Theory of interlayer exchange interactions in magnetic multilayers

This paper presents a review of the phenomenon of interlayer exchange coupling in magnetic multilayers. The emphasis is put on a pedagogical presentation of the mechanism of the phenomenon, which has been successfully explained in terms of a spin-dependent quantum confinement effect. The theoretical predictions are discussed in connection with corresponding experimental investigations.Comment: 18 pages, 4 PS figures, LaTeX with IOP package; v2: ref. added. Further (p)reprints available from http://www.mpi-halle.de/~theory

The circumstellar envelope of the C-rich post-AGB star HD 56126

We present a detailed study of the circumstellar envelope of the post-asymptotic giant branch ``21 micron object'' HD 56126. We build a detailed dust radiative transfer model of the circumstellar envelope in order to derive the dust composition and mass, and the mass-loss history of the star. To model the emission of the dust we use amorphous carbon, hydrogenated amorphous carbon, magnesium sulfide and titanium carbide. We present a detailed parametrisation of the optical properties of hydrogenated amorphous carbon as a function of H/C content. The mid-infrared imaging and spectroscopy is best reproduced by a single dust shell from 1.2 to 2.6 arcsec radius around the central star. This shell originates from a short period during which the mass-loss rate exceeded 10^(-4) M_sun/yr. We find that the strength of the ``21'' micron feature poses a problem for the TiC identification. The low abundance of Ti requires very high absorption cross-sections in the ultraviolet and visible wavelength range to explain the strength of the feature. Other nano-crystalline metal carbides should be considered as well. We find that hydrogenated amorphous carbon in radiative equilibrium with the local radiation field does not reach a high enough temperature to explain the strength of the 3.3-3.4 and 6-9 micron hydrocarbon features relative to the 11-17 micron hydrocarbon features. We propose that the carriers of these hydrocarbon features are not in radiative equilibrium but are transiently heated to high temperature. We find that 2 per cent of the dust mass is required to explain the strength of the ``30'' micron feature, which fits well within the measured atmospheric abundance of Mg and S. This further strengthens the MgS identification of the ``30'' micron feature.Comment: 20 Pages, 10 Figures, accepted for publication in Astronomy and Astrophysic

Theoretical prediction and experimental evidence of a novel oscillatory behaviour of interlayer magnetic coupling

The interlayer magnetic coupling between ferromagnetic layers separated by a non-magnetic spacer layer is investigated with emphasis on the role of the non-magnetic overlayer. By using a recently proposed theory of interlayer coupling, which expresses the coupling in terms of quantum interferences due to electron confinement, it is predicted that the coupling oscillates vs. overlayer thickness, with a period related to the Fermi surface of the latter. This prediction has been confirmed experimentally on Au/Co/Au/Co/Au(111) films, with an oscillation period vs. Au overlayer thickness of 5 atomic layers, in good agreement with the predicted value (4.8 atomic layers).

Modes of Hydrogen Incorporation in Hydrogenated Amorphous Carbon (a–C:H), Modifications with Annealing Temperature

In order to obtain more information about the nature of the hydrogen bonding and the thermal stability of a-C:H films, we have studied two different types of films labeled I and II deposited in a d.c. multipolar plasma system from pure methane at substrate bias equal to $-40$ and $-600$ V respectively. A combination of several complementary techniques has been carried out on the samples in their as-deposited state as well as after isochronal annealing cycles at increasing temperatures up to 650 $^{\circ}$C. The results clearly indicate that the two types of samples have very different H bonding and microstructure in their as-deposited state. They also behave differently upon annealing and still exhibit quite different microstructures and H bonding configurations up to high annealing temperature