There are electron-spin based quantum computation proposals that employ ferromagnet/semiconductor heterostructures in order to implement local magnetic field for single qubit operations. We examine how a ferromagnetic layer affects the coherent electron spin dynamics in a neighboring GaAs layer in ferromagnet/semiconductor heterostructures grown by molecular beam epitaxy (MBE).

Using time resolved Faraday rotation (TRFR), we observed that the spin dynamics are unexpectedly dominated by hyperpolarized nuclear spins that align along the ferromagnet's magnetization. The photoexcited carriers acquire spin-polarization from the ferromagnet, and dynamically polarize these nuclear spins. The resulting hyperfine fields are as high as 9000 gauss in small external fields, enabling ferromagnetic control of local electron spin coherence.

This figure shows TRFR scans at 5K with an applied field of 1000G for four different ferromagnetic layers. The top trace is for a control sample that does not have any ferromagnetic layer. Single layer refers to 1/2 monolayer (ML) of MnAs, "5 layers' to 5 periods of 1/2ML MnAs with GaAs spacings, "GaMnAs" sample has Mn concentration of 5%. The presence of an ferromagnetic layer strongly modifies the coherent electron spin precession.

Figure below shows how TRFR changes as a function of field for the single layer sample at 5K. As we scan the field, we see a hysteretic behavior, and a sharp change in TRFR appears at the coercive field of ferromagnetic layer.

When we fit the data and extract the effective field, we find a "dip' near zero field. In order to explain this dip (and other behaviors) we propose a model where nuclear spins are mediating the effect. Due to dipole-dipole interactions, nuclear spins cannot polarize near zero field. If the nuclei are mediating the effective field, this dip can be explained.

To check the validity of this "imprinting' on nuclear spins, we performed all-optical NMR on this sample. Using photo-elastic modulator to create an effective AC magnetic field for nuclear spins, we see peaks that correspond to Ga and As isotopes.

To learn more about our studies, please refer to 'Ferromagnetic Imprinting of Nuclear Spins in Semiconductors", R. K. Kawakami, Y. Kato, M. Hanson, I. Malajovich, J. M. Stephens, E. Johnston-Halperin, G. Salis, A. C. Gossard, and D. D. Awschalom, Science, vol. 294 , p.131 (2001)