Investigations of Lines in Domain Walls of Orthoferrites

Research project P 13846 Investigations of Lines in Domain Walls of Orthoferrites Hans HAUSER 11.10.1999 Bloch lines in ferromagnets are localized perturbations that retain their structure during the motion and represent examples of topological magnetic solitons. Their properties are being investigated intensively both experimentally and theoretically. Most of these investigations are performed on garnet films of various compositions. However, another kind of stable topological formations was predicted many years ago: in weak ferromagnets a resultant magnetization, according to symmetry rules, cannot be perpendicular to the plane of magnetization rotation in the domain wall (to this class belong orthoferrites, materials that are highly transparent in the near infrared range crystals and display very strong uniaxial magnetic anisotropy). Consequently, in lines separating parts of different handedness within a wall, the magnetization vector instead of rotation decreases in magnitude and passes through zero. For many years the very existence of such lines of zero magnetization (ZMLs) was questionable. In a recent experimental breakthrough, we obtained first dark field images of these lines in orthoferrites. In our photographs, boundaries between sections of different brightness represent lines of zero magnetization and are observed both in statics and in dynamics. The lines move under action of magnetic field pulses and experience displacements that persist after the acting magnetic field pulse. These results represent only a first step in the investigation of ZMLs and our proposed studies, including investigations of their generation, motion, interactions, should provide information on this new kind of the magnetic solitons. Domain walls are another concrete example of topological magnetic solitons. In our study we intend to investigate interrelations between these two kinds of solitons, first of all the influence of the lines on the domain wall dynamics. We have found nonlinearities in the domain wall dynamics in the subsonic range of the domain wall velocities, contradicting conventional expectations. We assume that these nonlinearities as well as those found earlier at supersonic velocities are associated with ZMLs and in our proposed work we intend to tackle this problem as well. In particular we want to study the dynamic creation of ZMLs and corresponding changes in the domain wall motion. Results of this study could be of interest also for practical applications. Domain widths in orthoferrites greatly exceed those in garnets, and for a long time, this was recognized as a fatal drawback of orthoferrites for their use in storage devices. But there are numerous applications (in optical communications and measurement techniques) where wide domains actually may prove to be an essential advantage. Most of the mentioned applications are based on the extremely fast domain wall motion in orthoferrites and need precise understanding of the domain wall dynamics. We also plan to contribute to the interpretation of the experimental results, taking advantage of a collaboration with a theoretical group actively promoting new studies of DW dynamics in weak ferromagnets and topological magnetic solitons. New approaches recently allowed to find surprising results, including a prediction of dynamic transformation of Bloch domain walls into Neel walls; this effect is related to a bifurcation of domain wall mobilities. We believe that it will be possible to extend these studies to include interactions of a moving wall with quasi-particles, as well as to develop a qualitative description of ZML-solitons in weak ferromagnets.