Theoretical description of magnetic Compton scattering and magnetic properties of Cr-chalcogenide compounds [Elektronische Ressource] / Diana Benea

Diana Benea

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• Physik

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• Publié par: ludwig-maximilians-universitat_munchen
• Publié le: 01 janvier 2004
• Nombre de lectures: 19
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

Dissertation zur Erlangung des Doktorgrades
der Fakultat¤ fur¤ Chemie und Pharmazie
der Ludwig Maximilians Universitat¤ Munchen¤
Theoretical description of magnetic Compton scattering
and magnetic properties of Cr-chalcogenide compounds
Diana Benea
aus
Klausenburg, Rumanien¤
2004Erklar¤ ung
Diese Dissertation wurde im Sinne vonx13 Abs. 3 bzw. 4 der
Promotionsordnung vom 29. Januar 1998 von Prof. Dr. H. Ebert
betreut.
Ehrenwortliche¤ Versicherung
Diese Dissertation wurde selbststandig,¤ ohne unerlaubte Hilfe
erarbeitet.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(Unterschrift des Autors)
Munchen,¤ 17. 04. 2004 Diana Benea
Dissertation eingereicht am 17. 04. 2004
1. Gutachter: Prof. Dr. H. Ebert
2. Prof. Dr. W. Bensch
Mundliche¤ Prufung¤ am 10. 12. 2004Contents
1 Introduction 13
1.1 Compton effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2 Positron annihilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.3 Cr-chalcogenide systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.4 Scope of the work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2 Theory 19
2.1 Density Functional Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.1 Non-relativistic Density Functional Theory . . . . . . . . . . . . . . . . 20
2.1.2 Relativistic Density Functional Theory . . . . . . . . . . . . . . . . . . . 29
2.2 Korringa-Kohn-Rostoker Green’s Function Method . . . . . . . . . . . . . . . 34
2.2.1 Green’s Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.2.2 The Calculation of Observables . . . . . . . . . . . . . . . . . . . . . . . 36
2.2.3 The single-site scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.2.4 The Green’s function . . . . . . . . . . . . . . . . . . . . . . . 44
2.2.5 Multiple scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.2.6 Scattering Green’s function . . . . . . . . . . . . . . . . . . . . 48
2.3 Treatment of Disordered Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . 52
2.3.1 The Coherent Potential Approximation Method . . . . . . . . . . . . . 52
2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3 Compton scattering 59
3.1 Compton scattering cross section . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.2 Magnetic Compton Scattering (MCS) . . . . . . . . . . . . . . . . . . . . . . . . 61
3.3 The expression for the magnetic Compton pro le . . . . . . . . . . . . . . . . 64
12 CONTENTS
3.4 Application to the transition metals Fe and Ni . . . . . . . . . . . . . . . . . . 68
3.4.1 Three-dimensional spin momentum density . . . . . . . . . . . . . . . 73
3.5 Application to disordered alloys . . . . . . . . . . . . . . . . . . . . . . . . . . 75
3.5.1 Compton anisotropic pro les in Ni Co alloy . . . . . . . . . . . . . 75x 1¡x
3.5.2 Magnetic Compton pro les in Fe Pt Invar alloy and ordered3
compound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
3.6 Magnetic Compton pro les for rare earth systems . . . . . . . . . . . . . . . . 85
3.6.1 Magnetic Compton pro le of Gd . . . . . . . . . . . . . . . . . . . . . . 85
3.6.2 pro le of Y Gd alloy . . . . . . . . . . . . . . 880:38 0:62
3.7 Magnetic Compton pro le of UFe . . . . . . . . . . . . . . . . . . . . . . . . . 902
3.7.1 The orbital polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
3.7.2 In uence of spin-orbit coupling and orbital polarization
on the MCP of UFe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 932
3.7.3 Individual atomic-type contributions on the magnetic Compton
pro le of UFe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 942
3.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
4 Positron annihilation 99
4.1 Introduction to positron annihilation . . . . . . . . . . . . . . . . . . . . . . . . 99
4.2 The electron-positron momentum density . . . . . . . . . . . . . . . . . . . . . 101
4.3 Electron-positron density of V . . . . . . . . . . . . . . . . . . . . 105
4.4 Comparison with Compton scattering . . . . . . . . . . . . . . . . . . . . . . . 108
4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
5 Ground-state properties of Cr-chalcogenide systems 111
