Spin reorientation has been observed in CoFe 2 O 4 thin single crystalline films epitaxially grown on ͑100͒ MgO substrate upon varying the film thickness. The critical thickness for such a spin-reorientation transition was estimated to be 300 nm. The reorientation is driven by a structural transition in the film from a tetragonal to cubic symmetry. At low thickness, the in-plane tensile stress induces a tetragonal distortion of the lattice that generates a perpendicular anisotropy, large enough to overcome the shape anisotropy and to stabilize the magnetization easy axis out of plane. However, in thicker films, the lattice relaxation toward the cubic structure of the bulk allows the shape anisotropy to force the magnetization to be in plane aligned. The importance of magnetic anisotropy is well recognized in many technical applications such as magnetic and magneto-optic recording. The large interest for high anisotropies is motivated by technological demands such as increasing the magnetic recording density. With large anisotropy, the superparamagnetic limit can be pushed down, and a stable magnetization can be promoted in ultrasmall nanosized magnetic structures, which are needed in advanced media for ultrahigh density recording. Besides the intrinsic anisotropy of the bulk, other sources of anisotropy may be enhanced in artificial structures and contribute to their magnetic properties. Depending on their relative orientations and magnitudes, the involved anisotropies may compete between each other, leading to spin-reorientation phenomena in the system. For example, the broken symmetry at the interfaces in ultrathin films generates a perpendicular anisotropy, which overcomes the shape anisotropy.1 However, increasing the layer thickness reduces the ratio between the surface and the volume atoms, leading to an in-plane alignment of the easy axis.2 In obliquely sputtered metallic thin films, we established the existence of an in-plane reorientation of magnetic anisotropy.3 Depending on the film thickness and due to the shadow effect during the growth, the layer can develop columns or nuclei able to confine the anisotropy parallel or perpendicular to the longitudinal direction ͑projection of the incident beam in the film plane͒.Ferrites cover a large family of oxides, including soft as well as hard magnetic materials. Hard ferrites such as the hexagonal ͑BaFe 12 O 19 ͒ and the spinel ͑CoFe 2 O 4 ͒ are particularly attractive for magnetic and magneto-optic recording applications due to their large magnetocrystalline anisotropy and high chemical stability. Recent studies demonstrated that integrating cobalt ferrite as a pinning layer in the spin valve architecture can strongly enhance the magnetoresistance effect of the sandwiched structure. 4 In epitaxial hexaferrite thin films, the uniaxial magnetocrystalline anisotropy is strong enough to dominate all the other sources of anisotropy and to keep the spin alignment constant regardless the film thickness and the preparation conditions. 5 However, cobalt ferrite rep...

The carbon in ancient carbonaceous chondritic meteorites is mainly in a hydrocarbon composite similar to terrestrial kerogen, a cross-linked structure of aliphatic and aromatic hydrocarbons. Until recently, the composite has been commonly thought to have been produced in the early solar nebula by a Fischer-Tropsch-type process, involving the catalytic synthesis of hydrocarbons from carbon monoxide and hydrogen on grain surfaces. Instead, the aromatic hydrocarbons may form in gas-phase pyrolysis of simple aliphatics like acetylene and methane by a mechanism developed recently to explain formation of soot in combustion and of aromatic molecules in circumstellar envelopes. Nonequilibrium chemical kinetic calculations indicate that this mechanism can produce meteoritic aromatics if the initial concentration of simple hydrocarbons in the solar nebula was sufficiently but not unreasonably high.

We present results of analyses of observations of the X-ray source 2A 1704ϩ241 with the ROSAT Position Sensitive Proportional Counter (PSPC) and the High Resolution Imager (HRI). The source 2A 1704ϩ241 was first associated with the M giant star HD 154791 based on observations with the HEAO 1 scanning modulation collimator and the Einstein IPC and analysis of a spectrum of HD 154791 obtained with the International Ultraviolet Explorer. This identification was unusual because there are few bright X-ray binaries associated with an M giant star. We observed 2A 1704ϩ241 with the PSPC and the HRI in order to determine more accurately the position of the X-ray source and in order to study the previously seen 900 s variability in the Einstein data. Based on the previous identification and determination of the position of MS 1703.7ϩ2417, an active galactic nucleus in the field, and the position of three previously unreported X-ray sources that we have associated with stars in the USNO-A2.0 catalog, we have greatly reduced the X-ray positional error of 2A 1704ϩ241. HD 154791 remains the prime candidate as the optical counterpart of the X-ray source. While the 50% modulation in the X-ray flux seen by the Einstein IPC is apparent in the ROSAT data, it appears to be at a slightly different frequency.

Reorientation of magnetic anisotropy has been observed in single-crystal CoFe 2 O 4 thin films deposited on (100) MgO substrate by pulsed laser deposition (PLD). The as-grown film exhibits a perpendicular anisotropy whereas after annealing the magnetization easy axis switches to be parallel to the film plane. The origin of such spin reorientation is explained in terms of competition between stress and magnetocrystalline anisotropies. The as-prepared film is under tensile stress, which induces a huge perpendicular uniaxial anisotropy dominating the in-plane magnetocrystalline component. Annealing releases the stress by relaxing the film lattice. Consequently, the perpendicular stress anisotropy is considerably reduced and magnetocrystalline anisotropy prevails, leading to an in-plane alignment of the easy axis.

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