  Study of the formation and evolution of large scale, ordered structures is an
enduring theme in science. The generation, evolution and control of large sized
magnetic domains are intriguing and challenging tasks, given the complex nature
of competing interactions present in any magnetic system. Here, we demonstrate
large scale non-coplanar ordering of spins, driven by picosecond, megagauss
magnetic pulses derived from a high intensity, femtosecond laser. Our studies
on a specially designed Yttrium Iron Garnet (YIG)/dielectric/metal film
sandwich target, show the creation of complex, large, concentric, elliptical
shaped magnetic domains which resemble the layered shell structure of an onion.
The largest shell has a major axis of over hundreds of micrometers, in stark
contrast to conventional sub micrometer scale polygonal, striped or bubble
shaped magnetic domains found in magnetic materials, or the large dumbbell
shaped domains produced in magnetic films irradiated with accelerator based
relativistic electron beams. Through micromagnetic simulations, we show that
the giant magnetic field pulses create ultrafast terahertz (THz) spin waves. A
snapshot of these fast propagating spin waves is stored as the layered onion
shell shaped domains in the YIG film. Typically, information transport via spin
waves in magnonic devices occurs in the gigahertz (GHz) regime, where the
devices are susceptible to thermal disturbances at room temperature. Our
intense laser light pulse - YIG sandwich target combination, paves the way for
room temperature table-top THz spin wave devices, which operate just above or
in the range of the thermal noise floor. This dissipation-less device offers
ultrafast control of spin information over distances of few hundreds of
microns.
