Strongly Inhomogeneous Spin Dynamics Induced by Ultrashort Laser Pulses with a Gradient Intensity Profile

The optical pump-probe technique is a common tool for the investigation of ultrafast spin dynamics, which usually utilizes single-diode detection averaging the dynamics over the pumped area. Using an 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 can result in a dramatic underestimate of the lifetime and amplitude of the laser-induced spin dynamics.