Propagation of magnetic vortices using nanocontacts as tunable attractors.

Magnetic vortices in thin films are in-plane spiral spin configurations with a core in which the magnetization twists out of the film plane. Vortices result from the competition between atomic-scale exchange forces and long-range dipolar interactions. They are often the ground state of magnetic dots, and have applications in medicine, microwave generation and information storage. The compact nature of the vortex core, which is 10-20 nm wide, makes it a suitable probe of magnetism at the nanoscale. However, thus far the positioning of a vortex has been possible only in confined structures, which prevents its transport over large distances. Here we show that vortices can be propagated in an unconstrained system that comprises electrical nanocontacts (NCs). The NCs are used as tunable vortex attractors in a manner that resembles the propelling of space craft with gravitational slingshots. By passing current from the NCs to a ferromagnetic film, circulating magnetic fields are generated, which nucleate the vortex and create a potential well for it. The current becomes spin polarized in the film, and thereby drives the vortex into gyration through spin-transfer torques. The vortex can be guided from one NC to another by tuning attractive strengths of the NCs. We anticipate that NC networks may be used as multiterminal sources of vortices and spin waves (as well as heat, spin and charge flows) to sense the fundamental interactions between physical objects and fluxes of the next-generation spintronic devices.

Magnetic logic using nanowires with perpendicular anisotropy.

In addition to a storage function through the magnetization of nanowires, domain wall propagation can be used to trigger magnetic logic functions. Here, we present a new way to realize a pure magnetic logic operation by using magnetic nanowires with perpendicular anisotropy. Emphasis is given on the generation of the logic function 'NOT' that is based on the dipolar interaction between two neighbouring magnetic wires, which favours the creation of a domain wall. This concept has been validated on several prototypes and the results fit well with the expectations.

Current-driven vortex oscillations in metallic nanocontacts.

We present experimental evidence of subgigahertz spin-transfer oscillations in metallic nanocontacts that are due to the translational motion of a magnetic vortex. The vortex is shown to execute large-amplitude orbital motion outside the contact region. Good agreement with analytical theory and micromagnetics simulations is found.

Line shape distortion in a nonlinear auto-oscillator near generation threshold: application to spin-torque nano-oscillators.

The power spectrum of an auto-oscillator with a large frequency nonlinearity in a noisy environment is calculated. The power spectrum becomes strongly non-Lorentzian, broadened, and asymmetric near the generation threshold. A Lorentzian spectrum is recovered far below and far above the threshold, which suggests that line shape distortions provide a signature of the threshold. We show that the developed theory adequately describes the observed behavior of a strongly nonlinear spin-torque nano-oscillator.

Single-shot time-resolved measurements of nanosecond-scale spin-transfer induced switching: stochastic versus deterministic aspects.

Using high bandwidth resistance measurements, we study the single-shot response of tunnel junctions subjected to spin torque pulses. After the pulse onset, the switching proceeds by a ns-scale incubation delay during which the resistance is quiet, followed by a 400 ps transition terminated by a large ringing that is damped progressively. While the incubation delay fluctuates significantly, the resistance traces are reproducible once this delay is passed. After switching, the time-resolved resistance traces indicate micromagnetic configurations that are rather spatially coherent.

Thermally activated depinning of a narrow domain wall from a single defect.

We describe the field induced depinning process of a magnetic domain wall (DW) from a single bidimensional nanometric defect. The DW propagates in a wire lithographed on a film with strong perpendicular anisotropy. We observe a statistical distribution of the relaxation time consistent with a Néel-Brown picture of magnetization reversal. This indicates that the nanometric DW can be considered as an ideal monodomain particle switching over a single energy barrier. Such a stochastic character of DW depinning has to be taken into account for spintronic applications.

Nanometer scale observation of high efficiency thermally assisted current-driven domain wall depinning.

Nanometer scale observation of the depinning of a narrow domain wall (DW) under a spin current is reported. We studied approximately 12 nm wide 1D Bloch DWs created in thin films exhibiting perpendicular magnetic anisotropy. Magnetotransport measurements reveal thermally assisted current-driven DW motion between pinning sites separated by as little as 20 nm. The efficiency of current-driven DW motion assisted by thermal fluctuations is measured to be orders of magnitude higher than has been found for in-plane magnetized films, allowing us to control DW motion on a nanometer scale at low current densities.

