Computational studies on the structure and stabilities of magnetic inter-metallic compounds [Elektronische Ressource] / vorgelegt von Hem Chandra Kandpal

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• Publié par: johannes_gutenberg-universitat_mainz
• Publié le: 01 janvier 2007
• Nombre de lectures: 26
• Langue: English
• Poids de l'ouvrage: 3 Mo

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Computational studies on the structure and stabilities of
magnetic inter-metallic compounds
Dissertation
zur Erlangung des Grades
“Doktor der Naturwissenschaften”
am Fachbereich Chemie, Pharmazie und Geowissenschaften
der Johannes Gutenberg-Universita¨t Mainz
vorgelegt von
Hem Chandra Kandpal
geboren in Almora (INDIA)
Mainz, 2006Dekan: Prof. Dr. Peter Langguth
Tag der mu¨ndlichen Pru¨fung: 20th July 2007
This work was carried out from October 2002 to December 2006 at the Institut fu¨r Anorganische
und Analytische Chemie, Johannes Gutenberg-Universita¨t, Mainz under the supervision of Prof.
Dr. Claudia Felser.I declare that I wrote this work myself and carried out the computational study de-
scribed in it, without using any other sources and aids than those that are stated.
Mainz, December 2006Thesis advisor Author
Claudia Felser Hem Chandra Kandpal
Computational studies on the structure and stabilities of magnetic inter-
metallic compounds
Abstract
The purpose of this thesis is to further the understanding of the structural, electronic and magnetic
properties of ternary inter-metallic compounds using density functional theory (DFT). Four main
problems are addressed. First, a detailed analysis on the ternary Heusler compounds is made. It
has long been known that many Heusler compounds (X YZ; X and Y transition elements, Z main2
group element) exhibit interesting half-metallic and ferromagnetic properties. In order to understand
these, the dependence of magnetic and electronic properties on the structural parameters, the type
of exchange-correlation functional and electron-electron correlation was examined. It was found
that almost all Co YZ Heusler compounds exhibit half-metallic ferromagnetism. It is also observed2
that X and Y atoms mainly contribute to the total magnetic moment. The magnitude of the
total magnetic moment is determined only indirectly by the nature of Z atoms, and shows a trend
consistent with Slater-Pauling behaviour in several classes of these compounds. In contrast to
experiments, calculations give a non-integer value of the magnetic moment in certain Co -based2
Heusler compounds. To explain deviations of the calculated magnetic moment, the LDA+U scheme
was applied and it was found that the inclusion of electron-electron correlation beyond the LSDA
and GGA is necessary to obtain theoretical description of some Heusler compounds that are half-
metallic ferromagnets. The electronic structure and magnetic properties of substitutional series of the
quaternary Heusler compound Co Mn Fe Si were investigated under LDA+U. The calculated2 1−x x
band structure suggest that the most stable compound in a half-metallic state will occur at an
intermediate Fe concentration. These calculated findings are qualitatively confirmed by experimental
studies.
Second, the effect of antisite disordering in the Co TiSn system was investigated theo-2
ivAbstract v
retically as well as experimentally. Preservation of half-metallicity for Co TiSn was observed with2
moderate antisite disordering and experimental findings suggest that the Co and Ti antisites disorder
amounts to approximately 10 % in the compound.
Third, a systematic examination was carried out for band gaps and the nature (covalent
or ionic) of bonding in semiconducting 8- and 18-electron or half-metallic ferromagnet half-Heusler
compounds. It was found that the most appropriate description of these compounds from the
viewpoint of electronic structures is one of a YZ zinc blende lattice stuffed by the X ion. Simple
valence rules are obeyed for bonding in the 8- and 18-electron compounds.
Fourth, hexagonal analogues of half-Heusler compounds have been searched. Three series
of compounds were investigated: GdPdSb, GdAuX (X = Mn, Cd and In) and EuNiP. GdPdSb is
suggested as a possible half-metallic weak ferromagnet at low temperature. GdAuX (X = Mn, Cd
and In) and EuNiP were investigated because they exhibit interesting bonding, structural and mag-
netic properties. The results qualitatively confirm experimental studies on magnetic and structural
behaviour in GdPdSb, GdAuX (X = Mn, Cd and In) and EuNiP compounds.Contents
Title Page . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvi
Dedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xviii
1 Introduction 1
1.1 First principles methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.1 Density functional theory (DFT) and local density approximation (LDA) . . 2
1.1.2 Local spin density approximation (LSDA) . . . . . . . . . . . . . . . . . . . . 5
1.1.3 Linear muffin-tin orbital (LMTO) highly localized basis functions . . . . . . . 6
1.1.4 Linear augmented plane wave method (WIEN2k) . . . . . . . . . . . . . . . . 6
1.1.5 Full relativistic Korringa-Kohn-Rostocker (KKR) method . . . . . . . . . . . 7
1.2 Crystal structures of Heusler, cubic C1 and hexagonal compounds . . . . . . . . . . 7b
1.2.1 Crystal structure of Heusler compounds . . . . . . . . . . . . . . . . . . . . . 7
1.2.2 C1 crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8b
1.2.3 Hexagonal analogues of cubic C1 compounds . . . . . . . . . . . . . . . . . . 8b
1.2.4 Crystal structure of GdAuX compounds . . . . . . . . . . . . . . . . . . . . . 10
1.3 Structure of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.4 Calculational parameters used in different Chapters . . . . . . . . . . . . . . . . . . . 12
2 Electronic and magnetic properties of the half-metallic, transition metal based
Heusler compounds 14
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.2 Non existing Heusler compounds, optimization and band structure . . . . . . . . . . 15
2.3 Results and discussions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.1 Magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3.2 Electronic structure and density of states . . . . . . . . . . . . . . . . . . . . 23
2.3.3 Other Co based half-metallic ferromagnets . . . . . . . . . . . . . . . . . . . 342
2.3.4 Other Heusler compounds exhibiting half-metallic ferromagnetism . . . . . . 35
2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3 Correlation in the transition-metal based Heusler compounds 37
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.2 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
viContents vii
3.2.1 Structure optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.2.2 Electronic structure and lattice parameter . . . . . . . . . . . . . . . . . . . . 39
3.2.3 Electron correlation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.2.4 Magnetic moment and minority gap in the LDA+U . . . . . . . . . . . . . . 45
3.2.5 Electronic structure in the LDA+U . . . . . . . . . . . . . . . . . . . . . . . 48
3.2.6 Electronic structure of Co Mn Fe Si with U . . . . . . . . . . . . . . . . . 542 1−x x
3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4 Electronic structure, magnetism, and disorder in the Heusler compound
Co TiSn 582
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.2 Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.3.1 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.3.2 Computation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
5 Covalentbondingand thenature oftheband gapsinsomecubicC1 compounds 69b
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.2 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.2.1 8-electron compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.2.2 18-electron compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
5.2.3 Magnetic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.2.4 MnNiSb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
6 GdPdSb: a possible half-metallic weak ferromagnet at low temperature 89
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . .