Researchers have uncovered unusual magneto-optical properties of magnetic nanostructures composed of Co-Pt nanolayers on top of a surface covered with either gold or silver nanoparticles. Based on their results, the researchers from the Akita Industrial Technology Center and the Chiba Institute of Technology in Japan propose a new type of magneto-plasmon sensor for chemical and biological sensing applications, such as bio-chemical imaging, surface-enhanced Raman scattering, and fluorescence observations. This device is anticipated to show improved sensitivity and detection limits compared to more conventional plasmon sensors. The study is published in a recent issue of the Journal of Applied Physics. 

Magneto-optic (MO) effects appear when light interacts with magnetic materials—for example, when changes in the magnetic field of a medium through which the light is transmitted influence light propagation. One of the most important MO phenomena is the magneto-optic Kerr effect (MOKE), which occurs when the plane of polarization of a light beam rotates during reflection from a magnetic material. Because the rotation is proportional to the magnetization of the material, MOKE experiments are popular for characterizing magnetic properties in nanostructures.

For this study, the samples under investigation consisted of a magnetic Co₈₀Pt₂₀ nanolayer with a hexagonal close-packed (hcp) crystal structure, covering/on top of nonmagnetic gold (Au) or silver (Ag) nanoparticles. An important feature of the new nanocomposite material was that the magnetic moments of the Co-Pt layers were aligned perpendicular to the plane of the layers.

“Co-Pt-based films have been used as perpendicular magnetic recording media for commercial HDD systems and the Ag and Au nanoparticles are known to be plasmonic materials,” says Haruki Yamane, first author of the article and research officer at the Akita Industrial Technology Center. Plasmons, coherent electron oscillations on the surface of a metal, can significantly enhance optical sensing signals and can also improve the MO activities on magnetic nanostructures, according to Yamane.

To build the CoPt–Ag and CoPt–Au nanostructures, the researchers fabricated the noble metal fine-grained structures by first depositing a 100 nm Ru underlayer on a 10-nm-thick ZnO seed layer. An extra 3-nm-thick ZnO intermediate layer was then added and a Ag or Au nanofilm was deposited on it. The nanograins were fabricated by thermal annealing of the nonmagnetic nanofilm. Finally sputtering was used to deposit a Co-Pt/ZnO layer (around 5 nm thick) on the noble metal grains

The size and shape of the grains were controlled by varying the thickness of the films in a range from 5 nm to 30 nm for Ag and from 3 nm to 15 nm for Au, with the annealing temperature ranging from 200°C to 500°C. The ZnO intermediate layer also helped to obtain fine-grained structures by reducing the size and increasing the packing density of the grains while improving the perpendicular magnetic properties of the CoPt layers. The surface agglomeration and the forming of fine grains was more effective for Ag than for Au. The Ru underlayer enhanced the magneto-plasmonic activities of the CoPt layer formed on the Ag grains.

The nanostructured material was then investigated under polar Kerr measurement conditions, an optical configuration suitable for observing the chemical and biological reactions at the sensor surface.

“This and our previous work are the first studies in which the polarization of the light is reversed by the localized plasmon resonance of the NM [nonmagnetic] nanoparticles, neighboring the magnetic material,” Yamane says. What the researchers observed was the unusual phenomenon in which, instead of having the MO properties (the shape of the MO hysteresis Kerr loop for example) monotonically changed by the applied magnetic field, they saw them increase at a low magnetic field.

“This work seems to give a new path for controlling the magneto-optical activities of perpendicular magnetic nanostructures, taking advantage of localized surface plasmon resonances, and is paving the way for new perspectives in chemical and biological sensing applications,” says Aristi Christofi, a postdoctoral researcher in the Physics Department of the National and Kapodistrian University of Athens, Greece, who did not participate in the study. “The new type of chemical sensor, that the authors propose, allows the realization of a new generation of magneto-plasmonic sensors,” Christofi says, “and we are really looking forward to seeing a prototype.”

Read the abstract in the Journal of Applied Physics.