Terahertz spin precession and coherent transfer of angular momenta in magnetic quantum wells

Femtosecond-resolved measurements of induced Faraday rotation reveal exchange coupling between the spin angular momenta of charge carriers and a sublattice of magnetic ions in semiconductor quantum wells. In transverse magnetic fields, we observe terahertz spin precession of photoinjected electrons, rapid spin relaxation of holes, and the coherent transfer of angular momentum to the magnetic sublattice via the ultrafast rotation of the local moments. The perturbed ions undergo free-induction decay, enabling time-domain all-optical electron-spin-resonance measurements in single magnetic planes.