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We discuss the theory of extrinsic contributions to the linewidth of ultrathin ferromagnetic films, magnetized parallel to their surfaces. We invoke defect-induced two magnon scatterings, rendered active by unique aspects of dipolar contributions to spin wave dispersion in two dimensions. A two magnon shift in resonance frequency is present as well. We summarize our principal earlier conclusions, and present new results on the in-plane anisotropy of linewidth, for samples with stepped surfaces.

The problem of dynamic symmetric branching of a tensile crack propagating in a brittle material is studied within Linear Elastic Fracture Mechanics theory. The Griffith energy criterion and the principle of local symmetry provide necessary conditions for the onset of dynamic branching instability and for the subsequent paths of the branches. The theory predicts a critical velocity for branching and a well defined shape parameterized by a branching angle and a curvature of the side branches. The model rests on a scenario of crack branching based on reasonable assumptions and on exact dynamic results for the anti-plane branching problem. Our results reproduce within a simplified 2D continuum mechanics approach the main experimental features of the branching instability of fast cracks in brittle materials.