A. T. G. Pym

Interface sharpening in CoFeB magnetic tunnel junctions

We report grazing incidence x-ray scattering evidence for sharpening of the interface between amorphous Co60Fe20B20 and AlOx during in situ annealing below the Co60Fe20B20 crystallization temperature. Enhancement of the interference fringe amplitude in the specular scatter and the absence of changes in the diffuse scatter indicate that the sharpening is not a reduction in topological roughness but a reduction in the width of the chemical composition profile across the interface. The temperature at which the sharpening occurs corresponds to that at which a maximum is found in the tunneling magnetoresistance of magnetic tunnel junctions.

Investigation of ultrafast demagnetization and cubic optical nonlinearity of Ni in the polar geometry

Femtosecond optical pump-probe experiments were performed upon a Ni(720 A)/Si(100) sample in the polar geometry with the pump beam close to normal incidence. A signal due to the ultrafast demagnetization effect was observed when the pump pulse was linearly polarized. When the pump was elliptically polarized, additional peaks were observed at zero time delay, resulting from the specular inverse Faraday effect (SIFE) and the specular optical Kerr effect (SOKE). By comparing measurements made with different pump helicities, the SIFE and SOKE peaks and the demagnetization signal were found to superpose in a linear fashion. From the dependence of the peak height upon the pump polarization, values of χxxyy=(1−3i)×10−10 rad cm3 erg−1 and χxyyx=(−9+2i)×10−12 rad cm3 erg−1 were deduced for the nonvanishing components of the local cubic susceptibility tensor. For applied fields less than the saturation value, the sudden reduction of the thin film demagnetizing field leads to an imbalance of the torques acting upon ...

Ultrafast third-order optical nonlinearity of noble and transition metal thin films

We present transient ellipsometry measurements made upon Au, Cu, Ag, Ni, Pd, Ti, Zr, and Hf thin films under identical experimental conditions. Using an elliptically polarized pump beam, we have simultaneously observed the specular inverse Faraday effect (SIFE) and specular optical Kerr effect (SOKE) contributions to the optical polarization response, in order to extract the real and imaginary parts of the non-vanishing components of the third-order optical susceptibility tensor. The signal magnitudes and the extracted tensor components show a systematic variation that reflects the underlying band structure of the different metals. A time delay observed between the SIFE and SOKE signals is interpreted in terms of lifetimes for the transient polarization that may be of comparable magnitude to the pulse width. This suggests that formulae derived in the continuous wave limit may not be applicable in the description of ultrafast nonlinearities in metallic samples. The implications of our results for different areas of applied optics are discussed.

The in-plane length scale of the conformal interface roughness as a function of bilayer repeat number in Co/Pd multilayers

Grazing incidence small angle x-ray scattering has been used to probe the in-plane length scale and scaling behaviour of the conformal interface roughness of sputtered Co/Pd multilayers. Scaling was seen in the intensity distribution through the Bragg sheets for large out-of-plane wavevectors. The scaling exponent showed power law variation with multilayer repeat number, the extreme values being close to those predicted by the Kardar–Parisi–Zhang and Tang–Alexander–Bruinsma models of film growth. The wavevector at which scaling broke down increased linearly with repeat number. A power law increase as a function of bilayer repeat number was found in the in-plane correlation length defined by fitting the scattering in the incidence plane to the Sinha model. Both approaches showed that the multilayer interfaces tended towards a two-dimensional character as the number of repeats increased.

Layer and interface structural changes in Co0.6Fe0.2B0.2/AlOx multilayers on annealing.

Sputtered multilayers of Co0.6Fe0.2B0.2 and AlOx have been measured using grazing incidence x-ray scattering to determine the changes in layer and interface structure during in situ annealing. We confirm our earlier deduction of a sharpening of the interfaces on annealing up to 400 °C. This sharpening arises from reduction in chemical intermixing, not from change in topological roughness and provides an explanation for the enhanced tunneling magnetoresistance. The annealing is shown to result in a decrease in Co0.6Fe0.2B0.2 mean layer thickness and increase in the layer density. There is a commensurate increase in AlOx mean layer thickness while at the same time there is a reduction in the layer density. The increase in thickness could explain the corresponding increase observed in the resistance-area product.