, 4 figure
Orthorhombic versus monoclinic symmetry of the charge-ordered state of NaV2O5
High-resolution X-ray diffraction data show that the low-temperature
superstructure of alpha-NaV2O5 has an F-centered orthorhombic 2a x 2b x 4c
superlattice. A structure model is proposed, that is characterized by layers
with zigzag charge order on all ladders and stacking disorder, such that the
averaged structure has space group Fmm2. This model is in accordance with both
X-ray scattering and NMR data. Variations in the stacking order and disorder
offer an explanation for the recently observed devils staircase of the
superlattice period along c.Comment: REVTEX, 4 pages including 2 figures, shortened, submitted to PR
Early stage scaling in phase ordering kinetics
A global analysis of the scaling behaviour of a system with a scalar order
parameter quenched to zero temperature is obtained by numerical simulation of
the Ginzburg-Landau equation with conserved and non conserved order parameter.
A rich structure emerges, characterized by early and asymptotic scaling
regimes, separated by a crossover. The interplay among different dynamical
behaviours is investigated by varying the parameters of the quench and can be
interpreted as due to the competition of different dynamical fixed points.Comment: 21 pages, latex, 7 figures available upon request from
[email protected]
Charge-ordering phase transition and order-disorder effects in the Raman spectra of NaV2O5
In the ac polarized Raman spectra of NaV2O5 we have found anomalous phonon
broadening, and an energy shift of the low-frequency mode as a function of the
temperature. These effects are related to the breaking of translational
symmetry, caused by electrical disorder that originates from the fluctuating
nature of the V {4.5+} valence state of vanadium. The structural correlation
length, obtained from comparisons between the measured and calculated Raman
scattering spectra, diverges at T< 5 K, indicating the existence of the
long-range charge order at very low temperatures, probably at T=0 K.Comment: 8 pages, 4 figures, new version, to appear in PR