Electron spins in semiconductors are prime candidates for quantum information storage and processing because they exhibit relatively long coherence times. To realize this goal, electronic and optical properties of heterostructures are designed to efficiently generate, transport, manipulate, and detect electron spins. A powerful tool for designing spin interactions in semiconductors involves magnetic doping, since magnetic ion and band electron spins may be strongly coupled. These exchange effects are often thousands of times larger than spin-orbit and hyperfine interactions in solely non-magnetic structures. Using nonequilibrium growth by molecular beam epitaxy, it is possible to realize metastable phases of magnetic ions alloyed with optoelectronic semiconductors. For example, Mn-doped GaAs remarkably combines semiconductivity with ferromagnetism, exhibiting a magnetic transition temperature (Tc) that is electrically tunable due to the carrier-mediated nature of the ferromagnetism. Although Tc has improved over the last ten years, defects inherent to nonequilibrium growth remain, limiting magnetic and optoelectronic quality. By exploring the phase diagram of GaMnAs over a broad range of magnetic doping, we have developed methods to largely remove these defects.
In the ferromagnetic regime, we utilize a combinatorial growth method to systematically reduce nonstoichiometric defects and synthesize material at the Mn-doping limits of ferromagnetism. At the dilute doping limit, we find growth conditions for producing GaMnAs with optoelectronic quality and spin lifetimes on par with non-magnetic heterostructures. Using this system we optically address and detect the spins of extremely small numbers of Mn ions. Surprisingly, we identify a new method for manipulating magnetic ions without magnetic fields. A dynamic exchange mechanism polarizes a few hundred Mn ions within GaAs quantum wells, a magnetization that can be optically oriented. We observe Mn ion spin coherence times exceeding 10 ns, suggesting that they may be useful systems for information processing. These studies have led to experiments currently probing single magnetic ions.