Half-metals at finite temperature
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Nijmegen (The Netherlands) : [S.n.]
Number of pages
Radboud University Nijmegen, Faculteit NWI, 4 december 2008
Promotor : Groot, R.A. de
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Electronic Structure of Materials
SubjectElectronic Structure of Materials
The research presented in this thesis is aimed at achieving finite temperature spin injection. The first chapters are dedicated to the half-metallic half-heuslers, in particular NiMnSb. The problem of creating half-metallic interfaces between NiMnSb and a semiconductor is addressed. It is shown that the symmetry in the bulk is the origin of the half-metallic properties, and that with careful engineering half-metallic interfaces can be constructed. The importance of impurities and nano-scale structuring is discussed, and they are used to improve the polarisation at finite temperature. Finally, thermal excitations in NiMnSb are investigated. The phonon spectrum is calculated and compared with experiment. The next three chapters present the alkali-sesquioxides. These are new ferromagnetic materials based on a 1:2 mixture of peroxides and superoxides stabilized by an alkali-metal.These compounds are unusual in that the magnetism originates from the 2p-electrons. Rubidium sesquioxide, Rb4O6, orders ferromagnetically, with a calculated Curie temperature of 302K. This is unexpectedly high for 2p-electron systems. Rubidium-sesquioxide also shows interesting physics at low temperatures, including a large electron-phonon coupling. Its phonon spectrum at normal conditions and under pressure has been calculated. Finally, oxides with a different superoxide/peroxide ratio, as well as sesquioxides with different cations were calculated. In the last chapter, the relation between the work function and stability of half-metallic CrO2 was investigated. We found a large anisotropy in the work function. Due to the formation of a surface dipole, the work function can be very different from any of the elements in the compound. The surfaces containing the elements with the lowest valency are the most stable, whereas the work function is determined by the electronegativity of the elements at the surface. We expect that the trend found for the work function anisotropy holds for interfaces, and it can be used to construct low resistance interfaces.
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