  Manipulation of magnetization with ultrashort laser pulses is promising for
information storage device applications. The dynamic of the magnetization
response depends on the energy transfer from the photons to the spins during
the initial laser excitation. A material of special interest for magnetic
storage is FePt nanoparticles , on which optical writing with optical angular
momentum was demonstrated recently by Lambert et al., although the mechanism
remained unclear. Here we investigate experimentally and theoretically the
all-optical switching of FePt nanoparticles. We show that the magnetization
switching is a stochastic process. We develop a complete multiscale model which
allows us to optimize the number of laser shots needed to write the
magnetization of high anisotropy FePt nanoparticles in our experiments. We
conclude that only angular momentum induced optically by the inverse Faraday
effect will provide switching with one single femtosecond laser pulse.
