  Ultra-thin magnetic dielectric films are of prime importance due to their
applications for nanophotonics and spintronics. Here, we propose an efficient
method for the magneto-optical investigation of ultra-thin magnetic films which
allows one to access their state of magnetization and magneto-optical
properties. It is based on the surface-plasmon-polariton-assisted transverse
magneto-optical Kerr effect (TMOKE). In our experiments, sub-100 nm-thick
bismuth-substituted lutetium iron-garnet films covered with a plasmonic gold
grating have been analyzed. The excitation of surface plasmon-polaritons
provides resonance enhancement of TMOKE up to 0.04 and makes it easily
detectable in the experiment. For films thicker than 40 nm, the TMOKE
marginally depends on the film thickness. A further decrease in the film
thickness diminishes TMOKE since for such thicknesses the surface
plasmon-polariton field partly penetrates inside the non-magnetic substrate.
Nevertheless, the TMOKE remains measurable even for few-nm-thick films, which
makes this technique unique for the magneto-optical study of ultra-thin films.
Particularly, the proposed method reveals that the off-diagonal components of
the magnetic film permittivity tensor grow slightly with the reduction of the
film thickness.
