arXiv subjects

D. Pescia

Publications and source records attributed to D. Pescia.

8 recordsLinked to original sources

arXiv2026

Lectures on ultrathin film ferromagnetism

In these Lecture Notes we review some of the fundamental principles that have emerged from research on the ferromagnetism of ultrathin films consisting of 3d transition-metal overlayers. Their growth is often layer-by-layer. This growth mode produces quantum wells along the vertical direction that profoundly impact any physical property of the materials. In addition, the vertical confinement establishes spin ensembles that extend to macroscopic distances along the in-plane directions and are finite along the vertical (perpendicular) direction, i.e. they are two-dimensional. Accordingly, they display ground state properties that originate from the two-dimensionality, such as ``dead'' magnetic layers or ``enhanced magnetic moments'', an oscillatory interlayer magnetic coupling and an anomalous perpendicular versus in-plane magnetic anisotropy that produces, in some specific situations, a perpendicular collective orientation of the spins. At finite temperatures, ferromagnetic order is observed to persist and an analysis of the magnetic order of ultrathin films in terms of the renormalization group provides a suitable framework for explaining this observation. The ferromagnetic order is lost at a phase transition which follows closely the two-dimensional Ising universality class, as shown by an accurate analysis of data in the vicinity of the critical point. The perpendicular spin orientation is often observed to turn in-plane by a reorientation phase transition which is also properly described by a renormalization group argument. Finally, the perpendicular spin orientation introduces topological excitations of the ferromagnetic order, consisting of stripes of reversed perpendicular spin direction. The stripe order undergoes a phase transition to the paramagnetic state that is not yet completely understood.

cond-mat.mes-hall↗

arXiv2026

Symmetry-assisted computation of the magnetic field at the site of a point dipole

In this paper, we revisit this concept of the magnetic field at the site of a magnetic dipole using symmetry arguments that are mathematically rigorous yet intuitive enough to be well-suited for upper-level undergraduate courses in electrodynamics and quantum mechanics. Furthermore, we present symmetry-based results that go beyond those discussed in the standard literature, which are directly relevant to fundamental problems in two-dimensional physics.

cond-mat.mes-hall↗

arXiv2024

On the spontaneous magnetization of two-dimensional ferromagnets

Ferromagnetism is typically discussed in terms of the exchange interaction and magnetic anisotropies. Yet real samples are inevitably affected by the magnetostatic dipole-dipole interaction. Because of this interaction, a theorem (R.B. Griffiths, Free Energy of interacting magnetic dipoles, Phys. Rev. 176, 655 (1968)) forbids a spontaneous magnetization in, nota bene, three-dimensional bodies. Here we discuss perpendicularly and in-plane magnetized ferromagnetic bodies in the shape of a slab of finite thickness. In perpendicularly magnetized slabs, magnetic domains are energetically favored when the lateral size is sufficiently large, i.e. there is no spontaneous magnetization. For in-plane magnetization, instead, spontaneous magnetization is possible below a critical thickness which, in real thin films, could be as small as few monolayers. At this critical thickness, we predict a genuine phase transition to a multi-domain state. These results have implications for two-dimensional ferromagnetism.

cond-mat.stat-mech↗

arXiv2016

Critical exponents and scaling invariance in the absence of a critical point

The paramagnetic-to-ferromagnetic phase transition is believed to proceed through a critical point, at which power laws and scaling invariance, associated with the existence of one diverging characteristic length scale -- the so called correlation length -- appear. We indeed observe power laws and scaling behavior over extraordinarily many decades of the suitable scaling variables at the paramagnetic-to-ferromagnetic phase transition in ultrathin Fe films. However, we find that, when the putative critical point is approached, the singular behavior of thermodynamic quantities transforms into an analytic one: the critical point does not exist, it is replaced by a more complex phase involving domains of opposite magnetization, below as well as $above$ the putative critical temperature. All essential experimental results are reproduced by Monte-Carlo simulations in which, alongside the familiar exchange coupling, the competing dipole-dipole interaction is taken into account. Our results imply that a scaling behavior of macroscopic thermodynamic quantities is not necessarily a signature for an underlying second-order phase transition and that the paramagnetic-to-ferromagnetic phase transition proceeds, very likely, in the presence of at least two long spatial scales: the correlation length and the size of magnetic domains.

