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Alteration of intrinsic properties of non-magnetized metals using light
Assistant Professor Justin Song from NTU’s School of Physical and Mathematical Sciences and Associate Professor Mark Rudner form Niels Bohr Institute published in Nature Physics a method that in principle could turn non-ferrous metals into magnets using laser light.

The properties of many materials are conventionally thought to be fixed, determined by the arrangement of its atoms at the nanoscale. For example, the configuration of atoms in a material dictates whether it conducts electricity easily or has non-conductive behaviour.

Song and Rudner wanted to explore how plasmons - local oscillations of charge in metals – and the intense oscillating electric fields they create, can be used to alter material properties.

Like how light consists of photons, the plasma oscillation consists of plasmons, a type of quasiparticle. Plasmons tend to oscillate and move in the same direction as the field that is driving it.

However, the scientists found that when the light irradiation is strong enough, the plasmons in a non-magnetic metallic disk can spontaneously rotate in either a left-handed or right-handed fashion, even when driven by linearly polarised light.

“This was a signature that the material’s intrinsic properties had been altered,” said Asst Prof Song. “We found that when a plasmon’s strong internal fields modify a material’s electronic band structure it would also transform the plasmon as well, setting up a feedback loop enabling the plasmon to spontaneously exhibit a chirality.”

This chiral motion of the plasmon produced a magnetisation which then made the non-magnetic metallic disk of their scheme, magnetic.

The scientists say that the key observation in their theoretical analysis is that intense plasmonic oscillating electric fields can modify the dynamics of the electrons in the metal.

Assoc Prof Rudner said: “From the point of view of an electron within a material, an electric field is an electric field: it doesn’t matter whether this oscillating field was produced from plasmons within the material itself or by a laser shining on the material.”

Song and Rudner used this insight to theoretically demonstrate the conditions when feedback from the internal fields of the plasmons could trigger an instability towards spontaneous magnetisation in the system. The team expects that this theoretical approach could be realised in a range of high quality plasmonic materials such as graphene.

Reference:

Mark S. Rudner & Justin C. W. Song (2019) Self-induced Berry flux and spontaneous non-equilibrium magnetism. Nature Physics

 

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