Interplay between charge, spin and orbital ordering in La_1tn1-1tnxSr_1tnxMnO_1tn3 manganites [Elektronische Ressource] / vorgelegt von Konstantin Istomin

Sujets

• Physik
• Astronomie

Informations

• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2003
• Nombre de lectures: 14
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

Interplay between Charge, Spin and Orbital
Ordering in La Sr MnO Manganites1−x x 3
Von der Fakult¨at fu¨r Mathematik, Informatik und
Naturwissenschaften
der Rheinisch- Westf¨alischen Technischen Hochschule Aachen
zur Erlangung des akademischen Grades eines Doktors
der Naturwissenschaften genehmigte Dissertation
vorgelegt von
Diplom-Physiker
Konstantin Istomin
aus Novosibirsk, Russische F¨oderation
Berichter: Universit¨atsprofessor Dr. Thomas Bru¨ckel,
Universit¨atsprofessor Dr. Bernd Bu¨chner.
Tag der mu¨ndlichen Pru¨fung: 14.03.2003
Diese Dissertation ist auf den Internetseiten der
Hochschulbibliothek online verfu¨gbar.2Contents
1 Introduction 3
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 The Influence of Cubic Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Jahn-Teller Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5 Influence of Super and Double Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5.1 Super Exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5.2 Double exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.5.3 Calculation of the total Hamiltonian . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.6 Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.7 Charge and Orbital Ordering in Manganites . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2 Review of Experimental Works 17
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2 Neutron diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3 High Energy X-ray Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.4 Theory of Resonant X-Ray Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.5 Review of RXS Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.6 Conclusions from the Experimental Review . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3 Preparation and Characterization 31
3.1 Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.2 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2.1 X-Ray Powder Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2.2 Atom Emission Spectroscopy with Inductively Coupled Argon Plasma . . . . . . . 34
3.3 Crystal Growing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.3.2 Experimental Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.3.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
12 CONTENTS
3.4 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4.1 Microprobe Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4.2 Magnetization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.4.3 Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4 Experiments with X-Ray Scattering 45
4.1 Resonant X-Ray Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.2 High-Energy X-Ray Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.3 Non-resonant X-Ray Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.4 Interpretation Of The Experimental Results and Discussion . . . . . . . . . . . . . . . . . 63
4.4.1 Resonant X-Ray Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.4.2 Charge Ordering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.4.3 Interplay Between Orbital Ordering and Lattice Distortions . . . . . . . . . . . . . 65
4.4.4 Interplay between Charge and Orbital Ordering . . . . . . . . . . . . . . . . . . . . 66
5 Experiments with Neutron Scattering 69
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.1.1 Elastic Nuclear and Magnetic Scattering . . . . . . . . . . . . . . . . . . . . . . . . 70
5.1.2 Inelastic Neutron Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.1.3 Magnetic Critical Neutron Scattering . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.1.4 Paramagnetic Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.2 Rules For Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.3 Diffuse Neutron Scattering Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5.4 Polarized Neutrons Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.5 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
6 Summary of Results 83
A RXS Mesh-Scans 85
B Neutron Scattering Data 89
B.1 List of Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
B.2 Acknowledgement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
B.3 Curriculum Vitae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105Chapter 1
Introduction
1.1 Introduction
Stronglycorrelatedelectronsystems, inwhichtheCoulombinteractionsbetweenelectrons
strongly inhibit their motion, are of great interest due to their possible applications in
technology. These materials are characterized by a wide variety of ground states, ranging
fromantiferromagnetictoferromagneticandfrominsulatingtosuperconducting. Inmany
cases transitions between these ground states can be driven by relatively small changes
in the chemical doping or temperature. The origin of such unusual sensitivity is believed
to be in the fact that not a single degree of freedom dominates the behavior, but rather a
number of correlated degrees of freedom. These can include the spin, charge, orbital and
lattice degrees of freedom. The ground state is then determined by the interplay between
the competing degrees of freedom. However, despite this qualitative understanding, the
complete description of the electronic behavior still does not exist. Investigation of this
problem remains one of the central tasks in condensed matter research today.
In this work interplay between charge, orbital and spin ordering in lightly doped
La A MnO with x ≈ 1/8 has been studied using both resonant and non-resonant1−x x 3
X-ray scattering as well as neutron scattering.
The outline of this work is as follows:
• Chapter 1 is an introduction with a basic theory of manganites and their most
important properties.
34 CHAPTER 1. INTRODUCTION
• Chapter 2 contains overview of experimental works devoted to studying of charge,
spin and orbital ordering in this system.
• In Chapter 3 sample preparation and characterization procedures are described.
• Experiments with X-ray scattering are presented and discussed in Chapter 4.
• Chapter 5 is devoted to experiments with neutron scattering.
• The results are summarized in Chapter 6
• Appendix contains some additional information such as resonant X-ray scattering
mesh-scans, neutron scattering patterns and credits.
1.2 Structure
The mixed-valence compounds of the type La A MnO , where A is a bivalent atom1−x x 3
(for example Sr, Ca, Ba, etc.,) are the most investigated among manganites. They have√
a typical Pbnm orthorhombic perovskite structure with space group Pbnm with aa 2×√
b 2×2c supercell. In the ideal perovskite structure the centre of the cell is randomly
occupied by a large lantanide cation or an alkali cation. At the corners there are smaller
Mn ions, the oxygen ions aresituated at the centres of the cubic edges. (See. Figure 1.1).
These compounds can be regarded as solid state solutions, for example of LaMnO3
2− 2−3+ 3+ 2+ 4+and SrMnO with La Mn O and Sr Mn O ion valence states, respectively.3 3 3
3+ 3+ 2−2+ 4+The intermediate compositions have a valence structure (La Sr )(Mn Mn O )1−x x 1−x x 3
4 3containing trivalent (3d ) and tetravalent (3d ) manganese ions. Thus, doping the parent
compoundLaMnO withabivalentelementatconcentration x producesanequalamount3
of holes in the 3d band of the material (for x< 0.5). For x> 0.5, the compound can be
regarded as the parent compound SrMnO doped with electrons of concentration 1−x.3
1.3 The Influence of Cubic Field
The ground states of the Mn-ions consist of five-fold degenerate 3d-orbits. If one such
ion is situated in octahedral oxygen neighborhood, it splits due to the influence of the1.4. JAHN-TELLER EFFECT 5
Figure 1.1: The idealized perovskite unit-cell of manganese and Jahn-Teller distortions.
cubic crystal field (CF) into two energy states, t and e , with two and three orbitals2g g
respec