D. Scholl

Ultraviolet photoablation of a plasma-synthesized fluorocarbon polymer

Plasma polymerized tetrafluoroethylene (PPTFE) is shown to undergo efficient 248 nm excimer laser ablation. The principle difference between this material and the analogous polytetrafluoroethylene (PTFE), which results in only poor quality ablation, is PPTFEs much greater absorption coefficient (7×104 vs. ∼102 cm−1). A plot of the ablation depth per pulse versus incident fluence indicates that the threshold for significant ablation occurs near 50 mJ/cm2, and that approximately 0.7 μm/pulse can be removed at 800 mJ/cm2. Near threshold, the ablation rate curve can be fit by a single Arrhenius-type exponential. This suggests that the removal process is at least partially governed by a photothermal process, similar to well-known laser induced thermal desorption experiments. In the very low fluence regime between 10 and 30 mJ/cm2, small removal rates are measured in a process likely dominated by non-thermal ablation. The paper concludes with a discussion of the high quality, micron-size features that can be directly patterned into PPTFE surfaces.

Magnetism in very thin films of permalloy measured by spin polarized cascade electrons

The spin polarizationP of the low energy cascade electrons excited with a primary unpolarized electron beam is measured with ultrathin films of permalloy (Ni80Fe20) as a function of film thickness, external magnetic field, and temperatureT. Surface adsorbates of small concentrations of less than 10% of a monolayer can change the Curie point and the saturation value ofP0(T→0) by as much as 30%. The Ta-substrate induces a magnetically dead region in permalloy. Conventional spin wave theory cannot account for the observed smallT-dependence of the magnetizationM. Films on a nonmagnetic substrate are compared to similar films coupled to bulk permalloy over an interface of Ta. TheT-dependence ofM with the coupled films can be explained by spin wave theory. At lowT, the films coupled to the bulk exhibit a faster decrease ofM than the uncoupled films. We propose that this thermal stabilization of the magnetization in very thin ferromagnetic films is due to quenching of the long wavelength spin modes.

Effects of film growth mode on magnetic exchange coupling in trilayer structures (abstract)

In situ temperature‐dependent spin‐polarized cascade electron spectroscopy is used to study exchange coupling processes between a magnetic substrate and a magnetic overlayer separated by a nonferromagnetic spacer layer. Relative Auger intensities of the overlayer/substrate configurations as a function of overlayer coverage together with a model by Ossicini are used to evaluate the degree to which these trilayer structures grow by a continuous layer growth mode. Measurement of the spin polarization and Auger spectroscopy are used during film growth as criteria of chemical film purity. Evidence will be given to show that the oscillatory magnetic coupling behavior as a function of Cr thickness of a NiFe/Cr/NiFe trilayer changes significantly depending on the deposition temperature, which may be associated with microstructural changes. Competing coupling mechanisms are thought to account for this behavior. Data will also be presented to suggest how trace quantities of residual gas sorbates, presumably acting ...