Magnetic and electronic properties of bimagnetic materials comprising cobalt particles within hollow silica decorated with magnetite nanoparticles

Bimagnetic materials were fabricated by decorating the external surface of rattle-type hollow silica microspheres (which themselves contain metallic cobalt nanoparticles) with magnetite nanoparticles; thus, each magnetic substance was spatially isolated by the silica shell. The amount of magnetite decoration on the co-occluded hollow silica was varied from 1 to 17 mass %. Magnetic and electronic properties of the resulting bimagnetic materials were characterized by superconducting quantum interference device measurements and X-ray absorption spectroscopy, respectively. The ferrous iron in the bimagnetic sample was slightly more oxidized than in the magnetite reference, probably from some charge-transfer because of the SiO2 surface contact, although the overall oxidation state of the samples is very similar to that of magnetite. The temperature dependence of the sample magnetization recorded with Zero Field Cooling and Field Cooling resulted in blocking temperatures for the bimagnetic materials that were close to that of magnetite nanoparticles (176K) and were lower than that for the bare Co-occluded hollow silica (which was above room temperature). Values of coercive force and exchange bias at 300K became quite small after decoration with only minimal amounts of magnetite nanoparticles (1-3 mass %) and were lower than those of magnetite. This is the first example of enhancing superparamagnetism by spatial separation of both Co and magnetite magnetic nanoparticles using a thin wall of diamagnetic silica.ArticleJOURNAL OF APPLIED PHYSICS. 114(12):124304 (2013)journal articl

Structure and Evolution of Nearby Stars with Planets. I. Short-Period Systems

Using the Yale stellar evolution code, we have calculated theoretical models for nearby stars with planetary-mass companions in short-period nearly circular orbits: 51 Pegasi, Tau Bootis, Upsilon Andromedae, Rho Cancri, and Rho Coronae Borealis. We present tables listing key stellar parameters such as mass, radius, age, and size of the convective envelope as a function of the observable parameters (luminosity, effective temperature, and metallicity), as well as the unknown helium fraction. For each star we construct best models based on recently published spectroscopic data and the present understanding of galactic chemical evolution. We discuss our results in the context of planet formation theory, and, in particular, tidal dissipation effects and stellar metallicity enhancements.Comment: 48 pages including 13 tables and 5 figures, to appear in Ap