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Soutenance de Thèse "Effects of nanoscale inhomogeneities on the magnetic properties of diluted magnetic systems"

26th of June, 2012. D420, 3rd Floor, Building D at 14h00.

"Effects of nanoscale inhomogeneities on the magnetic properties of diluted magnetic systems".

Abstract : This thesis is mainly devoted to the study of nanoscale inhomogeneities in diluted and disordered magnetic systems. The presence of inhomogeneities was detected experi- mentally in several disordered systems which in turn gave rise to various interesting and unexpected properties. In particular, the possibility of room-temperature ferromagnetism generated a huge thrust in these inhomogeneous materials for potential spintronics applications. However, a proper theoretical understanding of the underlying physics was a longstanding debate. In this manuscript we provide a detailed theoretical account of the effects of these nanoscale inhomogeneities on the magnetic properties of diluted systems. First we show the importance of disorder e-ects in these systems, and the need to treat them in an appropriate manner. The self-consistent local RPA (SC-LRPA) theory, based on -nite temperature Green’s function, is found to be the most reliable and accurate tool for this. We have successfully implemented the SC-LRPA to study the dynamical mag- netic properties of the 3D nearest-neighbor diluted Heisenberg model. The percolation threshold is found to be reproduced exactly in comparison with previous existing stud- ies. Following this, we discuss the essential role of a minimal model approach to study diluted magnetic systems. The one-band V -J model, has been used to calculate the Curie temperature and the spin excitation spectrum in (Ga,Mn)As. An excellent agreement isobtained with -rst principles based calculations as well as experiments. Finally we propose an innovative path to room-temperature ferromagnetism in these materials, by nanoscale cluster inclusion. We -nd a colossal increase in TC of up to 1600% compared to the ho- mogeneous case in certain cases. Also the spontaneous magnetization is found to exhibit anomalous non-mean—eld like behavior in the presence of inhomogeneities. In addition we observe a complex nature of the magnon excitation spectrum with prominent features appearing at high energies, which is drastically di-erent from the homogeneous case. Our study interestingly reveals a strong suppression of the spin-sti-ness in these inhomogeneous systems. The results indicate toward the strong complexities associated with the interplay/competition between several typical length scales. We believe this work would strongly motivate detailed experimental as well as theoretical studies in this direction in the near future.

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