Describing synchronization and topological excitations in arrays of magnetic spin torque oscillators through the Kuramoto model

The collective dynamics in populations of magnetic spin torque oscillators (STO) is an intensely studied topic in modern magnetism. Here, we show that arrays of STO coupled via dipolar fields can be modeled using a variant of the Kuramoto model, a well-known mathematical model in non-linear dynamics. By investigating the collective dynamics in arrays of STO we find that the synchronization in such systems is a finite size effect and show that the critical coupling-for a complete synchronized state-scales with the number of oscillators. Using realistic values of the dipolar coupling strength between STO we show that this imposes an upper limit for the maximum number of oscillators that can be synchronized. Further, we show that the lack of long range order is associated with the formation of topological defects in the phase field similar to the two-dimensional XY model of ferromagnetism. Our results shed new light on the synchronization of STO, where controlling the mutual synchronization of several oscillators is considered crucial for applications.Comment: Accepted for publication in Scientific Reports. Corrected typo in Eq.(9) from previous versio

Tiedotusvälineiden suhde tieteeseen

Tiedotusvälineiden yhteiskunnallinen tehtävä on myötävaikuttaa demokraattisen yhteiskunnan luomiseen ja ylläpitoon. Tiedotusvälineet suorittavat tehtävänsä taloudellisten realiteettien puitteissa.; eihän mikään tiedotusväline pystyy toimimaan tappiollisena loputtomiin, Yleisradion kaltaisia public serviceinstituutioita tai jäsenlehtityyppisiä julkaisuja lukuunottamatta. Miltei kaikkien tiedotusvälineiden omistajat odottavat, että ne nyt tai tulevaisuudessa pärjäävät vähintään omillaan

Thickness and temperature dependence of the magnetodynamic damping of pulsed laser deposited La0.7Sr0.3MnO3\text{La}_{0.7}\text{Sr}_{0.3}\text{MnO}_3 on (111)-oriented SrTiO3\text{O}_3

We have investigated the magnetodynamic properties of $\text{La}_{0.7}\text{Sr}_{0.3}\text{MnO}_3$ (LSMO) films of thickness 10, 15 and 30 nm grown on (111)-oriented SrTi$\text{O}_3$ (STO) substrates by pulsed laser deposition. Ferromagnetic resonance (FMR) experiments were performed in the temperature range 100--300 K, and the magnetodynamic damping parameter $\alpha$ was extracted as a function of both film thickness and temperature. We found that the damping is lowest for the intermediate film thickness of 15 nm with $\alpha \approx 2 \cdot 10^{-3}$, where $\alpha$ is relatively constant as a function of temperature well below the Curie temperature of the respective films.Comment: Accepted for publication in Journal of Magnetism and Magnetic Material

Brandrisker vid lägre syrehalter – Experiment i konkalorimetern

This report has been conducted to examine the advantages and disadvantages of using an open system for experiments in the cone calorimeter with a vitiated air enclosure at low oxygen levels. Eight different experimental set-ups were conducted in which two were designed according to ISO 5660, i.e. the cone calorimeter in its standard configuration. In the other experimental set-ups, a chamber which enclosed the specimen was placed under the exhaust hood. Using this chamber, experiments were performed with a reduced oxygen content, approximately 15 % by volume, which was obtained by adding a desired mixture of nitrogen and air. In some experimental set-ups, a chimney with a length of 30 cm was placed on top of the chamber. In all experimental set-ups, particle board was used as the test material and the irradiance levels were 25 or 50 kW/m2

Coverage-dependent frequency for Li-atom vibrations on Cu(111)

Electron-energy-loss spectra recorded for monolayer amounts of Li adsorbed on Cu(111) show a loss peak associated with Li vibrations perpendicular to the substrate. The loss energy shifts from 38 meV at low coverage to 43 meV at 0.3 ML and remains constant for coverages between 0.3 and 0.5 ML. The loss intensity passes a maximum at a Li coverage of 0.15 ML and gradually decreases such that it is difficult to resolve a loss peak at coverages above 0.5 ML. The high loss energy indicates that the adatom resides on the surfaces rather than in substitutional sites. The frequency shift is much too large to be explained by dipole-dipole interactions. The above results are obtained with the evaporation source loaded with the natural Li isotope mixture (92.6% 7Li, 7.4% 6Li). Measurements with 6Li show that the increase of the vibration frequency with increasing coverage is not an isotope effect