provided by the substitutional manganese, and as such they are known as compensating defects. The interstitial manganese also bond antiferromagnetically to substitutional manganese, removing the magnetic moment. Both these defects are detrimental to the ferromagnetic properties of the , and so are undesired. The temperature below which the transition from paramagnetism to ferromagnetism occurs is known as the Curie temperature, TC. Theoretical predictions based on the Zener model suggest that the Curie temperature scales with the quantity of manganese, so TC above 300°K is possible if manganese doping levels as high as 10% can be achieved. After its discovery by Ohno et al., the highest reported Curie temperatures in rose from 60°K to 110°K. However, despite the predictions of room-temperature ferromagnetism, no improvements in TC were made for several years. As a result of this lack of progress, predictions started to be made that 110°K was a fundamental limit for . The self-compensating nature of the defects would limit the possible hole concentrations, preventing further gains in TC. The major breakthrough came from improvements in post-growth annealing. By using annealing temperatures comparable to the growth temperature it was possible to pass the 110°K barrier. These improvements have been attributed to the removal of the highly mobile interstitial manganese. Currently, the highest reported values of TC in are around 173°K, still well below the much sought room-temperature. As a result, measurements on this material must be done at cryogenic temperatures, currently precluding any application outside of the laboratory. Naturally, considerable effort is being spent in the search for an alternative magnetic semiconductors that does not share this limitation. In addition to this, as molecular beam epitaxy techniques and equipment are refined and improved it is hoped that greater control over growth conditions will allow further incremental advances in the Curie temperature of . Properties Regardless of the fact that room-temperature ferromagnetism has not yet been achieved, magnetic semiconductors materials such as , have shown considerable success. Thanks to the rich interplay of physics inherent to magnetic semiconductors a variety of novel phenomena and device structures have been demonstrated. It is therefore instructive to make a critical review of these main developments. A key result in magnetic semiconductors technology is gateable ferromagnetism, where an electric field is used to control the ferromagnetic properties. This was achieved by Ohno et al. using an insulating-gate field-effect transistor with as the magnetic channel. The magnetic