5.1 Structural properties of binary CrX (X = S, Se, Te)
compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
5.2 Band-structure calculations of CrX (X = S, Se, Te) systems . . . . . . . . . . . . 115
5.2.1 Density of states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.2.2 Phase stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
5.2.3 Magnetic moments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
5.3 Non-collinear spin structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
5.3.1 Non-collinear spin structure in CrSe system . . . . . . . . . . . . . . . 121
5.3.2 spin structure in CrTe system . . . . . . . . . . . . . . . 124CONTENTS 3
5.4 Band-structure calculations for the system CrTe Se . . . . . . . . . . . . . . 1261¡x x
5.4.1 Magnetic moments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
5.4.2 ground state . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
5.5 Band-structure calculations for Cr (Te Se ) systems . . . . . . . . . . . . 1301+x 1¡y y 2
5.5.1 Structural properties of Cr (Te Se ) compounds . . . . . . . . . . 1301+x 1¡y y 2
5.5.2 Preferential site occupation . . . . . . . . . . . . . . . . . . . . . . . . . 131
5.5.3 Density of states and magnetic moments . . . . . . . . . . . . . . . . . 132
5.6 Band-structure calculations for (Cr Ti ) Te systems . . . . . . . . . . . . . . 135x 1¡x 5 8
5.6.1 Structural properties of (Cr Ti ) Te compounds . . . . . . . . . . . . 136x 1¡x 5 8
5.6.2 Preferential site occupation . . . . . . . . . . . . . . . . . . . . . . . . . 137
5.6.3 Density of states and magnetic moments . . . . . . . . . . . . . . . . . 139
5.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
6 Summary 145
A Green’s function in momentum representation (Compton) 147
B Matrix elements (Compton) 151
C Green’s function in momentum representation (Positron Annihilation) 155
C.1 Site-diagonal contribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
C.2 Site-off-diagonal . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
C.3 Lattice sum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
D Matrix elements (Positron Annihilation) 1634 CONTENTSList of Tables
3.1 Spin magnetic moments per formula unit in Fe Pt compound. Results are3
quoted as „ per formula unit of Fe Pt . The MCS (magnetic Comp-B 0:75 0:25
ton scattering) and VSM (vibrating sample magnetometer) experimental data
stem from Taylor et al. [1]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
3.2 Spin magnetic moments per Fe atom in Fe Pt compound. . . . . . . . . . . . 803
3.3 The theoretical spin magnetic moments in Gd . The SPR-KKR calculated mag-
netic moments are compared with FLAPW results of Kubo and Asano [2] re-
spectively with LMTO magnetic moments of Duffy et al. [3]. . . . . . . . . . . 85
3.4 LMTO [4] and SPR-KKR magnetic moments of Y Gd alloy. The SPR-0:38 0:62
KKR spin magnetic moments are presented in parenthesis. . . . . . . . . . . . 88
3.5 Magnetic moments of U and Fe in UFe compound. . . . . . . . . . . . . . . . 922
5.1 The experimental [5, 6, 7] lattice parameters for CrX (X = S, Se, Te) systems. . 115
5.2 The interatomic distances in the NiAs structure for CrX (X = S, Se,Te)
N is the number of neighbours and c and ab distinguish Cr-neighbours along
the c-axis and within the ab plane. . . . . . . . . . . . . . . . . . . . . . . . . . 115
5.3 The exchange-splitting of the Cr(3d) states of CrX (X = S, Se,Te) systems in
KKR and ASW calculations (Dijkstra et al. [8]). . . . . . . . . . . . . . . . . . . 116
5.4 Total energy difference E ¡ E (mRy) in CrX (X = S, Se, Te) systems fromFM AF
SPR-KKR calculations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
5.5 Magnetic moments in CrX (X = S, Se, Te) compounds resulting from SPR-
KKR, LMTO [9] and ASW [8] calculations. . . . . . . . . . . . . . . . . . . . . . 119
5.6 Magnetic moments of Cr in CrX (X = S, Se, Te) compounds resulted from
FM/AF SPR-KKR calculations. . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
5.7 The SPR-KKR-LSDA equilibrium lattice parameters of CrTe Se system0:30 0:70
for AF and FM, compared with the experimental values ([5, 6, 7]). . . . . . . 127
5.8 The SPR-KKR-GGA equilibrium lattice parameters of CrTe Se system0:30 0:70
for AF and FM, compared with the values ([5, 6, 7]). . . . . . . 129
56 LIST OF TABLES
5.9 The lattice parameters for Cr (Se/Te) non-stoichiometric trigonal compounds1+x 2
[10]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
5.10 Magnetic moments in trigonal Cr (Te Se ) non-stoichiometric compounds1+x 0:88 0:12 2
(in „ ). . .