Jean-Christophe Toussaint

Observation of Bloch-point domain walls in cylindrical magnetic nanowires

Topological protection is an elegant way of warranting the integrity of quantum and nanosized systems. In magnetism one example is the Bloch-point, a peculiar object implying the local vanishing of magnetization within a ferromagnet. Its existence had been postulated and described theoretically since several decades, however it has never been observed. We con rm experimentally the existence of Bloch points, imaged within domain walls in cylindrical magnetic nanowires, combining surface and transmission XMCD-PEEM magnetic microscopy. This opens the way to the experimental search for peculiar phenomena predicted during the motion of Bloch-point-based domain walls.

Controlled Switching of Néel Caps in Flux-Closure Magnetic Dots

While magnetic hysteresis usually considers magnetic domains, the switching of the core of magnetic vortices has recently become an active topic. We considered Bloch domain walls, which are known to display at the surface of thin films flux-closure features called Néel caps. We demonstrated the controlled switching of these caps under a magnetic field, occurring via the propagation of a surface vortex. For this we considered flux-closure states in elongated micron-sized dots, so that only the central domain wall can be addressed, while domains remain unaffected.

A convergent finite element approximation for Landau–Lifschitz–Gilbert equation

This paper describes a finite element formulation for Landau–Lifschitz–Gilbert equation (LLGE) that is proved to converge as the time and space steps tend to 0 toward a weak solution of LLGE. An order two (in time) scheme is also given and numerical results are presented showing the applicability of the method.

Dimensionality crossover in magnetism: from domain walls (2D) to vortices (1D).

Dimensionality crossover is a classical topic in physics. Surprisingly, it has not been searched in micromagnetism, which deals with objects such as domain walls (2D) and vortices (1D). We predict by simulation a second-order transition between these two objects, with the wall length as the Landau parameter. This was confirmed experimentally based on micron-sized flux-closure dots.

Angular dependence of magnetization switching for a multidomain dot: Experiment and simulation

We have measured the in-plane angular variation of nucleation and annihilation fields of a multidomain magnetic single dot with a micro-SQUID. The dots are Fe/ Mos110d self-assembled in ultrahigh vacuum, with submicron size and an elongated hexagonal shape. The angular variations were quantitatively reproduced by micromagnetic simulations. Discontinuities in the variations are observed, and shown to result from bifurcations related to the interplay of the nonuniform magnetization state with the shape of the dot.

Chiral nature of magnetic monopoles in artificial spin ice

Micromagnetic properties of monopoles in artificial kagome spin ice systems are investigated using numerical simulations. We show that micromagnetics brings additional complexity into the physics of these monopoles that is, by essence, absent in spin models: in addition to a fractionalized classical magnetic charge, monopoles in the artificial kagome ice are chiral at remanence. Our simulations predict that the chirality of these monopoles can be controlled without altering their charge state. This chirality breaks the vertex symmetry and triggers a directional motion of the monopole under an applied magnetic field. Our results also show that the choice of the geometrical features of the lattice can be used to turn on and off this chirality, thus allowing the investigation of chiral and achiral monopoles.

A pulsed field magnetometer for industrial use

Summary form only given. An industrial Pulsed Field Magnetometer (PFM) was developed, focusing on industrial needs. The system is to be used for the complete characterising of permanent magnets in a production line situation. The purpose is to create a fast, reliable and accurate magnetometer for quality control but also, in a later step as a standard measurement method for magnets.

Calibration of an industrial pulsed field magnetometer

Summary form only given. Based on the existing knowledge an industrial pulsed field magnetometer (PFM) was developed focusing on the industrial needs. The system shall be used for characterizing the hysteresis parameters (coercivity, remanence, energy product) of permanent magnets. The purpose is two use a fast and reliable magnetometer for the quality control but also in a later step as a standardized measurement method for magnets. For this purpose the best suited setup but also effects like that of eddy currents in such transient fields were studied in the past. It is the purpose of the present work to investigate a high accurate calibration procedure for such pulsed field magnetometers.

Phase diagram of magnetic domain walls in spin valve nano-stripes

We investigate numerically the transverse versus vortex phase diagram of head-to-head domain walls in Co/Cu/Py spin valve nano-stripes (Py: permalloy), in which the Co layer is mostly single domain while the Py layer hosts the domain wall. The range of stability of the transverse wall is shifted towards larger thickness compared to single Py layers, due to a magnetostatic screening effect between the two layers. An approached analytical scaling law is derived, which reproduces faithfully the phase diagram.

Improved coherence of ultrafast spin-transfer-driven precessional switching with synthetic antiferromagnet perpendicular polarizer

The coherence of the precessional switching was compared in planar spin-valves comprising either an additional simple perpendicular polarizer or a synthetic antiferromagnet perpendicular polarizer. A significant improvement in the precession coherence was observed experimentally in the second type of samples. Micromagnetic simulations were performed to study the effect of the stray field from the perpendicular polarizer. They provide an explanation for the gradual loss of coherence of the precession in terms of vortex formation, which occurs much faster when a simple perpendicular polarizer is used.