  By ultrafast x-ray diffraction we show that the laser-induced
magnetostructural phase transition in FeRh nanoislands proceeds faster and more
complete than in continuous films. We observe an intrinsic 8 ps timescale for
nucleation of ferromagnetic (FM) domains in both types of samples. For the
continuous film, the substrate-near regions, which are not directly exposed to
light, are only slowly transformed to the FM state by domain wall motion
following heat transport. In contrast, numerical modeling of the plasmonic
absorption in the investigated nanostructure reveals a strong contribution near
the FeRh/MgO interface. On average, the absorption is larger and more
homogeneous in the nanoislands, enabling the phase transition throughout the
entire volume at the intrinsic nucleation timescale.
