  The quest to improve density, speed and energy efficiency of magnetic memory
storage has led to exploration of new ways of optically manipulating magnetism
at the ultrafast time scale, in particular in ferrimagnetic alloys. While
all-optical magnetization switching is well-established on the femtosecond
timescale, lateral nanoscale confinement and thus potential significant
reduction of the size of the magnetic element remains an outstanding challenge.
Here we employ resonant electromagnetic energy-funneling plasmon nanoantennas
to influence the demagnetization dynamics of a ferrimagnetic TbCo alloy thin
film. We demonstrate how Ag nanoring-shaped antennas under resonant optical
femtosecond pumping reduce the overall magneto-optical response due to
demagnetization in the underlying films up to three times compared to
non-resonant illumination. We attribute such substantial reduction to the
nanoscale confinement of the demagnetization process. This is qualitatively
supported by the electromagnetic simulations that strongly evidence the optical
energy-funneling to the nanoscale from the nanoantennas into the ferrimagnetic
film. This is the first and defining step for reaching deterministic ultrafast
all-optical magnetization switching at the nanoscale in such systems, opening a
route to develop nanoscale ultrafast magneto-optics.
