2,371 research outputs found
Phase diagram of an Ising model for ultrathin magnetic films
We study the critical properties of a two--dimensional Ising model with
competing ferromagnetic exchange and dipolar interactions, which models an
ultra-thin magnetic film with high out--of--plane anisotropy in the monolayer
limit. In this work we present a detailed calculation of the phase
diagram, being the ratio between exchange and dipolar interactions
intensities. We compare the results of both mean field approximation and Monte
Carlo numerical simulations in the region of low values of ,
identifying the presence of a recently detected phase with nematic order in
different parts of the phase diagram, besides the well known striped and
tetragonal liquid phases. A remarkable qualitative difference between both
calculations is the absence, in this region of the Monte Carlo phase diagram,
of the temperature dependency of the equilibrium stripe width predicted by the
mean field approximation. We also detected the presence of an increasing number
of metastable striped states as the value of increases.Comment: 9 pages, 9 figure
Pattern formation in the dipolar Ising model on a two-dimensional honeycomb lattice
We present Monte Carlo simulation results for a two-dimensional Ising model
with ferromagnetic nearest-neighbor couplings and a competing long-range
dipolar interaction on a honeycomb lattice. Both structural and thermodynamic
properties are very similar to the case of a square lattice, with the exception
that structures reflect the sixfold rotational symmetry of the underlying
honeycomb lattice. To deal with the long-range nature of the dipolar
interaction we also present a simple method of evaluating effective interaction
coefficients, which can be regarded as a more straightforward alternative to
the prevalent Ewald summation techniques.Comment: 5 pages, 5 figure
Disorder-induced mechanism for positive exchange bias fields
We propose a mechanism to explain the phenomenon of positive exchange bias on
magnetic bilayered systems. The mechanism is based on the formation of a domain
wall at a disordered interface during field cooling (FC) which induces a
symmetry breaking of the antiferromagnet, without relying on any ad hoc
assumption about the coupling between the ferromagnetic (FM) and
antiferromagnetic (AFM) layers. The domain wall is a result of the disorder at
the interface between FM and AFM, which reduces the effective anisotropy in the
region. We show that the proposed mechanism explains several known experimental
facts within a single theoretical framework. This result is supported by Monte
Carlo simulations on a microscopic Heisenberg model, by micromagnetic
calculations at zero temperature and by mean field analysis of an effective
Ising like phenomenological model.Comment: 5 pages, 4 figure
Dall’inferno si leva un canto. Italo Calvino e «l’eco lontana del silenzio»
Nella produzione narrativa di Italo Calvino l’explicit può circoscrivere lo spazio verso cui il 'rumore' del mondo s’avanza a minacciare l’ordine della finzione – e, viceversa, il finale dell’opera può risultare l’ultimo baluardo per mezzo del quale il senso, di cui la finzione è la più strenua attestazione, si sforza di resistere al non senso che proviene dal caos della Storia. Il cielo di pietra, Un re in ascolto, racconti dedicati al valore del canto e, per contro, alla minaccia acustica scoperchiata dalle indiavolate metropoli del nostro tempo, insieme all’epilogo de Le città invisibili rappresentano tre casi emblematici in cui la finzione digrada verso la realtà marcando, su un piano finalmente sincronico, l’estrema frontiera da cui l’autore contempla la propria comunità d’appartenenza – ed eventualmente riflette sul potere, se non salvifico, per lo meno discriminante della parola scritta. Da questa prospettiva il silenzio è al contempo il presupposto e il movimento cui dovrebbe tendere ogni scrittura: costituisce in definitiva per Calvino l’opportunità di discernere per cogliere finalmente l’essenziale
Non glassy ground-state in a long-range antiferromagnetic frustrated model in the hypercubic cell
We analize the statistical mechanics of a long-range antiferromagnetic model
defined on a D-dimensional hypercube, both at zero and finite temperatures. The
associated Hamiltonian is derived from a recently proposed complexity measure
of Boolean functions, in the context of neural networks learning processes. We
show that, depending of the value of D, the system either presents a low
temperature antiferromagnetic stable phase or the global antiferromagnetic
order disappears at any temperature. In the last case the ground state is an
infinitely degenerated non-glassy one, composed by two equal size anti-aligned
antiferromagnetic domains. We also present some results for the ferromagnetic
version of the model.Comment: 8 pages, 5 figure
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