Magnetism in diluted magnetic semiconductors

The hope of attaining ferromagnetic order at room temperature and above has spurred a huge interest in the field of diluted magnetic semiconductors over the past couple of decades. However, it has been difficult to obtain Curie temperatures larger than 190 K measured in well annealed (Ga,Mn)As, in the dilute regime. Also a proper understanding of the origin and nature of ferromagnetism in these materials was often lacking. Here we will present a theoretical account of the magnetic properties in diluted systems and discuss the possibility to induce room-temperature ferromagnetism in these materials. We will show the importance of disorder effects and the need to treat them in an appropriate manner. The self-consistent local random phase approximation (SC-LRPA) theory, based on finite temperature Green's function, is shown to be an accurate and reliable tool for this. We will brief?y discuss the essential role of a minimal model approach to study diluted magnetic systems. The one-band "V-J Model", is used to calculate the Curie temperatures and the spin excitation spectrum in (Ga,Mn)As. An excellent agreement is obtained with first principles based calculations as well as experiments. Finally, we propose an innovative path to room-temperature ferromagnetism in these diluted materials, by the inclusion of nanoscale inhomogeneities. We find a colossal increase in TC of up to 1600% compared to the homogeneously diluted systems in certain cases. The spontaneous magnetization is found to exhibit an anomalous non-mean-field like behavior in presence of inhomogeneities. In addition, we also observe a complex nature of the spin excitation spectrum with prominent features appearing at high energies, which is drastically different from the homogeneous case. This study interestingly reveals a strong suppression of the spin-wave stiffness in these inhomogeneous diluted systems.