The thesis presented here deals electronic and magnetic properties of Gd doped GaN and Gadolinium pnictides. GdN is a ferromagnetic semiconductor while the other Gd-pnicitides GdX with X=P,As,Sb,Bi are all antiferromagnetic and semimetallic. Gd-doped GaN is ferromagnetic above room temperatures even in very dilute concentrations of Gd and has been claimed to have colossal magnetic moments. The main question we address in this thesis is: what are the origins of magnetism in terms of the electronic structure ofthe materials?
We begin our study with bulk GdN. Our main contribution on the electronic structure,
is a study of the optical interband transitions, which in future work may become useful
to experimentally validate the calculated electronic band structure.
Next we revisit the calculation of the magnetic exchange interactions in the Gd-pnictides. The full-potential linearized mu±n-tin orbital calculations performed here
provide new insights by analyzing the distribution of induced magnetic moments on Gd-d
and N-p sites.
Secondly, an alternative linear response Green's function approach in the atomic sphere approximation is applied here. The results between the two approaches are carefully
compared.
Finally, in the third part of the thesis we turn to the challenging Gd-doped GaN system. Our investigation of this system starts by examining the models that have been proposed to address the origin of colossal magnetism. We then focus on the origins of ferromagnetism rather than the colossal moments in the rest of the study.
Our work proposes two alternative defects that could contribute to the magnetism while at the same time demonstrating that they are energetically favorable: N interstitials in octahedral sites and octahedral interstitial O impurities. We find that these interstitial
atoms induce strong ferromagnetic coupling between the Gd atoms but also are themselves coupled ferromagnetically to the Gd. The distance dependence of the interactions is
investigated.