Measurement of the dynamical dipolar coupling in a pair of magnetic nano-disks using a Ferromagnetic Resonance Force Microscope

International audienceWe perform an extensive experimental spectroscopic study of the collective spin-wave dynamics occurring in a pair of magnetic nano-disks coupled by the magneto-dipolar interaction. For this, we take advantage of the stray field gradient produced by the magnetic tip of a ferromagnetic resonance force microscope (f-MRFM) to continuously tune and detune the relative resonance frequencies between two adjacent nano-objects. This reveals the anti-crossing and hybridization of the spin-wave modes in the pair of disks. At the exact tuning, the measured frequency splitting between the binding and anti-binding modes precisely corresponds to the strength of the dynamical dipolar coupling $\Omega$. This accurate f-MRFM determination of $\Omega$ is measured as a function of the separation between the nano-disks. It agrees quantitatively with calculations of the expected dynamical magneto-dipolar interaction in our sample

Magnetic Vortex Core Reversal by Excitation of Spin Waves

Micron-sized magnetic platelets in the flux closed vortex state are characterized by an in-plane curling magnetization and a nanometer-sized perpendicularly magnetized vortex core. Having the simplest non-trivial configuration, these objects are of general interest to micromagnetics and may offer new routes for spintronics applications. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field core toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was established. At frequencies more than an order of magnitude higher vortex state structures possess spin wave eigenmodes arising from the magneto-static interaction. Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited. These results are confirmed by micromagnetic simulations which clearly show the selection rules for this novel reversal mechanism. Our analysis reveals that for spin wave excitation the concept of a critical velocity as the switching condition has to be modified.Comment: Minor corrections and polishing of previous versio

Universal Vectorial and Ultrasensitive Nanomechanical Force Field Sensor

Miniaturization of force probes into nanomechanical oscillators enables ultrasensitive investigations of forces on dimensions smaller than their characteristic length scale. Meanwhile it also unravels the force field vectorial character and how its topology impacts the measurement. Here we expose an ultrasensitive method to image 2D vectorial force fields by optomechanically following the bidimensional Brownian motion of a singly clamped nanowire. This novel approach relies on angular and spectral tomography of its quasi frequency-degenerated transverse mechanical polarizations: immersing the nanoresonator in a vectorial force field does not only shift its eigenfrequencies but also rotate eigenmodes orientation as a nano-compass. This universal method is employed to map a tunable electrostatic force field whose spatial gradients can even take precedence over the intrinsic nanowire properties. Enabling vectorial force fields imaging with demonstrated sensitivities of attonewton variations over the nanoprobe Brownian trajectory will have strong impact on scientific exploration at the nanoscale

Spin torque resonant vortex core expulsion for an efficient radio-frequency detection scheme

Spin-polarised radio-frequency currents, whose frequency is equal to that of the gyrotropic mode, will cause an excitation of the core of a magnetic vortex confined in a magnetic tunnel junction. When the excitation radius of the vortex core is greater than that of the junction radius, vortex core expulsion is observed, leading to a large change in resistance, as the layer enters a predominantly uniform magnetisation state. Unlike the conventional spin-torque diode effect, this highly tunable resonant effect will generate a voltage which does not decrease as a function of rf power, and has the potential to form the basis of a new generation of tunable nanoscale radio-frequency detectors

Optimal control of vortex core polarity by resonant microwave pulses

In a vortex-state magnetic nano-disk, the static magnetization is curling in the plane, except in the core region where it is pointing out-of-plane, either up or down leading to two possible stable states of opposite core polarity p. Dynamical reversal of p by large amplitude motion of the vortex core has recently been demonstrated experimentally,raising fundamental interest for potential application in magnetic storage devices. Here we demonstrate coherent control of p by single and double microwave pulse sequences, taking advantage of the resonant vortex dynamics in a perpendicular bias magnetic field. Optimization of the microwave pulse duration required to switch p allows to experimentally infer the characteristic decay time of the vortex core in the large oscillation regime. It is found to be more than twice shorter than in the small oscillation regime, raising the fundamental question of the non-linear behaviour of magnetic dissipation

Measurement of the dynamical dipolar coupling in a pair of magnetic nanodisks using a ferromagnetic resonance force microscope

We perform a spectroscopic study of the collective spin-wave dynamics occurring in a pair of magnetic nanodisks coupled by the magnetodipolar interaction. We take advantage of the stray field gradient produced by the magnetic tip of a ferromagnetic resonance force microscope to continuously tune and detune the relative resonance frequencies between two adjacent nano-objects. This reveals the anticrossing and hybridization of the spin-wave modes in the pair. At the exact tuning, the measured frequency splitting between the binding and antibinding modes corresponds to the strength of the dynamical dipolar coupling Ω. This accurate ferromagnetic resonance force microscope determination of Ω is measured versus the separation between the nanodisks. It agrees quantitatively with calculations of the expected dynamical magnetodipolar interaction in our sample

Remodelling of Cortical Actin Where Lytic Granules Dock at Natural Killer Cell Immune Synapses Revealed by Super-Resolution Microscopy

Super-resolution 3D imaging reveals remodeling of the cortical actin meshwork at the natural killer cell immune synapse, which is likely to be important for secretion of lytic granules

A Fuzzy Linguistic Summarization Technique for TV Recommender Systems

is resulting in a soaring number of broadcast programs available to viewers. To alleviate this problem, Personal Video Recorders (multimedia platforms which record TV programs on a hard disk) should integrate a recommender system, which purpose is to filter programs according to their relevance. These systems are based on a user profile, acting as a representative for the user's interests. An important research issue resides in going beyond explicitly userdefined profiles