GaMnAs is a ferromagnetic semiconductor which is considered to be an ideal candidate for spintronic applications. However, the bottleneck is that its highest critical temperature so far achieved is about 200 K. The reason for this obstacle is that the properties of GaMnAs, despite a decade of research, are not fully understood, thus preventing realistic strategies for fabrication of GaMnAs with a significantly higher Curie temperature.
This talk will address the unresolved issue of ferromagnetism in GaMnAs concerning the question of whether the ferromagnetic order is mediated by valence band holes or by holes residing in the impurity band. The results to be presented are based on the investigation of a wide range of GaMnAs samples using a comprehensive set of experiments that include magnetization, electrical transport and magneto-optics, along with studies of microscopic composition by channeling Rutherford back-scattering and particle-induced x-ray emission. These experiments show unambiguously that the holes underlying ferromagnetic order in GaMnAs reside in the impurity band; and that it is not only the concentration of Mn and of holes, but also the specific location of the Fermi level in the impurity band that establishes the Curie temperature of this material. Since the location of the Fermi level can be controlled by a variety of means both during and after growth, this new understanding automatically provides practical guidelines for increasing the critical temperature of this material.