Transport Properties of Ferromagnetic Semiconductors

This chapter has covered several areas of the rich physics of transport effects in (Ga,Mn)As materials and microdevices. (Ga,Mn)As is a remarkable system which allows to study basic transport phenomena in extrinsic semiconductors over many decades of doping densities. Particularly intriguing in these systems is the interplay between charge doping by Mn-acceptors, strong SO-coupling in the hole bands, and the interaction between hole spins and Mn d-shell local moments. While the conduction in (Ga,Mn)As has a well understood impurity band character at low dopings and a disordered valence band character at high dopings, the intermediate dopings around 1% of Mn are intriguing and difficult to model. On the other hand it is exactly this doping regime in which ferromagnetism sets on and where future research will likely uncover new interesting physics which is undetectable in the more conventional systems with shallow, hydrogenic-like nonmagnetic dopands. Another potentially fruitful and only partially understood area of research is related to the role played by the internal exchange field and SO-coupling in low-temperature, quantum-coherent magnetotransport. Ferromagnetic semiconductor materials and microdevices in which these fields and other doping characteristics are largely tunable represent ideal systems to study these phenomena. The most extensively explored transport characteristics of (Ga,Mn)As and related ferromagnetic semiconductors are the extraordinary MR effects. (Ga,Mn)As has provided an unprecedented physical insight into the anomalous Hall and AMR effects in standard ohmic devices and led to the discovery of AMR in the tunneling and CB regimes. Many results of the extraordinary MR studies in ferromagnetic semiconductors may be directly relevant to room-temperature metal ferromagnet systems with strong SO-coupling and may therefore lead to new technological applications even prior to the realization of high Curie temperature ferromagnetic semiconductors. Other favorable characteristics, such as the sensitivity of micromagnetic and magnetotransport coefficients to doping and electrical gating, and the higher integrability due to low saturation moments are specific to the class of dilute moment ferromagnetic semiconductors among which (Ga,Mn)As has become a prototypical material. © 2008 Elsevier Inc. All rights reserved.