Theoretical Studies of Dipolar and Exchange Effects in Creating Magnetic Rogue Waves

Abstract

Magnetic rogue waves, like ocean rogue waves, are large deviations from equilibrium which are localized in time and space. They can be created in thin magnetic films by a time-reversal process consisting of a recording step and a reconstruction step. During the recording step, an initial rogue wave is allowed to decay and the microwave fields at various sites are recorded. In reconstruction step, these fields are played backwards in time at the recording sites, ultimately recreating the initial rogue wave as seen in Fig. 1. This technique gives the ability to control the magnitude and position of the rogue waves, which may make them suitable for application in spin wave logic devices and for encryption and secure communications. We study the efficiency of constructing rogue waves. For a Permalloy film which is 30 nm thick, the peak to noise ratio (PNR) of the reconstructed rogue waves can vary by a factor of 5 depending on the location of sensors with respect to the magnetic field. This is caused by the emission of caustic beams as the initial wave evolves in time. The best location is with the sensors perpendicular to the applied field. We also study the relative importance of dipolar and exchange coupling in creating rogue waves. We explore a geometry of a square magnetic film (side length 300 nm) separated from a surrounding magnetic rim by a gap of 10 nm. This gap prevents exchange coupling between the two regions. The initial rogue wave, a small square (25 nm on a side) where the magnetization is raised out of plane by a maximum of 2 degrees, is established in the inner region, but all the recording sensors are in the outer region. It is clear in Fig. 2 that the system in the middle of the recording phase is dominated by short wavelength modes, where exchange is important. However, in the playback, it is the long wavelength dipolar fields which are important in reconstructing the rogue wave.