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P. Mohn:
"Magnetic Semiconductors: Electronic Structure and Magnetic Moment Formation in Mn-Ga-As Alloys";
Vortrag: Seminar Institut für Allgemeine Physik (IAP), TU Wien; 07.06.2005.

Calculations of the electronic and magnetic structure of the cubic phases of Mn-Ga-As will be presented. It is found that for high coordination numbers like in the bcc and fcc environment, the Ga/As atoms form broad metallic like sp-bands which interact with Mn. For most of these systems antiferromagnetic structures are found to be the most stable ones. The reason for this behavior seems to be, that in the AF structure the magnetic moments are usually larger than for the FM ones. Since the interaction between Mn and the Ga/As states is small in general, the enlarged moment leads to a gain in exchange energy and thus to a stabilization of that sort of spin-ordering which simply has the larger magnetic moment.
For the ZnS structure the situation is much more subtle. Due to the fourfold coordination, As forms a sp3 hybrid, which then interacts withMn in different ways, depending on the spin ordering. For FM and [111] ordering, the As 4p-band follows the spin polarization of Mn and splits accordingly causing AsMn to form a half metallic ferromagnet. For the [001] spin ordering, where As is positioned in a spin compensated site, the fact that spin up and spin down As have to be equal, leads to an enhanced interaction with Mn causing a metallic state with a reduced magnetic moment and a much larger total energy. However, since the most stable modification is found to be the FM one, AsMn could be suggested as the ideal interface for spin injection.
Calculations of Mn-GaAs super-cells allow formulating a possible mechanism to enhance the magnetic ordering temperature. Since Tc is found to depend strongly on the number of excess electrons in the system, alloying constituencies with fewer electrons than As could help so stabilize the FM state and thus might increase the magnetic ordering temperature.
A new class of half metallic ferromagnets is the s-p systems like CaAs. In the Wurtzite and Zinkblende structure these systems order magnetically although no d-electrons are present.