430 research outputs found
Multiferroicity in spin ice: towards a magnetic crystallography of Tb2Ti2O7 in a field
We combine two aspects of magnetic frustration, multiferroicity and emergent
quasi-particles in spin liquids, by studying magneto-electric monopoles. Spin
ice offers to couple these emergent topological defects to external fields, and
to each other, in unusual ways, making possible to lift the degeneracy
underpinning the spin liquid and to potentially stabilize novel forms of charge
crystals, opening the path to a "magnetic crystallography". In developing the
general phase diagram including nearest-neighbour coupling, Zeeman energy,
electric and magnetic dipolar interactions, we uncover the emergence of a
bi-layered crystal of singly-charged monopoles, whose stability, remarkably, is
strengthened by an external [110] magnetic field. Our theory is able to account
for the ordering process of Tb2Ti2O7 in large field for reasonably small
electric energy scales.Comment: 10 pages, 10 figure
Reentrance of disorder in the anisotropic shuriken Ising model
For a material to order upon cooling is common sense. What is more seldom is
for disorder to reappear at lower temperature, which is known as reentrant
behavior. Such resurgence of disorder has been observed in a variety of
systems, ranging from Rochelle salts to nematic phases in liquid crystals.
Frustration is often a key ingredient for reentrance mechanisms. Here we shall
study a frustrated model, namely the anisotropic shuriken lattice, which offers
a natural setting to explore an extension of the notion of reentrance between
magnetic disordered phases. By tuning the anisotropy of the lattice, we open a
window in the phase diagram where magnetic disorder prevails down to zero
temperature. In this region, the competition between multiple disordered ground
states gives rise to a double crossover where both the low- and
high-temperature regimes are less correlated than the intervening classical
spin liquid. This reentrance of disorder is characterized by an entropy
plateau, a multi-step Curie law crossover and a rather complex diffuse
scattering in the static structure factor. Those results are confirmed by
complementary numerical and analytical methods: Monte Carlo simulations,
Husimi-tree calculations and an exact decoration-iteration transformation.Comment: 16 pages, 13 figure
Living on the edge : ground-state selection in quantum spin-ice pyrochlores
The search for new quantum phases, especially in frustrated magnets, is
central to modern condensed matter physics. One of the most promising places to
look is in rare-earth pyrochlore magnets with highly-anisotropic exchange
interactions, materials closely related to the spin ices Ho2Ti2O7 and Dy2Ti2O7.
Here we establish a general theory of magnetic order in these materials. We
find that many of their most interesting properties can be traced back to the
accidental degeneracies where phases with different symmetry meet. These
include the ordered ground state selection by fluctuations in Er2Ti2O7, the
dimensional-reduction observed in Yb2Ti2O7, and the absence of magnetic order
in Er2Sn2O7.Comment: A long-paper version of this preprint, "Living on the Edge", appears
as arXiv:1603.09466 [accepted for publication in Physical Review B]. The text
of v2 is otherwise unchanged from v1 (Submitted on 14 Nov 2013
Classical spin liquids in stacked triangular lattice Ising antiferromagnets
We study Ising antiferromagnets that have nearest-neighbour interactions on
multilayer triangular lattices with frustrated ( and ) stacking, and
make comparisons with the unfrustrated () stacking. If interlayer
couplings are much weaker than in-plane ones, the paramagnetic phase of models
with frustrated stackings has a classical spin-liquid regime at low
temperature, in which correlations are strong both within and between planes,
but there is no long-range order. We investigate this regime using Monte Carlo
simulations and by mapping the spin models to coupled height models, which are
treated using renormalisation group methods and an analysis of the effects of
vortex excitations. The classical spin-liquid regime is parametrically wide at
small interlayer coupling in models with frustrated stackings. By contrast, for
the unfrustrated stacking there is no extended regime in which interlayer
correlations are strong without three-dimensional order.Comment: 25 pages, 21 figures; version to appear in Physical Review B,
