Dynamics and factors controlling regularity of thermomagnetically written domains

Abstract

The reproducibility of thermomagnetically written domains in rare earth–transition metal thin films was studied and as a result films capable of supporting very regular domains were fabricated. To understand the origins of the domains’ size and shape variations, we first studied the nucleation and growth of domains by using a high-speed magneto-optical sampling camera having a 10-ns exposure time. It is observed that the domain is nucleated within 20 ns after the leading edge of the write pulse. The domain grows in size until 50–150 ns after termination of the write pulse, then shrinks, and finally reaches an equilibrium size. For very short write pulses, no shrinkage of the domain is observed after termination of the write pulse. It is found that increased laser power or longer laser pulse duration increases the final domain size, but not in proportion to total energy applied. The domain growth rate varies from film to film, indicating it is influenced by the intrinsic magnetic parameters. The reproducibility and regularity of the final stable domains were then studied as a function of writing and magnetic parameters. Although both influence the regularity of nucleated domains, the latter have been found to have more profound influences than the former. Films with compensation temperature above ambient temperature, larger perpendicular anisotropy constant, smaller magnitude of magnetization, and smaller domain aspect ratio nucleate domains with a higher degree of reproducibility. Higher coercivity films also support more regular domains. Photographs of domains nucleated with different parameters demonstrate these trends. The experimental observations can be qualitatively explained by analyzing the stiffness of the domain wall defining the domains. The wall stiffness is determined by the two competing forces derived from wall energy and demagnetizing energy. When wall energy dominates, the stiffness is large and domains tend to be circular and highly reproducible. When demagnetizing energy dominates, wall stiffness is low and domains become irregular and less reproducible in shape.