Invited Paper MI-FrM1
High-Efficiency Spin Injection through the Depleted Edge of a Magnetic Semiconductor

Friday, October 19, 2007, 8:00 am, Room 619

Dilute ferromagnetic semiconductors are composed of magnetic dopants (such as Mn) that interact strongly with each other through a host nonmagnetic semiconductor (such as GaAs) over distances of order one nanometer to establish the ferromagnetic state. The interaction is mediated by holes, which at low concentrations are bound to the dopants and at high concentrations become mobile. Theoretical and experimental studies of the Curie temperature and carrier spin polarization of Ga_1-xMn_xAs find them to depend strongly on the hole density, and a local mean-field theory has been developed that quantitatively accounts for many of the bulk properties of these materials in terms of the mean hole density. However, the properties near the edges of magnetic semiconductors, where the carrier concentration and dopant concentration are changing rapidly over the interaction's length scale of a nanometer, cannot be accounted for within a local mean-field theory. A theory of magnetic interactions in the highly depleted regime has been built on the foundation of a quantitatively-accurate theory of the interaction energy of a single pair of widely-separated Mn dopants in GaAs. Predictions from this theory of the interaction between Mn dopants have been confirmed by experimental measurements via scanning tunneling microscopy. This theory also provides a new explanation of the origin of the unusual magnetic anisotropies in strained low-doped (even insulating) ferromagnetic Ga_1-xMn_xAs. The resulting theory for the edges of a magnetic semiconductor suggests that the carrier spin polarization at those edges should be much larger than in the bulk of the material, and may even approach 100%. Measurements of carrier transport across highly-depleted Ga_1-xMn_xAs suggest that these very high spin polarizations are real, and that they may provide an alternate pathway to nearly 100% efficient spin injection.