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Etude théorique de la corrélation entre le magnétisme et les propriétés des défauts dans le fer, le chrome et leurs alliages
| Content Provider | Semantic Scholar |
|---|---|
| Author | Soulairol, Romain |
| Copyright Year | 2011 |
| Abstract | This PhD work is devoted to the study of the correlation between the magnetism and the properties of defects in 3d metals, mainly ironand chromium-based systems. Its main objective is to clarify not only the effect of the magnetic structure on the properties of defects, but also the role of those defects in a possible modification of the magnetic structures. To reach that objective, we have used two complementary approaches based on Density Functional Theory (DFT) as well as a Tight Binding (TB) approach. We have shown, in the first part of the study, how those different approaches, used with finely-tunned and justified approximations, could describe the magnetic properties of bulk Fe and Cr. For the latter, standard DFT and TB calculations accurately describe the magnetic characteristics of the experimental ground state, namely spin density wave (SDW), although they predict a lowest energy state with AF configuration. In the following parts, we have considered various types of defects (point and extended defects) often found in real materials. Concerning chromium, first of all, we have observed that either formation and migration of a vacancy or dissolution of impurities such as iron or copper become anisotropic in the SDW magnetic structure. Reciprocally, such point defects can activate a mechanism of destabilization of the SDW by migration and annihilation of nodes. We have moreover checked that the calculation of defects properties was not affected by the energy difference between AF and SDW phases as predicted by DFT. Concerning α-iron, the solution enrgy of 3d impurities such as vanadium, chromium, manganese or cobalt depends on two main terms : a chemical contribution, linked to the difference between the number of d electrons of iron and solute and to the mean number of d electrons in the alloy, and a magnetic contribution that reveals to be predominant in Fe-Cr. The third part of this work is dedicated to the correlation between magnetism and extended defects. We have shown that, for chromium surfaces as for Fe/Cr interfaces, the magnetism induces an inversion of energetical stability between (100) and (110) orientations. This inversion is related, in the Fe/Cr interfaces, to the existence of magnetic frustrations due first and second nearest-neighbour interactions, in particular for the (110) orientation. This frustration at (110) interface is also the driving force that induce a non collinear magnetic configuration. Its structure is moreover in agreement with experimental findings that observed a perpendicular coupling between Fe and Cr magnetic moments across the (110) interface. In the meanwhile, we have confirmed that out-of-plane SDW was stable near (100) Cr surface or Fe/Cr interface, whatever is the thickness of the chromium slab. At variance, near (110) Cr surface or Fe/Cr interface, only the in-plane SDW remains stable for thin films of chromium, in agreement with experimental data. In the latter case, as for the non collinear configuration, the partial relaxation of magnetic frustration induces a decrease of interface energy. All these results enhance the fact that layered-AF or NM structures of Cr, which are only approximations of real chromium ground state, do not enable an accurate and complete description of the properties of point and extended defects. As a result, it seems mandatory to consider explicitely the SDW structure, as we have done all along this PhD work. Small iron or chromium clusters in Fe-Cr system are an intermediate case between the behaviour of these metals as impurities and infinite interfaces that characterize large precipitates. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://iramis.cea.fr/spec/GMT/docs/these_R_Soulairol.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
