| Using femtosecond lasers and ultrafast optical techniques, we have investigated the time-evolution of the spins of electrons, holes, and excitons which are optically injected into magnetic semiconductor quantum wells. Here, the spins of the mobile electronic carriers can directly couple to the spins of the local magnetic moments (Mn{dollar}sp{lcub}2+{rcub}){dollar} present in the quantum structure, leading to new channels for spin relaxation, decoherence, and angular momentum transfer. Timescales, strengths, and physical manifestations of these dynamic spin-spin interactions are measured in real-time with femtosecond resolution using a method of ultrafast Faraday rotation.; Model systems in which to study the interaction of electronic spins with embedded local moments are realized in a new class of "digital" magnetic heterostructures: II-VI ZnSe/ZnCdSe single quantum wells containing discrete mono- and submonolayer planes of MnSe. Strong coupling between excitons and local moments is observed, resulting in large effective exciton g-factors (g {dollar}sim{dollar} 500) and enhanced Faraday rotation. The fractional planes of magnetic material can be considered nearly ideal 2-D spin distributions, and the statistics of Mn spin clustering in the 2D planes is studied through photoluminescence Zeeman shifts in high magnetic fields (30T).; In longitudinal applied magnetic fields (Faraday geometry), the monotonic exciton spin relaxation is rapid ({dollar}<{dollar}5ps) and found to depend solely on the magnitude of the exciton Zeeman splitting, regardless of the particular digital magnetic environment. No longlived spin-dependent imprint on the magnetic sublattice is measured. By contrast, in transverse magnetic fields the electron spins are found to precess at THz frequencies, enabling measurement of the electron spin decoherence time separate from the spin relaxation of the holes. Furthermore, the data indicate that the embedded Mn{dollar}sp{lcub}2+{rcub}{dollar} sublattice undergoes an ultrafast coherent rotation about the transient exchange field of the spin polarized holes. The perturbed Mn{dollar}sp{lcub}2+{rcub}{dollar} spin ensemble subsequently undergoes a measurable free-induction decay, permitting all-optical time-domain electron paramagnetic resonance studies of fractional-monolayer magnetic planes. |
