Spintronic Materials

Most electronic devices rely upon the charge of the electron to process information and the spin degree of freedom to store the information. The field of spintronics looks to combine the spin and charge degrees of freedom to obtain devices with new functionalities. Ferromagnetic semiconductors, such as GaAs doped with Mn, are promising new materials in this regard, since they may be combined with conventional semiconductors, and (unlike the II-VI materials) may be doped.

There is a significant body of theoretical literature for these materials, including first principles calculations, mean-field theory for the Zener model, finite-sized Monte Carlo calculations, spin-wave modeling, and DMFA calculations. Most of these calculations neglect the role of spin-orbit coupling, and yield results which are similar to the physics of a conventional ferromagnet. Recently, Zarand and Janko introduced a new twist. Using perturbation theory in $J_c/E_F$ including the effects of the large spin-orbit coupling, they found that the spins tended to align perpendicular to the axis connecting them, leading to frustration in a system of randomly distributed spins.

Although many experiments point to the importance of both strong exchange $J_c$ and spin-orbit coupling, most theoretical approaches have avoided this regime. A large $J_c$ causes an impurity band to form. Although the role of the impurity band is still controversial, an array of experimental probes including photoemission, infrared spectroscopy, spectroscopic ellipsometry, scanning tunneling microscopy , and photoluminescence techniques, display features characteristic of an impurity band. There is also experimental evidence for frustration and a non-colinear magnetic state.

In a series of papers, we used the DMFA to explore the double-exchange model, or the generalized double-exchange model with both strong coupling $J$ and strong spin-orbit with heavy and light hole bands. Here, we identified the materials properties that optimize the transition temperature $T_c$ for a generalized double-exchange model. We reach the surprising conclusion that $T_c$ achieves a maximum when the band angular momentum $J$ equals 3/2 and when the masses in the $J_z=\pm 1/2$ and $\pm 3/2$ subbands are equal. However, we also find that $T_c$ is significantly reduced as the ratio of the masses decreases from one. This suggests that in semiconductors with p bands, such as the currently studied Mn-doped Ge and GaAs semiconductors, $T_c$ may be optimized by tuning the band masses through strain engineering or artificial nanostructures. However, semiconductors with s or d bands with nearly equal effective masses might prove to have higher $T_c$'s.