The optical pump-probe technique is a common tool for investigation of
ultrafast spin dynamics, which usually utilizes single-diode detection
averaging the dynamics over the pumped area. Using ultrafast imaging technique,
we show experimentally that a femtosecond laser pulse with a gradient
distribution of intensity efficiently excites strongly inhomogeneous spin
dynamics on spatial scales much smaller than the pump spot size. The mechanism
responsible for the inhomogeneous distribution is based on temperature
gradients and corresponds to a sign change of the torque derivative in
different areas of the pump. We argue that the observed phenomenon is general
for the systems with competitive magnetic anisotropies. Overlooking this effect
in the majority of pump-probe experiments may result in a dramatic
underestimation of the live time and amplitude of the laser-induced spin
dynamics.

Cet article explore les excursions dans le temps et leurs implications.

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