Direct optical probing of ultrafast spin dynamics in a magnetic semiconductor

We uncovered the spin dynamics involved in the birth and growth of giant spin polarons in a magnetic semiconductor. For this purpose, we developed a measurement technique which provides direct access to the spin dynamics, irrespective of phonons and carriers involved in the process. Moreover, we solved the Landau-Lifshitz equation in the specific scenario of spin polarons; this solution fits our data excellently and demonstrates that the spin polaron growth slows down dramatically when the sample is cooled in the paramagnetic phase. Finally, temperature-dependent Monte Carlo simulations were performed; these are in excellent agreement with the observed slowdown, which demonstrates that fluctuations in the Weiss field play a decisive role in spin coherence generation induced by light in magnetic materials.