B. Van Waeyenberge

Magnetic vortex core reversal by excitation with short bursts of an alternating field

The vortex state, characterized by a curling magnetization, is one of the equilibrium configurations of soft magnetic materials and occurs in thin ferromagnetic square and disk-shaped elements of micrometre size and below. The interplay between the magnetostatic and the exchange energy favours an in-plane, closed flux domain structure. This curling magnetization turns out of the plane at the centre of the vortex structure, in an area with a radius of about 10 nanometres—the vortex core. The vortex state has a specific excitation mode: the in-plane gyration of the vortex structure about its equilibrium position. The sense of gyration is determined by the vortex core polarization. Here we report on the controlled manipulation of the vortex core polarization by excitation with small bursts of an alternating magnetic field. The vortex motion was imaged by time-resolved scanning transmission X-ray microscopy. We demonstrate that the sense of gyration of the vortex structure can be reversed by applying short bursts of the sinusoidal excitation field with amplitude of about 1.5 mT. This reversal unambiguously indicates a switching of the out-of-plane core polarization. The observed switching mechanism, which can be understood in the framework of micromagnetic theory, gives insights into basic magnetization dynamics and their possible application in data storage.

Polarization Selective Magnetic Vortex Dynamics and Core Reversal in Rotating Magnetic Fields

We report on the observation of magnetic vortex dynamics in response to rotating magnetic fields in submicron platelets. Unlike linear fields or spin polarized currents, which excite both vortex core polarization states, an in-plane rotating field can selectively excite one of the polarization states. We demonstrate by direct imaging with time-resolved scanning x-ray microscopy that the rotating field only excites the gyrotropic mode if the rotation sense of the field coincides with the vortex gyration sense and that such a field can selectively reverse the vortex polarization.

Time-resolved X-ray microscopy of spin-torque-induced magnetic vortex gyration

Time-resolved x-ray microscopy is used to image the influence of alternating high-density currents on the magnetization dynamics of ferromagnetic vortices. Spin-torque-induced vortex gyration is observed in micrometer-sized permalloy squares. The phases of the gyration in structures with different chirality are compared to an analytical model and micromagnetic simulations, considering both alternating spin-polarized currents and the currents Oersted field. In our case the driving force due to spin-transfer torque is about 70% of the total excitation while the remainder originates from the currents Oersted field. This finding has implications to magnetic storage devices using spin-torque driven magnetization switching and domain-wall motion.

Vortex core switching by coherent excitation with single in-plane magnetic field pulses

The response of magnetic vortex cores to subnanosecond in-plane magnetic field pulses was studied by time-resolved x-ray microscopy. Vortex core reversal was observed and the switching events were located in space and time. This revealed a mechanism of coherent excitation by the leading and trailing edges of the pulse, lowering the field amplitude required for switching. The mechanism was confirmed by micromagnetic simulations and can be understood in terms of gyration around the vortex equilibrium positions, displaced by the applied field.

Annealing behaviour of reactor pressure-vessel steels studied by positron-annihilation spectroscopy, Mössbauer spectroscopy and transmission electron microscopy

The annealing behaviour of commonly used reactor pressure-vessel steels was studied using positron-annihilation spectroscopy (PAS) (lifetime and Doppler broadening techniques), transmission Mossbauer spectroscopy (MS), integral low-energy electron MS (ILEEMS) and transmission electron microscopy (TEM). The results of these methods applied to different types of steels are discussed. Some significant differences in the experimental results are observed between the Russian (3 types) and Western Europe (5 types) steels and between individual specimens within these two groups. All specimens were annealed in vacuum and studied after this thermal treatment. It was confirmed that the heat affected zone (HAZ) is the most sensitive region for thermal and neutron irradiation-induced embrittlement in the reactor. Positron-annihilation lifetime measurements on HAZ specimens annealed at successively higher temperatures show the drastic increase in the vacancy-type defect formation between 525°C and 600°C. Therefore these specimens were selected for further detailed studies by TEM.

Direct observation of the vortex core magnetization and its dynamics

Square-shaped thin film structures with a single magnetic vortex were investigated using a scanning transmission x-ray microscope. The authors report on the direct observation of the vortex core in 500×500nm2, 40nm thick soft magnetic Ni–Fe samples. The static configuration of the vortex core was imaged as well as the gyrotropic motion of the core under excitation with an in-plane alternating magnetic field. This enabled them to directly visualize the direction of the out-of-plane magnetization in the vortex core (up or down). The reversal of the core was effected by short bursts of an alternating magnetic field. An asymmetry appears in the core’s trajectory for its orientation pointing up and down, respectively.

Vortex dynamics in Permalloy disks with artificial defects: suppression of the gyrotropic mode

The dynamics of magnetic vortices in thin Permalloy disks having artificial defects in the form of small holes at different locations within the disk has been investigated by means of frequency-domain spatially resolved ferromagnetic resonance. It is found that the vortex can be effectively captured by such a defect. Consequently the commonly observed gyrotropic vortex motion in an applied microwave field of 1mT is suppressed. However, if in addition a static magnetic field of at least 4.3mT is applied, the vortex core is nucleated from the artificial defect and a modified gyrotropic motion starts again.

Chiral symmetry breaking of magnetic vortices by sample roughness

Finite-element micromagnetic simulations are employed to study the chiral symmetry breaking of magnetic vortices, caused by the surface roughness of thin-film magnetic structures. An asymmetry between vortices with different core polarizations has been experimentally observed for square-shaped platelets. For example, the threshold fields for vortex core switching were found to differ for core up and down. This asymmetry was, however, not expected for these symmetrically shaped structures, where both core polarizations should behave symmetrically. Three-dimensional finite element simulations are employed to show that a small surface roughness can break the symmetry between vortex cores pointing up and down. A relatively small sample roughness is found to be sufficient to reproduce the experimentally observed asymmetries. It arises from the lack of mirror-symmetry of the rough thin-film structures, which causes vortices with different handedness to exhibit asymmetric dynamics.

A new view on positronium in polymers

Abstract A new model is presented describing the annihilation of positronium in polymers. Ps is formed in the bulk and is trapped into the free volume holes with epithermal initial energy where it thermalizes towards its ground state. This model was verified by interpreting lifetime spectra in an amorphous cross-linked polyurethane.

Regarding the Néel relaxation time constant in magnetorelaxometry

Magnetorelaxometry (MRX) is a sensitive measurement technique frequently employed in biomedical applications for imaging magnetic nanoparticles (MNP). In this article, we employ a first principles model to investigate the effects of different iron oxide MNP sample properties on the Neel relaxation time constant τN in magnetorelaxometry. Using this model, we determined that dipolar interactions start to have an impact on the MRX signal from Fe concentrations of 100 mmol/l and result in a smaller τN. Additionally, the micromagnetic damping constant, closely related to τN, was found to be between 0.0005 and 0.002 by comparison to an MRX measurement of iron oxide particles. This is significantly lower compared to the bulk value of 0.07 for this material.