Fluctuations and Instabilities of Ferromagnetic Domain Wall pairs in an External Magnetic Field

Soliton excitations and their stability in anisotropic quasi-1D ferromagnets are analyzed analytically. In the presence of an external magnetic field, the lowest lying topological excitations are shown to be either soliton-soliton or soliton-antisoliton pairs. In ferromagnetic samples of macro- or mesoscopic size, these configurations correspond to twisted or untwisted pairs of Bloch walls. It is shown that the fluctuations around these configurations are governed by the same set of operators. The soliton-antisoliton pair has exactly one unstable mode and thus represents a critical nucleus for thermally activated magnetization reversal in effectively one-dimensional systems. The soliton-soliton pair is stable for small external fields but becomes unstable for large magnetic fields. From the detailed expression of this instability threshold and an analysis of nonlocal demagnetizing effects it is shown that the relative chirality of domain walls can be detected experimentally in thin ferromagnetic films. The static properties of the present model are equivalent to those of a nonlinear sigma-model with anisotropies. In the limit of large hard-axis anisotropy the model reduces to a double sine-Gordon model.Comment: 15 pages RevTex 3.0 (twocolumn), 9 figures available on request, to appear in Phys Rev B, Dec (1994

ON COMPARISONS BETWEEN COMPUTED AND OBSERVED GRAIN BOUNDARY STRUCTURES AND PROPERTIES IN METALS

A detailed assessment is given of the extent to which agreement has been obtained between experimentally observed and calculated structures and/or properties of grain boundaries in metals. All of the calculations employed pair-wise interaction models. Published work as well as new results obtained by the authors are cited. Topics include : boundary "width" ; primary and secondary relaxations (i.e., grain boundary dislocations) ; magnitude of the boundary energy ; lattice translations across the boundary and atomic positions in the boundary core ; and fast diffusion in boundaries. It is concluded in general that the pair-potential modeling has been a valuable tool in grain boundary research and has often produced detailed results which are in at least semi-quantitative agreement with observations. However, in some cases the agreement is poor and, as might be expected, it is clear that the technique is not capable of producing realistic results in all cases. Various problems are pointed out and discussed

Recognizing the Visual Focus of Attention for Human Robot Interaction

Sheikhi S, Odobez J-M. Recognizing the Visual Focus of Attention for Human Robot Interaction. In: Salah AA, Ruiz-del-Solar J, Meriçli Ç, Oudeyer P-Y, eds. Human Behavior Understanding. Lecture Notes in Computer Science. Vol 7559. Berlin, Heidelberg: Springer Berlin Heidelberg; 2012: 99-112.We address the recognition of people’s visual focus of attention (VFOA), the discrete version of gaze that indicates who is looking at whom or what. As a good indicator of addressee-hood (who speaks to whom, and in particular is a person speaking to the robot) and of people’s interest, VFOA is an important cue for supporting dialog modelling in Human-Robot interactions involving multiple persons. In absence of high definition images, we rely on people’s head pose to recognize the VFOA. Rather than assuming a fixed mapping between head pose directions and gaze target directions, we investigate models that perform a dynamic (temporal) mapping implicitly accounting for varying body/shoulder orientations of a person over time, as well as unsupervised adaptation. Evaluated on a public dataset and on data recorded with the humanoid robot Nao, the method exhibit better adaptivity and versatility producing equal or better performance than a state-of-the-art approach, while the proposed unsupervised adaptation does not improve results