includes minor correction
Crystal Shape-Dependent Magnetic Susceptibility and Curie Law Crossover in the Spin Ices Dy2Ti2O7 and Ho2Ti2O7
We present an experimental determination of the isothermal magnetic
susceptibility of the spin ice materials Dy2Ti2O7 and Ho2Ti2O7 in the
temperature range 1.8-300 K. The use of spherical crystals has allowed the
accurate correction for demagnetizing fields and allowed the true bulk
isothermal susceptibility X_T(T) to be estimated. This has been compared to a
theoretical expression based on a Husimi tree approximation to the spin ice
model. Agreement between experiment and theory is excellent at T > 10 K, but
systematic deviations occur below that temperature. Our results largely resolve
an apparent disagreement between neutron scattering and bulk measurements that
has been previously noted. They also show that the use of non-spherical
crystals in magnetization studies of spin ice may introduce very significant
systematic errors, although we note some interesting - and possibly new -
systematics concerning the demagnetizing factor in cuboidal samples. Finally,
our results show how experimental susceptibility measurements on spin ices may
be used to extract the characteristic energy scale of the system and the
corresponding chemical potential for emergent magnetic monopoles.Comment: 11 pages, 3 figures 1 table. Manuscript submitte
Spin ice under pressure: symmetry enhancement and infinite order multicriticality
We study the low-temperature behaviour of spin ice when uniaxial pressure
induces a tetragonal distortion. There is a phase transition between a Coulomb
liquid and a fully magnetised phase. Unusually, it combines features of
discontinuous and continuous transitions: the order parameter exhibits a jump,
but this is accompanied by a divergent susceptibility and vanishing domain wall
tension. All these aspects can be understood as a consequence of an emergent
SU(2) symmetry at the critical point. We map out a possible experimental
realisation
A Three Dimensional Kasteleyn Transition: Spin Ice in a [100] Field
We examine the statistical mechanics of spin-ice materials with a [100]
magnetic field. We show that the approach to saturated magnetisation is, in the
low-temperature limit, an example of a 3D Kasteleyn transition, which is
topological in the sense that magnetisation is changed only by excitations that
span the entire system. We study the transition analytically and using a Monte
Carlo cluster algorithm, and compare our results with recent data from
experiments on Dy2Ti2O7.Comment: 4 pages, 5 figure
Curie-law crossover in spin liquids
The Curie-Weiss law is widely used to estimate the strength of frustration in
frustrated magnets. However, the Curie-Weiss law was originally derived as an
estimate of magnetic correlations close to a mean-field phase transition, which
-- by definition -- is absent in spin liquids. Instead, the susceptibility of
spin liquids is known to undergo a Curie-law crossover between two magnetically
disordered regimes. Here, we study the generic aspect of the Curie-law
crossover by comparing a variety of frustrated spin models in two and three
dimensions, using both classical Monte Carlo simulations and analytical Husimi
tree calculations. Husimi tree calculations fit remarkably well the simulations
for all temperatures and almost all lattices. We also propose a Husimi Ansatz
for the reduced susceptibility , to be used in complement to the
traditional Curie-Weiss fit in order to estimate the Curie-Weiss temperature
. Applications to materials are discussed.Comment: 26 pages, 15 figure
Analysis of a fully packed loop model arising in a magnetic Coulomb phase
The Coulomb phase of spin ice, and indeed the Ic phase of water ice,
naturally realise a fully-packed two-colour loop model in three dimensions. We
present a detailed analysis of the statistics of these loops, which avoid
themselves and other loops of the same colour, and contrast their behaviour to
an analogous two-dimensional model. The properties of another extended degree
of freedom are also addressed, flux lines of the emergent gauge field of the
Coulomb phase, which appear as "Dirac strings" in spin ice. We mention
implications of these results for related models, and experiments.Comment: 5 pages, 4 figure
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