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 $(\delta,T)$ phase diagram, $\delta$ 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 $\delta$, 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 $\delta$ increases.Comment: 9 pages, 9 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

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

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