cond-mat.stat-mech↗

arXiv2013

Scale invariance of a diodelike tunnel junction

We measure the current vs voltage (I-V) characteristics of a diodelike tunnel junction consisting of a sharp metallic tip placed at a variable distance d from a planar collector and emitting electrons via electric-field assisted emission. All curves collapse onto one single graph when I is plotted as a function of the single scaling variable Vd^{-λ}, d being varied from a few mm to a few nm, i.e., by about six orders of magnitude. We provide an argument that finds the exponent λ within the singular behavior inherent to the electrostatics of a sharp tip. A simulation of the tunneling barrier for a realistic tip reproduces both the scaling behavior and the small but significant deviations from scaling observed experimentally.

cond-mat.mes-hall↗

arXiv2010

A Scaling Hypothesis for Modulated Systems

We propose a scaling hypothesis for pattern-forming systems in which modulation of the order parameter results from the competition between a short-ranged interaction and a long-ranged interaction decaying with some power $α$ of the inverse distance. With L being a spatial length characterizing the modulated phase, all thermodynamic quantities are predicted to scale like some power of L. The scaling dimensions with respect to L only depend on the dimensionality of the system d and the exponent α. Scaling predictions are in agreement with experiments on ultra-thin ferromagnetic films and computational results. Finally, our scaling hypothesis implies that, for some range of values α>d, Inverse-Symmetry-Breaking transitions may appear systematically in the considered class of frustrated systems.

cond-mat.soft↗

arXiv2010

(Ir-)Reversibility and thermal equilibrium in magnetic domain pattern formation in ultra-thin ferromagnetic films

We investigate the details of pattern formation and transitions between different modulated phases in ultra-thin Fe films on Cu(001). At high temperature, the transitions between the uniform saturated state, the bubble state and the striped state are completely reversible, while at low temperature the bubble phase is avoided. The observed non-equilibrium behavior can be qualitatively explained by considering the intrinsic energy barriers appearing in the system due to the competition between the short-ranged exchange and the long-ranged dipolar interactions. Our experiments suggest that the height of these energy barriers is related to the domain size and is therefore strongly temperature dependent.

cond-mat.mes-hall↗

arXiv2009

One-dimensional Ising ferromagnet frustrated by long-range interactions at finite temperatures

We consider a one-dimensional lattice of Ising-type variables where the ferromagnetic exchange interaction J between neighboring sites is frustrated by a long-ranged anti-ferromagnetic interaction of strength g between the sites i and j, decaying as |i-j|^-alpha, with alpha>1. For alpha smaller than a certain threshold alpha_0, which is larger than 2 and depends on the ratio J/g, the ground state consists of an ordered sequence of segments with equal length and alternating magnetization. The width of the segments depends on both alpha and the ratio J/g. Our Monte Carlo study shows that the on-site magnetization vanishes at finite temperatures and finds no indication of any phase transition. Yet, the modulation present in the ground state is recovered at finite temperatures in the two-point correlation function, which oscillates in space with a characteristic spatial period: The latter depends on alpha and J/g and decreases smoothly from the ground-state value as the temperature is increased. Such an oscillation of the correlation function is exponentially damped over a characteristic spatial scale, the correlation length, which asymptotically diverges roughly as the inverse of the temperature as T=0 is approached. This suggests that the long-range interaction causes the Ising chain to fall into a universality class consistent with an underlying continuous symmetry. The e^(Delta/T)-temperature dependence of the correlation length and the uniform ferromagnetic ground state, characteristic of the g=0 discrete Ising symmetry, are recovered for alpha > alpha_0.

cond-mat.stat-mech↗