Strain dependence of the acoustic properties of amorphous metals below 1K: Evidence for the interaction between tunneling states

We have conducted a thorough study of the acoustic properties between 10^-4 and 1 Kelvin for the amorphous metal Zr_x Cu_1-x (x=0.3 and x=0.4), by measuring the relative change of sound velocity dv/v and internal friction Q^-1 as a function of temperature and also of the applied strain, in both superconducting and normal state. We have found that when plotted versus the ratio of strain energy to thermal energy, all measurements display the same behavior: a crossover from a linear regime of ``independent'' tunneling systems at very low strains and/or high enough temperatures to a nonlinear regime where dv/v and Q^-1 depend on applied strain and the tunneling systems cannot be considered as independent.Comment: 4 pages, 4 figures (submitted to PRL

Anisotropy, hysteresis, and morphology of self-patterned epitaxial Fe/MgO/GaAs films

The dependence of the effective anisotropy and of the magnetization reversal processes on the deposition temperature of a series of epitaxial Fe/MgO/GaAs films, grown by a combination of laser ablation and sputtering, has been discussed and correlated to their local morphology and crystallinity. The transverse susceptibility measurements have evidenced the presence of a fourfold effective anisotropy for all films, irrespective of their deposition temperature and local morphology, with a constant value very close to that of single-crystalline iron. The magnetization processes evolve with increasing deposition temperature from a nucleation-expansion-propagation mechanism, taking place in smooth continuous films, to pinning, occurring in self-patterned films with homogeneous distributions of defects, and then to fully isotropic reversal, for tiled films composed of rectangular elements.The authors thank E. Navarro for the AFM measurements and the Spanish CICyT for financial support.Peer reviewe