Nanograins of noble metals influence magneto-optical properties of CoPt films

Materials that demonstrate magneto-optical effects have inherent magnetism that influences the behavior of light reflecting off of them. These materials can change the amplitude of polarized light, rotate the plane of polarized light, or transform linearly polarized light into elliptically polarized light. These magneto-optical properties have been used to create high density data storage disks, isolators for telecommunications, and chemical sensors.

Materials that combine magnetic transition metals and noble metals have enhanced magneto-optical effects due to interactions with plasmons — coherent surface electron-density oscillations — from the noble metals. Yamane et al. sputter a perpendicular magnetic Co₈₀Pt₂₀ nanolayer on silver or gold nanostructured films and observe a curious magneto-optical behavior due to localized surface plasmons.

To make the material, the authors first sputter a thin film of silver or gold on a 100-nanometer-thick metal underlayer. Next, they thermally anneal and form noble metal nanoparticles on the underlayer. They control the surface morphology by varying the as-deposited thicknesses of silver and gold films and the annealing temperature. Finally, they sputter a 5-nanometer CoPt film on the nanostructured films. The team measures the magneto-optical properties under a polar Kerr configuration, in which the incident light is directly irradiated along the surface-normal.

The localized surface plasmons from the nanoparticles produce the phase reversal on the magneto-optical effect of CoPt. This magneto-plasmonic phenomenon strongly depends on the size and material of nanoparticles and on the external environment.

The authors further propose a new parameter for chemical and biological sensors. The perpendicular magneto-plasmonic materials could be used to make simpler and more stable sensors for biochemical imaging and surface-enhanced fluorescence or Raman spectroscopy.

Source: “Magneto-plasmonics on perpendicular magnetic nanostructures consisting of a CoPt layer and noble-metal grains,” by H. Yamane, K. Takeda, Y. Isaji, Y. Yasukawa, and M. Kobayashi, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5036983 .