Dynamic Domains in Strongly Driven Ferromagnetic Films

The structures occurring in a transversely pumped, dissipative ferromagnetic film are investigated. The dynamics of the magnetization density is modelled using the Landau-Lifshitz equation. The model includes the exchange field, easy-axis anistropy and the demagnetizing part of the dipole field. In addition a saturating static field normal to the film and a resonant pump field in the plane of the film are applied. The investigation is carried out both by numerical and analytical methods. Two types of solution are found. Firstly, the homogeneous solution, where the magnetization rotates uniformly throughout the film at the frequency of the pump field. Secondly, dynamic domains are found. In the reference frame rotating with the driving field, dynamic domains have a stationary domain structure, like that known from static domains, yet in the lab frame their position is stationary while the magnetization within the domains rotates at different angles. The homogeneous solution is computed analytically and its stability investigated by means of a complete linear stability analysis. The features of the dynamic domain solution far from the domain wall are discussed and it is seen that dynamic domains merge into one another as the amplitude of the pump field is increased, eventually returning to the homogeneous solution. However, knowledge of the solution far from the wall does not suffice to determine whether the wall between these two domains is moving (an unstable solution) or stationary (a stable solution). By an expansion around the limiting case of no damping, the structure of the wall is determined, for both planar and cylindrical domain walls. Now it is known that static 180-degree domain walls will travel through a ferromagnetic probe at a velocity proportional to the total internal magnetic field in the easy-axis direction (Walker solution). Yet we are able to show that application of a transverse pump field at an amplitude above a certain critical threshold can call a halt to this travelling motion and allow stationary dynamic domains to form in the probe