1,329 research outputs found
Competition between supersolid phases and magnetisation plateaux in the frustrated easy-axis antiferromagnet on a triangular lattice
The majority of magnetic materials possess some degree of magnetic
anisotropy, either at the level of a single ion, or in the exchange
interactions between different magnetic ions. Where these exchange interactions
are also frustrated, the competition between them and anisotropy can stabilize
a wide variety of new phases in applied magnetic field. Motivated by the
hexagonal delafossite 2H-AgNiO 2, we study the Heisenberg antiferromagnet on a
layered triangular lattice with competing first- and second-neighbour
interactions and single-ion easy-axis anisotropy. Using a combination of
classical Monte Carlo simulation, mean-field analysis, and Landau theory, we
establish the magnetic phase diagram of this model as a function of temperature
and magnetic field for a fixed ratio of exchange interactions, but with values
of easy-axis anisotropy D extending from the Heisenberg (D =0) to the Ising
(D=∞) limits. We uncover a rich variety of different magnetic phases.
These include several phases which are magnetic supersolids (in the sense of
Matsuda and Tstuneto or Liu and Fisher), one of which may already have been
observed in AgNiO 2. We explore how this particular supersolid arises through
the closing of a gap in the spin-wave spectrum, and how it competes with rival
collinear phases as the easy-axis anisotropy is increased. The finite
temperature properties of this phase are found to be different from those of
any previously studied magnetic supersolid.Comment: 25 pages; 29 figures; minor revisions; accepted for publication in
Phys. Rev.
A Monte Carlo study of critical properties of strongly diluted magnetic semiconductor (Ga,Mn)As
Within a Monte Carlo technique we examine critical properties of diluted bulk
magnetic semiconductor (Ga,Mn)As modeled by a strongly diluted ferromagnetic
Heisenberg spin- system on a face centered cubic lattice. We
assumed that 5\% of Ga atoms is substituted by Mn atoms and the interaction
between them is of the RKKY-type. The considered system is randomly quenched
and a double average was performed: firstly, over the Boltzmann probability
distribution and secondly - over 2048 configurations related to the quenched
disorder. We estimated the critical temperature: K, which is in
agreement with the experiment. The calculated high value of critical exponent
seems to point to a possibility of non-universal critical behavior.Comment: 4 pages, 6 figure
Loop algorithm for classical Heisenberg models with spin-ice type degeneracy
In many frustrated Ising models, a single-spin flip dynamics is frozen out at
low temperatures compared to the dominant interaction energy scale because of
the discrete "multiple valley" structure of degenerate ground-state manifold.
This makes it difficult to study low-temperature physics of these frustrated
systems by using Monte Carlo simulation with the standard single-spin flip
algorithm. A typical example is the so-called spin ice model, frustrated
ferromagnets on the pyrochlore lattice. The difficulty can be avoided by a
global-flip algorithm, the loop algorithm, that enables to sample over the
entire discrete manifold and to investigate low-temperature properties. We
extend the loop algorithm to Heisenberg spin systems with strong easy-axis
anisotropy in which the ground-state manifold is continuous but still retains
the spin-ice type degeneracy. We examine different ways of loop flips and
compare their efficiency. The extended loop algorithm is applied to the
following two models, a Heisenberg antiferromagnet with easy-axis anisotropy
along the z axis, and a Heisenberg spin ice model with the local
easy-axis anisotropy. For both models, we demonstrate high efficiency of our
loop algorithm by revealing the low-temperature properties which were hard to
access by the standard single-spin flip algorithm. For the former model, we
examine the possibility of order-from-disorder and critically check its
absence. For the latter model, we elucidate a gas-liquid-solid transition,
namely, crossover or phase transition among paramagnet, spin-ice liquid, and
ferromagnetically-ordered ice-rule state.Comment: 12 pages, 11 figures, accepted for publication in Phys. Rev.
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