Deroughening of domain wall pairs by dipolar repulsion.

As a magnetic domain wall propagates under small fields through a random potential, it roughens as a result of weak collective pinning, known as creep. Using Kerr microscopy, we report experimental evidence of a surprising deroughening of wall pairs in the creep regime, in a 0.5 nm thick Co layer with perpendicular anisotropy. A bound state is found in cases where two rough domains nucleated far away from one another and first growing under the action of a magnetic field eventually do not merge. The two domains remain separated by a strip of unreversed magnetization, characterized by flat edges and stabilized by dipolar fields. A creep theory that includes dipolar interactions between domains successfully accounts for (i) the domain wall deroughening as the width of the strip decreases and (ii) the quasistatic and dynamic field dependence of the strip width s.

Domain wall creep in magnetic wires.

The dynamics of a 1D domain wall (DW) in magnetic wires patterned in 2D ultrathin Co films is studied as a function of the wire width w0. The DW velocity v(H) is hugely reduced when w0 is decreased, and its field dependence is consistent with a creep process with a critical exponent micro=1/4. The effective critical field scales as (1/w0). Measurements of v(H) in wires with controlled artificial defects show that this arises from the edge roughness introduced by patterning. We show that the creep law can be renormalized by introducing a topologically induced critical field proportional to (1/w0).

Ordering intermetallic alloys by ion irradiation: a way to tailor magnetic media.

We show how, combining He ion irradiation and thermal mobility below 600 K, the transformation from chemical disorder to order in thin films of an intermetallic ferromagnet (FePd) may be triggered and controlled. Kinetic Monte Carlo simulations show that the initial directional short range order determines the transformation. Magnetic ordering perpendicular to the film plane was achieved, promoting the initially weak magnetic anisotropy to the highest values known for FePd films. Applications to ultrahigh density magnetic recording are suggested.

Phase coherent precessional magnetization reversal in microscopic spin valve elements.

We evidence multiple coherent precessional magnetization reversal in microscopic spin valves. Stable, reversible, and highly efficient magnetization switching is triggered by transverse field pulses as short as 140 ps with energies down to 15 pJ. At high fields a phase coherent reversal is found revealing periodic transitions from switching to nonswitching under variation of pulse parameters. At the low field limit the existence of a relaxation dominated regime is established allowing switching by pulse amplitudes below the quasistatic switching threshold.

Quasiballistic magnetization reversal.

We demonstrate a quasiballistic switching of the magnetization in a microscopic magnetoresistive memory cell. By means of time resolved magnetotransport, we follow the large angle precession of the free layer magnetization of a spin valve cell upon application of transverse magnetic field pulses. Stopping the field pulse after a 180 degrees precession rotation leads to magnetization reversal with reversal times as short as 165 ps. This switching mode represents the fundamental ultrafast limit of field induced magnetization reversal.

Effect of monocular deprivation on NMDAR1 immunostaining in ocular dominance columns of the marmoset Callithrix jacchus.

We previously showed that immunoreactivity to N-Methyl-D-aspartate (NMDA) receptors in primary visual cortex of Callithrix jacchus is regulated by visual activity during the second and third postnatal months (Fonta et al., 1997). The purpose of the present study was to show that the columnar pattern of high and low NMDAR1 immunoreactivity observed in monocularly deprived animals corresponds to ocular dominance columns linked to the nondeprived and deprived eye, respectively. We compared cortical distribution of NMDAR1 receptors and the projection zones of thalamic afferents, revealed by transneuronal transport of tritiated proline, in 2-month-old, either monocularly deprived or control, marmosets. The data show that ocular dominance columns exist in 2-month-old marmosets and that a 2-week monocular deprivation by means of eyelid suture leads to a modification of the thalamo-cortical afferents organization. Experiments of neuronal tracing and immunohistochemistry performed on the same animals demonstrated that cortical domains with decreased NMDAR1 level correspond to the deprived eye columns. These investigations, coupled to the previous results, strongly suggest that the NMDA receptors, regulated by visual activity, are involved in the refining of ocular dominance columns in the primary visual cortex of juvenile marmoset.

