464 research outputs found
Order from structural disorder in pyrochlore antiferromagnet
Effect of structural disorder is investigated for an pyrochlore
antiferromagnet with continuous degeneracy of classical ground states. Two
types of disorder, vacancies and weakly fluctuating exchange bonds, lift
degeneracy selecting the same subset of classical ground states. Analytic and
numerical results demonstrate that such an "order by structural disorder"
mechanism competes with the effect of thermal and quantum fluctuations. Our
theory predicts that a small amount of nonmagnetic impurities in
will stabilize the coplanar ()
magnetic structure as opposed to the () state found in
pure material
Low-field behavior of an XY pyrochlore antiferromagnet: emergent clock anisotropies
Using as a motivation, we investigate finite-field
properties of pyrochlore antiferromagnets. In addition to a
fluctuation-induced six-fold anisotropy present in zero field, an external
magnetic field induces a combination of two-, three-, and six-fold clock terms
as a function of its orientation providing for a rich and controllable
magnetothermodynamics. For , we predict a new phase transition
for . Re-entrant transitions are also found for . We extend these results to the whole family the
pyrochlore antiferromagnets and show that presence and number of low-field
transitions for different orientations can be used for locating a given
material in the parameter space of anisotropic pyrochlores. Finite-temperature
classical Monte Carlo simulations serve to confirm and illustrate these
analytic predictions.Comment: 11 pages, accepted version with supplemental materia
Magnetocaloric effect in two-dimensional spin-1/2 antiferromagnets
The magnetocaloric effect is studied at the transition to saturation in the
antiferromagnetic spin-1/2 Heisenberg model on the simplest two-dimensional
lattices, namely the square and the triangular lattice. Numerical results are
presented for the entropy which are consistent with identical universal
properties. However, the absolute values of the entropy are bigger on the
geometrically frustrated triangular lattice than on the non-frustrated square
lattice, indicating that frustration improves the magnetocaloric properties.Comment: 2 pages, 2 figures included, to appear in Physica B (proceedings of
SCES'05
Field-induced decay dynamics in square-lattice antiferromagnet
Dynamical properties of the square-lattice Heisenberg antiferromagnet in
applied magnetic field are studied for arbitrary value S of the spin. Above the
threshold field for two-particle decays, the standard spin-wave theory yields
singular corrections to the excitation spectrum with logarithmic divergences
for certain momenta. We develop a self-consistent approximation applicable for
S >= 1, which avoids such singularities and provides regularized magnon decay
rates. Results for the dynamical structure factor obtained in this approach are
presented for S = 1 and S = 5/2.Comment: 12 pages, 11 figures, final versio
Magnetic excitations in dipolar pyrochlore antiferromagnet GdSnO
The spin dynamics in the geometrically frustrated pyrochlore antiferromagnet
is studied by means of the electron spin resonance. In the
ordered phase ( K), we have detected three gapped resonance modes.
Their values agree well with the developed spin-wave theory which takes into
account the Heisenberg nearest-neighbor exchange, the single-ion anisotropy and
the long-range dipolar interactions. The theory also predicts a fourth
lowest-frequency gap, which lies beyond the experimental range of frequencies,
but determines the exponential decrease of the specific heat at low
temperature.Comment: 8 pages, 5 figure
Finite-temperature order-disorder phase transition in a frustrated bilayer quantum Heisenberg antiferromagnet in strong magnetic fields
We investigate the thermodynamic properties of the frustrated bilayer quantum
Heisenberg antiferromagnet at low temperatures in the vicinity of the
saturation magnetic field. The low-energy degrees of freedom of the spin model
are mapped onto a hard-square gas on a square lattice. We use exact
diagonalization data for finite spin systems to check the validity of such a
description. Using a classical Monte Carlo method we give a quantitative
description of the thermodynamics of the spin model at low temperatures around
the saturation field. The main peculiarity of the considered two-dimensional
Heisenberg antiferromagnet is related to a phase transition of the hard-square
model on the square lattice, which belongs to the two-dimensional Ising model
universality class. It manifests itself in a logarithmic (low-)temperature
singularity of the specific heat of the spin system observed for magnetic
fields just below the saturation field
Enhanced magnetocaloric effect in frustrated magnets
The magnetothermodynamics of strongly frustrated classical Heisenberg
antiferromagnets on kagome, garnet, and pyrochlore lattices is examined. The
field induced adiabatic temperature change (dT/dH)_S is significantly larger
for such systems compared to ordinary non-frustrated magnets and also exceeds
the cooling rate of an ideal paramagnet in a wide range of fields. An
enhancement of the magnetocaloric effect is related to presence of a
macroscopic number of soft modes in frustrated magnets below the saturation
field. Theoretical predictions are confirmed with extensive Monte Carlo
simulations.Comment: 7 page
Excitonic ferromagnetism in the hexaborides
A ferromagnet with a small spontaneous moment but with a high Curie
temperature can be obtained by doping an excitonic insulator made from a spin
triplet exciton condensate. Such a condensate can occur in a semimetal with a
small overlap or a semiconductor with a small bandgap. We propose that it is
responsible for the unexpected ferromagnetism in the doped hexaboride material
Ca_{1-x}La_xB_6.Comment: 4 pages, 3 figure
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