Jean-Claude Serge Lévy

Forbidden frequency gaps in magnonic spectra of ferromagnetic layered composites

Abstract Spin wave spectra in layered composite materials are computed by means of a transfer matrix method with evidence for existence of numerous frequency gaps. Essential properties of these spectra are successfully explained within the framework of approximate models of independent films or an effective medium, according to the strength (strong and weak, respectively) of the magnetization and exchange contrasts; this includes an evidence of the existence of resonance states, for which both materials are simultaneously excited.

Magnetic structures of Ising and vector spins monolayers by Monte-Carlo simulations

Abstract We derive the optimal magnetic structures for monolayers of either square or triangular lattice symmetry with evidence for morphological differences. The interplay between short-range exchange and long-range dipolar forces leads to quite different results for Ising spins and vector spins. For the Ising model, spin domains with parallel stripes, chevron patterns and labyrinths at different scales and with thermal disorder are deduced. For the vector model with a weak perpendicular anisotropy, the spins are planar and form a lattice of vortices of both signs. Such a structure remains stable even under a large perpendicular magnetic field, whereas a weak in-plane magnetic field is sufficient to obtain a uniform magnetic domain. For a sufficiently large perpendicular anisotropy, a mixed structure appears that includes spin vortex areas surrounding spin-up and spin-down areas.

Purely dipolar versus dipolar-exchange modes in cylindrical nanorods

Properties of purely dipolar and dipolar-exchange modes in finite circular rods are investigated numerically, with the sample static magnetization assumed to be uniform and parallel to the rod axis; we study magnetic excitations propagating along this direction. In particular, we demonstrate size-exchange effects induced by modifying the rod aspect ratio (the diameter to length ratio) and the strength of the exchange coupling. Untypical groups of purely dipolar modes are shown to occur in smooth rods: bulk-dead modes, characterized by a dead-amplitude region inside the rod, and comb modes, in which a nearly “dead” region is found at the sample borders, while numerous (“comb-like”) oscillations take place in the bulk. These two mode groups fade to disappear when the sample geometry evolves toward the thin-film one, or when the exchange becomes dominant. The existence of these untypical modes is related to local demagnetizing field profile nonhomogeneity, particularly to two edge wells found in the field pr...

Localization properties of pure magnetostatic modes in a cubic nanograin

We investigate the dynamical properties of a system of interacting magnetic dipoles disposed in sites of an sc lattice and forming a cubic-shaped sample of size determined by the cube edge length (N-1)a (a being the lattice constant, N representing the number of dipolar planes). The dipolar field resulting from the dipole-dipole interactions is calculated numerically in points of the axis connecting opposite cube face centers (central axis) by collecting individual contributions to this field coming from each of the N atomic planes perpendicular to the central axis. The applied magnetic field is assumed to be oriented along the central axis, magnetizing uniformly the whole sample, all the dipoles being aligned parallelly in the direction of the applied field. The frequency spectrum of magnetostatic waves propagating in the direction of the applied field is found numerically by solving the Landau-Lifshitz equation of motion including the local (nonhomogeneous) dipolar field component; the mode amplitude spatial distributions (mode profiles) are depicted as well. It is found that only the two energetically highest modes have bulk-extended character. All the remaining modes are of localized nature; more precisely, the modes forming the lower part of the spectrum are localized in the subsurface region, while the upper-spectrum modes are localized around the sample center. We show that the mode localization regions narrow down as the cube size, N, increases (we investigated the range of N=21 to N=101), and in sufficiently large cubes one obtains practically only center-localized and surface-localized magnetostatic modes.

Does the generation of surface spin-waves hinge critically on the range of neighbour interaction?☆

Abstract We perform a numerical investigation of the spin-wave band structures throughout the two-dimensional Brillouin zone in magnetic (cubic) thin films for the surface orientations sc(001) and bcc(001) admitting the coexistence of exchange nearest and next-nearest neighbour interactions and elucidate the individual roles of these neighbourhoods and their perpendicular versus oblique near-surface disposition in the generation of surface spin-waves.

Stability of the Landau state in square two-dimensional magnetic nanorings

We use a microscopic theory taking into account dipolar and nearest-neighbour exchange interactions to explore spin-wave excitations in two-dimensional square-shaped magnetic nanorings with the Landau state assumed as a magnetic state. From the spin-wave spectra, we determine the range of the dipolar-to-exchange interaction ratio in which the assumed state is stable. Various types of localized spin waves prove responsible for the transition to a new magnetic configuration. We found the transition forced by predominating exchange interactions size-independent in a wide range of both external and internal size of the ring.

The Role of Next-Nearest Neighbours for the Existence Conditions of Subsurface Spin Waves in Magnetic Films

Spin-wave energies and respective band structures throughout the two-dimensional Brillouin zone are investigated in magnetic (cubic) thin films for the surface orientations sc(110), bcc(110) and fcc(110). We apply the Heisenberg localized spin model assuming exchange nearest (NN) and next-nearest (NNN) neighbour interactions and elucidate the role of the geometrical disposition of the NN and NNN neighbourhoods for the emergence of surface (SSW) and subsurface spin waves (SSSW). The necessary condition for the emergence of SSSW is found to require the NNN bonds to be cut at the surface obliquely thereto.

Simulated growth of layers on a substrate with mismatch: structural studies

Abstract High temperature deposition of metallic materials on a (111) face of a fcc substrate, followed by a slow cooling down to a given temperature, is simulated by means of a Monte-Carlo algorithm with Lennard-Jones interatomic pair potentials. Adsorption and growth modes on the surface are studied in order to determine whether the growth is three- or two-dimensional, according to relevant parameters such as lattice mismatch and relative atomic binding energy. For a ± 10% mismatch it is found that the Stranski-Krastanov process starts early and is later healed by the appearance of bridges between islands, after a deposition of about ten monolayers. The interlayer distance undergoes oscillations as a function of the layer number. This is observed for a ± 10% mismatch as well as for a 5% mismatch.

Self-locking of a modulated single overlayer in a nanotribology simulation

Abstract Molecular dynamics simulations of a single layer with +10% mismatch deposited on a (001) surface of an fcc substrate are reported. Lennard–Jones pair interactions are used. At equilibrium, the overlayer exhibits a triangular structure, one edge of which is parallel to the [ 1 10] direction of the fcc substrate lattice. This direction displays a modulation of the interatomic distances. Constant external forces are then applied parallel to the modulation axis, which is the easy sliding direction; an amplitude of 0.2σ/e per atom is enough to initiate sliding motion. A spectacular reorganization of the monolayer then occurs with a 90° rotation of the modulation direction and a self-locking of the motion. A microscopic mechanism for this gradual reorganization is suggested.

Structural transitions in clusters

Abstract Monatomic clusters are studied by Monte Carlo relaxation using generalized Lennard-Jones potentials. A transition from an icosahedral symmetry to a crystalline symmetry with stacking faults is always observed. Bcc-based soft atom clusters are found to have a lower energy than the corresponding hcp and fcc ones below the melting point.