Message #31065

[HereForNow]

Why It Can Work!

Astronomical observations and astrophysical theory tell us that the solar system formed by the collapse of a vast molecular cloud. A portion of the cloud collapsed into the primitive Sun surrounded by a disk of gas and dust. Planets, asteroids, and comets formed in this disk. One of the most exciting discoveries made by meteoriticists in NASA's Cosmochemistry Program is that meteorites contain tiny grains that once inhabited the interstellar cloud. These grains, called presolar grains or stardust, are typically only a few micrometers in diameter. They survived cloud collapse and heating in the accretion disk surrounding the nascent Sun. These relicts from stars give us a close-up look at the grains that inhabit interstellar space and offer a highly informative complement to astronomical observations of stars and interstellar clouds. They are minuscule bits of stars available for close study.

Evidence that the grains are presolar comes from the relative abundances of the isotopes of common elements, such as silicon, oxygen, and carbon. The abundances of the isotopes differ from all samples of typical solar system material as found on the planets, asteroids (as sampled by meteorites), and comets. The aberrant isotopic compositions are caused by nuclear reactions in dying and exploding stars. All the isotopes of elements other than hydrogen and helium are synthesized by nuclear reactions in the interiors of stars. The isotopes are expelled into interstellar space by stellar winds or monumental explosions. Many condense into dust grains. These products of the life and death of stars mixed into the cloud from which the Sun developed, forming the raw materials for the solar system. Thus, the solar system is a mixture of materials from countless stars. During the formation of the solar system, most of the material was homogenized, giving the normal solar system isotopic compositions for the elements. A small percentage of grains escaped homogenization, giving us a window into the nature of stellar evolution and interstellar clouds.

Now....

The next generation of materials for magnetic recording media will require i) the design of regular arrays of ferromagnetic nanoparticles with well controlled morphology and behaviour and ii) to physically separate these particles, either by vacuum or by a nonmagnetic material to fully discriminate the bits of information. Other requirements are the chemical stability, the obtention of a definite direction for magnetization, the mecanical stiffness… The growth of carbon nanotubes by a catalytic CVD process requires the presence of nanoscaled transition metals particles (Fe, Co, Ni), which are ferromagnetic. These particles are encapsulated after growth on the top of the nanotubes. Moreover in appropriate deposition conditions they took a very anisotropic shape, and the particle fullfills the nanotube which is a nonmagnetic material. Various other carbon nanostructures can be grown (nanocones, carbon nanofibers, …) depending on the experimental CVD and the metallic dispersion parameters. The magnetic properties of these arrays of Co nanoparticles encapsulated into carbon were investigated by SQUID and MFM. Different magnetic behaviour were evidenced - Superparamagnetic behaviour for small particles (5 à 8 nm) encapsulated into non oriented nanotubes - Strong magnetic anisotropy in the plan of the substrate for nanoparticles (~30 nm) encapsulated at the top of nanocones, due to an exchange coupling between a metallic core and a thin antiferromagnetic CoO external layer. - Strong magnetic anisotropy perpendicular to the plan of the substrate of metallic nanowires (diameter~25 nm and aspect ratio from 1/4 to 1/10), induced by the cork-like shape of the nanoparticles. In this case the coercitive field (750 Oe), the magnetic anisotropy combined with high density (1010particles/cm2) and the weak dipolar interactions evidenced by MFM are very attractive for dense storage media.