IPMAGNA · Imaging the Plasmonic Activity of Magnetic Nanostructures

Imaging the Plasmonic Activity of Magnetic Nanostructures

This project has been focused on the study of nanostructures that exhibit magneto-optic (MO) properties and localised surface plasmon resonances (LSPRs), constituting what is known as magnetoplasmonic systems. The typical magnetoplasmonic nanostructure is composed of a smart combination of a noble metal (such as gold or silver) with a ferromagnetic metal (such as iron, cobalt or nickel). The main novelty of our approach was using magnetic force microscopy (MFM) measurements to be performed upon illumination: the illuminated light would excite the LSPRs and the magnetic component of their electromagnetic field distribution would be detected by its interaction with the probe of the MFM equipment. Two kinds of nanostructures have been studied: nanodisks and nanoholes. The nanostructure arrays have been fabricated either as initial continuous thin films that were subsequently patterned using lithography and etching, or by lithography, evaporation and lift-off. In any case, the material deposition was performed in an ultrahigh vacuum chamber equipped with magnetron sputtering and thermal evaporation. By tuning the composition and dimensions of the nanostructures, we were able to tune the wavelength of the LSPRs with that of the laser used in the MFM experiments, as checked by optical extinction spectroscopy. There has been a big effort to improve the experimental technique chosen to detect the magnetic component of the LSPRs, i.e. MFM upon illumination. The first step consisted on building up an optical excitation setup in the vicinity of the sample stage. Then, optical microstructured masks were used to illuminate only the nanostructure arrays, since there was an undesired interference between the excitation laser and the laser used in the MFM equipment to detect the tip-sample interaction. Moreover, modulated excitation and lock-in detection was used to enhance the signal-to-noise ratio: the excitation light passed through an acousto-optic modulator at 50 kHz and the detection was tuned to the same frequency. In fact, several activities have been done to improve the MFM detection technique. On the one hand, we have bought commercial atomic force microscope (AFM) probes (i.e. silicon cantilevers with a pyramidal tip at the end, typically used for AFM) and such probes have been customised in two ways: 1) We have tailored the resonance frequency of the cantilevers by removing silicon using a focused ion beam (FIB); this helped us to shift it near 50 kHz and consequently improve the signal Q times, Q being the quality factor of the resonance. 2) We have made a selective magnetic layer deposition using magnetron sputtering: this allowed us for finely tuning the magnetic moment of the tip. Finally, a new detection mode has been explored: instead of the standard flexural mode of the cantilevers, we have also used the first torsional resonance (TR) mode. This TR-MFM technique not only improved about 20 % the lateral resolution but also allowed for decreasing the tip-sample distance to only 2 nm. However, in spite of all these experimental developments, we haven't been able to obtain a reliable magnetic signature associated to the LSPRs of the nanostructures. We are now convinced that strong laser power is needed to achieve such phenomenon. The scientific activities carried out within this project have been summarised in one article already published in Nanotechnology (impact factor: 3.979) and another two papers that will be submitted in the near future, as well as in 9 communications in scientific conferences. We think that the TR-MFM technique can have a significant impact not only in academic research but also in industry, since MFM is frequently used to check magnetic storage media.

Data: CORDIS, © European Union