Stability of magnetic vortex in soft magnetic nano-sized circular cylinder

Stability of magnetic vortex with respect to displacement of its center in a nano-scale circular cylinder made of soft ferromagnetic material is studied theoretically. The mode of vortex displacement producing no magnetic charges on the cylinder side is proposed and the corresponding absolute single-domain radius of the cylinder is calculated as a function of its thickness and the exchange length of the material. In cylinders with the radii less than the single-domain radius the vortex state is unstable and is absolutely prohibited (except if pinned by material imperfections), so that the distribution of the magnetization vector in such cylinders in no applied magnetic field is uniform (or quasi-uniform). The phase diagram of nano-scale cylinders including the stability line and the metastability region obtained here is presented.Comment: 3 pages, 2 figures, RevTex 4, presented at JEMS'01, accepted to JMM

Thermal Evolution and Light Curves of Young Bare Strange Stars

The cooling of a young bare strange star is studied numerically by solving the equations of energy conservation and heat transport for both normal and superconducting strange quark matter inside the star. We show that the thermal luminosity from the strange star surface, due to both photon emission and e+e- pair production, may be orders of magnitude higher than the Eddington limit, for about one day for normal quark matter but possibly for up to a hundred years for superconducting quark matter, while the maximum of the photon spectrum is in hard X-rays with a mean energy of ~ 100 keV or even more. This differs both qualitatively and quantitatively from the photon emission from young neutron stars and provides a definite observational signature for bare strange stars. It is shown that the energy gap of superconducting strange quark matter may be estimated from the light curves if it is in the range from ~ 0.5 MeV to a few MeV.Comment: Ref [10] added and abstract shortened. 4 pages, 3 figures, revtex4. To be published in Phys. Rev. Letter

Magnetars and Gamma Ray Bursts

In the last few years, evidences for a long-lived and sustained engine in Gamma Ray Bursts (GRBs) have increased the attention to the so called millisecond-magnetar model, as a competitive alternative to the standard collapsar scenario. I will review here the key aspects of the {\it millisecond magnetar} model for Long Duration Gamma Ray Bursts (LGRBs). I will briefly describe what constraints, present observations put on any engine model, both in term of energetic, outflow properties, and the relation with the associated Supernova (SN). For each of these I will show how the millisecond magnetar model satisfies the requirements, what are the limits of the model, how can it be further tested, and what observations might be used to discriminate against it. I will also discuss numerical results that show the importance of the confinement by the progenitor star in explaining the formation of a collimated outflow, how a detailed model for the evolution of the central engine can be built, and show that a wide variety of explosive events can be explained by different magnetar parameters. I will conclude with a suggestion that magnetars might be at the origin of the Extended Emission (EE) observed in a significant fraction of Short GRBs.Comment: 8 pages; to appear in Proceedings of IAU 279 "Death of Massive Stars: Supernovae and Gamma-ray Bursts

On the possible observational manifestation of supernova shock impact on the neutron star magnetosphere

Impact of supernova explosion on the neutron star magnetosphere in a massive binary system is considered. The supernova shock striking the NS magnetosphere filled with plasma can lead to the formation of a magnetospheric tail with significant magnetic energy. The magnetic field reconnection in the current sheet formed can convert the magnetic energy stored in the tail into kinetic energy of accelerated charged particles. Plasma instabilities excited by beams of relativistic particles can lead to the formation of a short pulse of coherent radio emission with parameters similar to those of the observed bright extragalactic millisecond radio burst (Lorimer et al. 2007).Comment: 8 pages, Astron. Lett. in pres

Magnetic penetration-depth measurements of a suppressed superfluid density of superconducting Ca0.5_{0.5}Na0.5_{0.5}Fe2_2As2_2 single crystals by proton irradiation

We report on the dramatic effect of random point defects, produced by proton irradiation, on the superfluid density $\rho_{s}$ in superconducting Ca$_{0.5}$Na$_{0.5}$Fe$_2$As$_2$ single crystals. The magnitude of the suppression is inferred from measurements of the temperature-dependent magnetic penetration depth $\lambda(T)$ using magnetic force microscopy. Our findings indicate that a radiation dose of 2$\times10^{16}$cm$^{-2}$ produced by 3 MeV protons results in a reduction of the superconducting critical temperature $T_{c}$ by approximately 10%. % with no appreciable change in the slope of the upper critical fields. In contrast, $\rho_{s}(0)$ is suppressed by approximately 60%. This break-down of the Abrikosov-Gorkov theory may be explained by the so-called "Swiss cheese model", which accounts for the spatial suppression of the order parameter near point defects similar to holes in Swiss cheese. Both the slope of the upper critical field and the penetration depth $\lambda(T/T_{c})/\lambda(0)$ exhibit similar temperature dependences before and after irradiation. This may be due to a combination of the highly disordered nature of Ca$_{0.5}$Na$_{0.5}$Fe$_2$As$_2$ with large intraband and simultaneous interband scattering as well as the $s^\pm$-wave nature of short coherence length superconductivity

Induced scattering of short radio pulses

Effect of the induced Compton and Raman scattering on short, bright radio pulses is investigated. It is shown that when a single pulse propagates through the scattering medium, the effective optical depth is determined by the duration of the pulse but not by the scale of the medium. The induced scattering could hinder propagation of the radio pulse only if close enough to the source a dense enough plasma is presented. The induced scattering within the relativistically moving source places lower limits on the Lorentz factor of the source. The results are applied to the recently discovered short extragalactic radio pulse.Comment: submitted to Ap

Evolution and stability of a magnetic vortex in small cylindrical ferromagnetic particle under applied field

The energy of a displaced magnetic vortex in a cylindrical particle made of isotropic ferromagnetic material (magnetic dot) is calculated taking into account the magnetic dipolar and the exchange interactions. Under the simplifying assumption of small dot thickness the closed-form expressions for the dot energy is written in a non-perturbative way as a function of the coordinate of the vortex center. Then, the process of losing the stability of the vortex under the influence of the externally applied magnetic field is considered. The field destabilizing the vortex as well as the field when the vortex energy is equal to the energy of a uniformly magnetized state are calculated and presented as a function of dot geometry. The results (containing no adjustable parameters) are compared to the recent experiment and are in good agreement.Comment: 4 pages, 2 figures, RevTe