The magnetization reversal processes are discussed for exchangecoupled ferromagnetic hard/soft bilayers made from Co 0.66 Cr 0.22 Pt 0.12 (10 and 20 nm)/Ni (from 0 to 40 nm) films with out-of-plane and in-plane magnetic easy axes respectively, based on room temperature hysteresis loops and first-order reversal curve analysis. On increasing the Ni layer thicknesses, the easy axis of the bilayer reorients from out-of-plane to in-plane. An exchange bias effect, 2 consisting of a shift of the in-plane minor hysteresis loops along the field axis, was observed at room temperature after in-plane saturation. This effect was associated with specific ferromagnetic domain configurations experimentally determined by polarized neutron reflectivity. On the other hand, perpendicular exchange bias effect was revealed from the out-of-plane hysteresis loops and it was attributed to residual domains in the magnetically hard layer.
A detailed investigation on the ion kinetic energy distributions of ions ejected in the nanosecond pulsed laser ablation of aluminum is reported. For laser fluences just over threshold, the emerging ions fit shifted neat Maxwell-Boltzmann-Coulomb (MBC) distributions. For fluences higher than ∼1.3 J/cm 2 , the Al + distributions split into two MBC contributions peaked at different energies. It is demonstrated that the observed Al + ion distribution has two components, one fast, correlated with the direct multiphoton laser ionization, and the other slow, associated with electron-Al 0 collisions in the solid. A similar behavior is observed at higher fluences for all Al ion distributions indicating that the electron-impact ionization of Al rate constants is faster than that of recombination and other possible collision channels. In addition, the linear relationship between the Coulomb velocities and the ion charges and the behavior of Coulomb energy of the ions versus the laser fluence suggest the appearance of an electric field within the metal/laser interaction volume that impels the ions up to the high velocities measured. A discussion of the application of this type of mechanisms to other metals is advanced.
Velocity and kinetic energy distributions ͑VDs, KEDs͒ of metal ions generated by nanosecond ͑ns͒ pulsed laser ablation under high vacuum have been determined using an electrostatic analyzer plus time-of-flight coupled system. A number of metal and alloy targets, principally involving Fe and Ni, have been studied at different laser fluences. At low fluence, the ion distributions have been shown to fit single Maxwell-Boltzmann-Coulomb ͑MBC͒ distributions; for medium and higher fluences, each ion distribution is found to comprise that of the surviving "precursor" ion, itself, overlapped with sidebands which arise from ion-electron recombination and/or ionization. The so-called surviving "precursor" ion of a distribution is that which underwent no change of charge. The Coulomb velocities of the surviving "precursor" ions and those of the ion products resulting from ion-electron collisions have been compared. Ion velocities are correlated with the local electric field resulting from ejection of the photoelectrons following laser ablation. Under identical conditions of laser fluence, the ions are seen to experience an electrical field nearly independent of their charge. The transit times of ions in the plasma have been estimated to be of the order of 1 ps. An overall quantitative mechanism for metal ablation on this basis is presented, including the ejection time for photoelectrons and differences in ion distributions resulting from employing laser pulses in the nanosecond ͑ns͒ and femtosecond ͑fs͒ regimes.
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