Apertureless scanning near-field magneto-optical microscopy of magnetic multilayers.

We imaged magnetic domains in Pt/Co/Pt multilayers using an apertureless scanning near-field optical microscope operating in reflection mode. As the magneto-optical effects are weak for this kind of structure, a polarization modulation technique with a photoelastic modulator was used to reveal the contrast between magnetic domains. In the case of a Pt/Co/Pt trilayer structure, a strong improvement in lateral resolution is observed compared with far-field magneto-optical images and good sensitivity is achieved. In the case of a Pt/[Co/Pt]Pt multilayer structure, stripe domains of 200 nm width could be resolved, in good agreement with images obtained by magnetic force microscopy on the same structure.

Magneto-optical contrast in near-field optics.

We propose a simple calculation of near-field magneto-optical (MO) images based on the beam propagation method. We calculate both Faraday rotation and circular dichroism contrasts of planar magnetic structures such as as-grown thin films and ion-irradiated samples. High-contrast near-field MO images are obtained, in good agreement with our experimental observations.

Planar patterned magnetic media obtained by ion irradiation

By ion irradiation through a lithographically made resist mask, the magnetic properties of cobalt-platinum simple sandwiches and multilayers were patterned without affecting their roughness and optical properties. This was demonstrated on arrays of 1-micrometer lines by near- and far-field magnetooptical microscopy. The coercive force and magnetic anisotropy of the irradiated regions can be accurately controlled by the irradiation fluence. If combined with high-resolution lithography, this technique holds promise for ultrahigh-density magnetic recording applications.

N-methyl-D-aspartate subunit R1 involvement in the postnatal organization of the primary visual cortex of Callithrix jacchus.

It has been demonstrated that the primary visual cortex is highly sensitive to manipulations of the visual environment during a specific, early, postdevelopmental period: the critical period. Pharmacological studies have shown that N-methyl-D-aspartate (NMDA) receptors are involved in the plasticity of the visual cortex just as they are involved in the induction of long-term potentiation (LTP), another activity-dependent form of plasticity. The setting up of synaptic connectivity in the neocortex may rely on LTP-like mechanisms. By using immunohistochemistry techniques, we tested the hypothesis of the role of subunit R1 of NMDA (NMDAR1) receptors in the thalamocortical afferent segregation into ocular-dominance columns in the New World monkey, Callithrix jacchus. We employed early and short (2 weeks) monocular-deprivation periods at different ages of postnatal development (17, 46, 67, 107, and 188 postnatal days). We observed heterogeneous distribution of NMDAR1 in the layer IVC receiving the thalamic inputs if the deprivation was realized between the ages of 46 and 107 days. Layers IVCalpha and IVCbeta were involved differently as a function of the deprivation age. The striped pattern lost its differential intensity with the postnatal age. These results are compared with the ocular-dominance pattern evolution described in other works on this primate. They provide evidence of the NMDAR1 role in the modular organization, within time limits, during the postnatal development of the primary visual cortex.

Prenatal lesioning of vestibular organ by aminoglycosides.

The effects of gentamicin administrated to gravid mice on the vestibular hair cells of their offspring were investigated. Scanning electron microscopy showed the effect to be dose-dependent and age-dependent, and there was a different sensitivity to toxicity for each of the three types of vestibular receptors. The destructive action of gentamicin on the vestibule before birth provides a mechanism to study the influence of the vestibular system on the central nervous system throughout its development.

Nanoscale Magnetic Domains in Mesoscopic Magnets

The basic magnetic properties of three-dimensional nanostructured materials can be drastically different from those of a continuous film. High-resolution magnetic force microscopy studies of magnetic submicrometer-sized cobalt dots with geometrical dimensions comparable to the width of magnetic domains reveal a variety of intricate domain patterns controlled by the details of the dot geometry. By changing the thickness of the dots, the width of the geometrically constrained magnetic domains can be tuned. Concentric rings and spirals with vortex configurations have been stabilized, with particular incidence in the magnetization reversal process as observed in the ensemble-averaged hysteresis loops.