Investigation of electronic and magnetic structure of advanced magnetic materials [Elektronische Ressource] / by Vasile Rednic

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Sujets

• Physik

Informations

• Publié par: universitat_osnabruck
• Publié le: 01 janvier 2010
• Nombre de lectures: 25
• Langue: English
• Poids de l'ouvrage: 6 Mo

Extrait

Investigation of electronic and magnetic
structure of advanced magnetic materials


A thesis presented to

Fachbereich Physik Faculty of Physics
Universität Osnabrück Babes-Bolyai University






by
Vasile REDNIC

Scientific supervisors:



Prof. Dr. Manfred NEUMANN Prof. Dr. Marin COLDEA




Osnabrück
January 2010
Contents

Introduction .......................................................................................................................................1
1. Theoretical Considerations ....................................................................................................................... 3
1.1 General issues of magnetism ............................................................................................................... 3
1.1.1 The origin of atomic moments ........................................................................................................... 3
1.1.2 Classes of magnetic materials ............................................................................................................ 6
1.2 Magnetic properties of metallic systems ......................................................................... 9
1.2.1 Localized moments in solids .............................................................................................................. 9
1.2.2 Itinerant-electron magnetism ........................................................................................................... 12
1.2.3 Local moments in metals ................................................................................................................. 16
1.2.4 Self-Consistent Renormalization (SCR) Theory of spin fluctuations of ferromagnetic
metals………………………………………………………………………………………….25
1.3 Photoelectron Spectroscopy .............................................................................................................. 26
1.3.1. Physical principles of the technique ................................................................................................ 27
1.3.2 Theory of photoelectron spectroscopy ............................................................................................. 29
1.3.3 Photoelectron spectroscopy models ................................................................................................. 31
1.3.4 Spectral Characteristics .................................................................................................................... 32
2. Preparation and Characterization techniques ...................................................................................... 40
2.1 Sample preparation ........................................................................................................................... 40
2.2 Structure analysis ............................................................................................................................... 40
2.3 Magnetic measurements .................................................................................................................... 42
2.3.1 Vibrating sample magnetometer ...................................................................................................... 42
2.3.2 SQUID magnetometer .................................................................................................................. 44
2.3.3 Weiss balance ................................................................................................................................... 45
2.4 X- ray photoelectron spectroscopy ................................................................................................... 47
2.5 Electronic structure calculations ................................................................................... 50
3. Electronic structure and magnetic properties of Mn Al Ni alloys ................................................... 52 1-x x 3
3.1 Structural characterization ............................................................................................................... 53
3.2 XPS spectra ......................................................................................................................................... 54
3.3 Magnetic measurements .................................................................................................................... 58
3.4 Electronic structure calculations ...................................................................................................... 63
3.5 Conclusions ...................................................................................................................... 68
4. Electronic structure and magnetic properties of Mn Al Ni alloys .................................................... 70 1-x x
i
4.1 Structural characterization ............................................................................................................... 71
4.2 XPS spectra ......................................................................................................................................... 72
4.3 Magnetic measurements .................................................................................................................... 79
4.4 Conclusions ......................................................................................................................................... 86

5. Electronic structure and magnetic properties of Ni Mn Al alloys .................................................... 87 1-x x
5.1 Structural characterization ............................................................................................................... 88
5.2 XPS spectra ......................................................................................................................................... 89
5.3 Magnetic measurements .................................................................................................................... 91
5.4 Conclusions ........................................................................................................................................ .96
6. Crystallographic and electronic structure of Ni Al Mn alloys ................................................. …98 0.7-x x 0.3
6.1 Structural characterization ............................................................................................................... 99
6.2 XPS spectra ....................................................................................................................................... 100
6.3 Magnetic measurements .................................................................................................................. 104
6.4 Conclusions ....................................................................................................................................... 107
Summary ........................................................................................................................................109
References ............................................................................................................................................... 111
List of Figures ......................................................................................................................................... 116
List of Tables ........................................................................................................................ 118
List of Publications .................................................................................................................................... 119
List of Conference Contributions ............................................................................................................. 120
Acknowledgements .................................................................................................................................... 122










ii Introduction

The problem of local moments confined to the transition metals (T) sites, i.e., localized
behaviour in some aspects of itinerant electrons, is one of the most important issues in the
physics of the magnetic alloys and intermetallic compounds. It was found experimentally that
under certain conditions the magnetic moment of a transition metal remains localized when
solute in another transition metal. The condition for the existence of the local moment at the T
site is πΔ / U < 1, where Δ is the width of the d states (corresponds to the virtual bound states in
the Friedel’s model) and U is the Coulomb correlation energy between d electrons.
1/2 The 3d band width Δ = Z J depends on the number of near-neighbours Z with d orbitals h
and the hopping integral J , which is very sensitive to the distance between the atoms. On the ohn
other hand, the strength and the sign of the interaction between the neighbouring local moments
are determined by the occupation fraction of d-orbitals and the orientation of these orbitals in the
lattice. By alloying with other elements, the vicinity of the transition metal atom is changing.
This leads to structural modifications with remarkable variations in the electronic structure and
magnetic properties of the parent compound.
The understanding and prediction of the pr