Abstract
The design and fabrication of a novel plasmonic cavity, intended to allow far-field recovery of signals arising from near field magneto-optical interactions, is presented. Finite element modeling is used to describe the interaction between a gold film, containing cross-shaped cavities, with a nearby magnetic under-layer. The modeling revealed strong electric field confinement near the center of the cross structure for certain optical wavelengths, which may be tuned by varying the length of the cross through a range that is compatible with available fabrication techniques. Furthermore, the magneto optical Kerr effect (MOKE) response of the composite structure can be enhanced with respect to that of the bare magnetic film. To confirm these findings, cavities were milled within gold films deposited upon a soluble film, allowing relocation to a ferromagnetic film using a float transfer technique. Cross cavity arrays were fabricated and characterized by optical transmission spectroscopy prior to floating, revealing resonances at optical wavelengths in good agreement with the finite element modeling. Following transfer to the magnetic film, circular test apertures within the gold film yielded clear magneto-optical signals even for diameters within the sub-wavelength regime. However, no magneto-optical signal was observed for the cross cavity arrays, since the FIB milling process was found to produce nanotube structures within the soluble under-layer that adhered to the gold. Further optimization of the fabrication process should allow recovery of magneto-optical signal from cross cavity structures.
Highlights
As magnetic hard disk drive capacities increase and the realisation of practical spintronic devices is demonstrated,[1,2] there is an urgent need to observe dynamic magnetic phenomena with increased spatial resolution
Deep nanoscale spatial resolution can be obtained in wide-field or scanning transmission measurements, or else x-ray magnetic circular dichroism (XMCD) can be combined with the imaging capability of photoemission electron microscopy (PEEM),[5] or holography techniques.[6]
Representative magneto optical Kerr effect (MOKE) hysteresis loops obtained from the circular apertures are shown in Figure 12, with the full set presented in the supplementary material SM 3 Figure 7
Summary
As magnetic hard disk drive capacities increase and the realisation of practical spintronic devices is demonstrated,[1,2] there is an urgent need to observe dynamic magnetic phenomena with increased spatial resolution. 055207-5 Loughran et al Figure 4 shows the field distribution for the 260 nm arm length model at the three resonant wavelengths. For the the middle wavelength resonance the electric field is strongly localized near the center of the cross on the dielectric side, where the magnetic layer is located, and its amplitude is enhanced in this region.
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