A team of researchers at the Tata Institute of Fundamental Research, Mumbai and the Indian Institute of Technology, Kanpur have used extremely strong magnetic pulses to line up spins in a magnetic film on a scale never achieved before.

They demonstrate beautiful concentric circular patterns of spins as large as hundreds of micrometers. The natural scale for such patterns is typically sub-micrometre. Creation of such large scale ordered spin structures is potentially useful for electronic devices in the terahertz frequency range.

How is this achieved? The team used a high intensity, femtosecond laser to create a hot, dense plasma on a solid surface which in turn generates the giant magnetic pulse. The magnetic film, made of yttrium iron garnet (YIG) is hosted in a clever design to obviate the damaging effects of the plasma and is exposed to the pulse.

The induced spin patterns are analysed by magneto-optical microscopy. In a big surprise, these onion ring shape structures are found to be very robust and stay 'arrested' as long as ten days!

How do we understand such large scale spin formations? YIG is a 'soft' magnetic material and micromagnetic simulations show that the giant magnetic field pulse creates ultrafast, terahertz (THz) spin waves in the film. A snapshot of these fast-propagating spin waves (magnons) is stored as layered onion shell shaped domains in the YIG film.

Typically, information transport via spin waves in magnonic devices occurs in the gigahertz regime, where devices are susceptible to thermal disturbances at room temperature. The intense laser light pulse - YIG sandwich target combination, paves the way for room temperature table-top THz spin wave devices, operating 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.

The study of patterns and symmetries is an enduring theme in science and our quest for understanding natural patterns can be tremendously aided if we can create ordered structures on a scale that is not naturally found. This study is a step in that direction.

Research Report: "Macroscopic, layered onion shell like magnetic domain structure generated in YIG films using ultrashort, megagauss magnetic pulses"

Related Links
Tata Institute Of Fundamental Research
Understanding Time and Space

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