Ferromagnetic Imprinting of Nuclear Spins in Semiconductors Roland Kawakami Department of Physics U. of California, Santa Barbara, CA Optically-generated electron spins in semiconductors show remarkable resilience against environmental decoherence, making it possible to envision a new class of magnetoelectronics based on the coherent superposition of quantum spin states. To examine how a ferromagnetic layer may be used to control coherent electron spins in a neighboring GaAs semiconductor, we fabricated a series of hybrid magnetic/GaAs structures using the following ferromagnets: (Ga,Mn)As, MnAs, digital (Ga,Mn)As [1], and a single layer of Mn delta-doping. Ultrafast optical pump-probe measurements on these samples reveal that the dynamics of coherent electron spins in the GaAs layer are strongly affected by the ferromagnet, but not through fringe fields or direct exchange interactions. Unexpectedly, the ferromagnet causes nuclear spins in the GaAs layer to become hyperpolarized (~17%) and align with the magnetization [2]. These polarized nuclei, in turn, generate large effective magnetic fields on the coherent electron spins through the hyperfine interactionas high as 9000 G in small external fields (< 1000 G)thereby leading to electron spin precession in the GaAs layer that is dominated by interactions with nuclei. Thus, ferromagnetic control of electron spin coherence is achieved by 'imprinting' nuclear spins in the GaAs layer.