  The development of spintronic technology with increasingly dense, high-speed,
and complex devices will be accelerated by accessible microscopy techniques
capable of probing magnetic phenomena on picosecond time scales and at deeply
sub-micron length scales. A recently developed time-resolved magneto-thermal
microscope provides a path towards this goal if it is augmented with a
picosecond, nanoscale heat source. We theoretically study adiabatic
nanofocusing and near-field heat induction using conical gold plasmonic
antennas to generate sub-100 nm thermal gradients for time-resolved
magneto-thermal imaging. Finite element calculations of antenna-sample
interactions reveal focused electromagnetic loss profiles that are either
peaked directly under the antenna or are annular, depending on the sample's
conductivity, the antenna's apex radius, and the tip-sample separation. We find
that the thermal gradient is confined to 40 nm to 60 nm full width at half
maximum for realistic ranges of sample conductivity and apex radius. To
mitigate this variation, which is undesirable for microscopy, we investigate
the use of a platinum capping layer on top of the sample as a thermal
transduction layer to produce heat uniformly across different sample materials.
After determining the optimal capping layer thickness, we simulate the
evolution of the thermal gradient in the underlying sample layer, and find that
the temporal width is below 10 ps. These results lay a theoretical foundation
for nanoscale, time-resolved magneto-thermal imaging.
