Spin engineering in magnetic quantum well structures

Molecular beam epitaxy grown heterostructures are used to engineer spin interactions between carriers in semiconductor quantum wells and magnetic ions. In contrast to the spin-orbit and hyperfine interactions, the exchange couplings present in magnetically doped semiconductors are orders of magnitude larger in energy. The confinement afforded in quantum wells is used to enhance, localize, and control the exchange interactions both extrinsically and intrinsically. Ultrafast optical spectroscopies monitor the spin dynamics of electrons and local magnetic moments confined to the wells revealing the effects of the exchange interaction on the spin polarization, lifetimes, and precession frequencies.; In parabolically-graded (ZnCdSe) magnetic quantum wells, a gate bias shifts the confinement potential minimum, thus changing the effective exchange coupling. In these structures, both electron and magnetic ion spin dynamics are electrically controlled at close to THz frequencies with spin interactions in the meV scale. A gate bias is again used to alter the exchange coupling in (GaAs) quantum wells with a ferromagnetic barrier. The wave function overlap between spins in a ferromagnetic layer and carrier spins in a quantum well is electrically controlled, thus providing a tunable spin polarization at low magnetic fields. Lastly, quantum confinement is used to engineer the intrinsic conduction band exchange coupling in GaMnAs and InGaMnAs. The magnitude of the exchange coupling increases with confinement, which is systematically altered by adjusting the well width, barrier height, and doping density. These measurements reveal an antiferromagnetic exchange coupling in the conduction band that is increased by one-dimensional confinement. From emission spectroscopies we observe the effect of exchange coupling on the valence band and the dynamics of magnetic ion spins.; Our results demonstrate carrier spin polarization and manipulation in the solid state using the exchange interaction. Electrons and holes are confined at nanometer length scales along one direction and forced to interact with magnetic ions doped within or nearby these wells. The engineering of exchange couplings in quantum wells may offer a pathway for robust spintronic devices at practical magnetic fields and temperatures.