3720

178 دكتري

دكتري

فيزيك ماده چگال محاسباتي

اصفهان: دانشگاه صنعتي اصفهان، دانشكده فيزيك

1385

[ده]، 147، [II]ص.: مصور، جدول، نمودار

ص. ع. به فارسي و انگليسي

هادي اكبر زاده

كيوان اسفرجاني

26/08/86

ناصر نفري، رضا عسگري، محمد رضا محمدي زاده

1396/03/08

كتابنامه

فيزيك

فيزيك

ID178 دكتري

به فارسي و انگليسي : قابل رؤيت در نسخه ديجيتال

Abstract The remarkable success of the local density and generalized gradient approximations withinKohn Sham approach have led to widespread interest in Density Functional Theory DFT as themost promising approach for theoretical study of materials Wien2k package employs the highly ac curate all electron full potential linearized augmented plane wave plus local orbital method to solvethe single particle Kohn Sham equations We have used Wien2k to study two interesting magneticmaterials half metal Co2 MnSi Heusler alloy and antiferromagnetic dilute Cr alloys Theoretical prediction of half metallicity in ferromagnetic full Heusler Co2 MnSi alloy along withits high measured curie temperature has made this alloy a promising material for spintronic applica tions In a ferromagnetic half metal the majority spins have metallic behavior while the Fermi energyis located in a gap of the minority band structure We have studied the structural electronic andmagnetic properties of the ideal and modi ed Co2 MnSi 001 surfaces For the ideal MnSi and CoCoterminations we found that some surface states arising mainly from surface or subsurface cobaltbreak half metallicity In the MnSi termination surface exchange enhancement is the main origin ofthe surface states while in CoCo termination the potential rise up at the surface push some minoritystates into the half metallic gap In order to study disorder e ects we have investigated the stabil ity the electronic and the magnetic properties of Co2 MnSi 001 thin lms for 13 di erent modi edterminations in a 1 1 supercell The phase diagram obtained by ab initio atomistic thermodynamicsshows that in practice the ideal MnSi modi ed pure Mn or pure Si terminated surfaces can bestabilized under suitable conditions Analyzing the surface band structure we found that the pureMn termination due to its strong surface subsurface coupling preserves the half metallicity of thesystem while surface states appear for the other terminations The second subject is magnetic properties of dilute impurities in chromium host Cr alloys re veal great variety of antiferromagnetic properties originating from the peculiar magnetic behavior ofsimple bcc chromium an incommensurate spin density wave ground state In this research we haveinvestigated Fe Ta and 4d impurities Nb I in a commensurate antiferromagnetic Cr host Thee ects of these impurities on the structural electronic and magnetic properties of chromium are dis cussed with special emphasis on the hyper ne interaction It is shown that the lled d shell 4d atomsare nonmagnetic with weak bonding while other impurities have considerable magnetic moments andstronger coupling with the nearest neighbors Two di erent magnetic states were identi ed for Feimpurity in the Cr host one with high and the other with low magnetic moments the latter beingtheoretically more stable in agreement with experimental observations Our calculated hyper ne elds on the impurities are highly overestimated compared with the experimental results that isoriginated from GGA approximation while LDA is not able to stabilize antiferromagnetism in Cralloys Such overestimation can be improved by implementing a simple model based on the scalingbehavior of magnetic properties in chromium The overestimation factor obtained by this model canbe used for predicting hyper ne eld values on 4d impurities in a Cr host

هادي اكبر زاده

كيوان اسفرجاني