All-Optical Studies of Ultrafast Magnetization Dynamics

All-optical techniques exploiting femtosecond laser pulses have opened the way towards the exploration of the ultimate limits of magnetization dynamics. Although part of the interest is curiosity driven, a better understanding of the dynamic behavior of spin systems is considered of extreme relevance for future progress in (high data rate) magnetic recording and spintronic applications as well. In this presentation, the application of pulsed laser strategies to spin dynamical phenomena will be introduced. After a general introduction of the magneto-optical Kerr effect, "all-optical" experimental pump-probe schemes will be explained, and compared to other techniques to probe fs magnetization dynamics. In particular, I will address the question: "What is it that we measure in such time-resolved magneto-optical experiments? Is it magnetism or optics?". It will be shown that extreme care has to be taken when interpreting experiments at the sub-ps time scales.Moreover, it will be demonstrated that a short laser pulse triggers a variety of processes, each of which allows to access specific spin-dependent dynamics in the ferromagnet. During the first hundreds of femtoseconds, the response can be dominated by band filling effects, which affects the MO response even while conserving the net magnetic moment. When taking care, a genuine and ultrafast demagnetization causing a (partial) loss of the magnetization within a few hundred femtoseconds, can be demonstrated unambiguously. On a much longer time scale, a coherent GHz precession of the magnetization vector is observed, launched by the sudden magnetic perturbation right after excitation.