Die magnetischen Eigenschaften von Li2CuO2 sind seit mehr als zwei Jahrzehnten Gegenstand theoretischen und experimentellen Interesses. Über die genaue Natur der magnetischen Wechselwirkungen in diesem Isolator konnte jedoch keine Einigkeit erzielt werden. Während das Material von Seiten theoretischer Untersuchungen als quasi-eindimensionaler Magnet mit starken ferromagnetischen Kopplungen entlang der Kette verstanden wurde, legten experimentelle Studien dominierende dreidimensionale Zwischenkettenkopplungen nahe.
Im Rahmen dieser Dissertation werden auf der Grundlage von Untersuchungen des magnetischen Anregungsspektrums mittels inelastischer Neutronenstreuung und dessen Analyse innerhalb eines Spinwellenmodels die führenden magnetischen Wechselwirkungen in Li2CuO2 bestimmt. Es wird zweifelsfrei nachgewiesen, dass das Material eine quasi-eindimensionale Spinkettenverbindung darstellt. Insbesondere kann die Konkurrenz von ferro- und antiferromagnetischen Wechselwirkungen entlang der Ketten nachgewiesen werden. Die Anwendbarkeit einer Spinwellenanalyse dieses niedrigdimensionalen Spin-1=2 Systems wird gezeigt. Das magnetische Phasendiagramm wird mittels Messungen von spezifischer Wärme, thermischer Ausdehnung und Magnetostriktion sowie der Magnetisierung in statischen und gepulsten Magnetfeldern untersucht und im Bezug auf die Austauschwechselwirkungen diskutiert. Aufgrund seiner einfachen kristallographischen und magnetischen Struktur stellt Li2CuO2 ein potentiell wertvolles Modellsystem in der Klasse der Spinkettenverbindungen mit konkurrierenden ferro- und antiferromagnetischen Wechselwirkungen dar.:1. Motivation 9
I. Introduction 13
2. Li2CuO2 15
2.1. ... as a cuprate 15
2.2. ... as a quasi-one dimensional magnet 17
2.3. Literature review on magnetic properties 21
2.3.1. Crystallographic structure 21
2.3.2. Magnetic structure and magnetic properties 21
3. Employed experimental techniques 25
3.1. Thermodynamic studies 25
3.2. Inelastic magnetic neutron scattering 28
4. Sample properties and characterization 33
4.1. A sample for INS 33
4.2. Crystal growth and characterization 36
II. Inelastic neutron scattering studies 41
5. Magnon excitations & spin-wave analysis 43
5.1. Linear spin-wave model 44
5.2. Experimental setup 46
5.3. Magnon dispersion for q || b* 46
5.3.1. Thermal neutrons 46
5.3.2. Cold neutrons 49
5.4. Magnon dispersion for q b* 52
5.5. Magnon dispersion at the zone boundary 55
5.6. Spin-wave analysis 58
5.7. Frustration of inter-chain couplings 64
6. Low energy excitations 69
III. Studies on thermodynamic properties 79
7. The magnetic phase diagram 81
7.1. High temperature short range order 83
7.2. The antiferromagnetic phase 85
7.3. The meta-magnetic transition and intermediate phase 88
7.4. Low temperature anomalies 93
7.4.1. Weak ferromagnetism 94
7.4.2. Anomalous magnetization at low T 100
8. Magnetization studies 105
8.1. Magnetization M(T) 105
8.2. Magnetization M(H) 110
9. Analysis of the magnetic specific heat 117
9.1. Estimate of phononic specific heat 118
9.2. Fluctuations, correlations near TN 120
9.3. Entropy 124
9.4. Specific heat at low temperature 129
10. Magneto-elastic coupling 133
10.1. Remarks on the measurement setup 133
10.2. Uniaxial pressure dependence of TN 135
10.3. Exploration of the easy axis magnetic phase diagram 139
10.3.1. Low temperature magnetostriction 139
10.3.2. Comprehensive survey of thermal expansion and magnetostriction data 141
10.3.3. A phenomenological model 146
10.4. Thermal expansion in magnetic fifields along the hard axes 150
IV. Conclusion 153
11.Summary and Outlook 155
V. Appendix 159
A. Supplementary data 161
A.1. Excitation spectrum in applied magnetic fifield 161
A.2. Low temperature specific heat 163
A.3. Pressure dependence of TN for H||a = 12T 163
A.4. Pressure dependence of magnetization 164
Bibliography 180 / The magnetic properties of Li2CuO2 have attracted interest since more than two decades, both in theory and experiment. Despite these efforts, the precise nature of the magnetic interactions in this insulator remained an issue of controversial debate. From theoretical studies, the compound was understood as a quasi-one-dimensional magnet with strong ferromagnetic interactions along the chain, while in contrast, experimentally studies suggested dominant three-dimensional inter-chain interactions.
In this thesis, the leading magnetic exchange interactions of Li2CuO2 are determined on the basis of a detailed inelastic neutron scattering study of the magnetic excitation spectrum, analyzed within spin-wave theory. It is unequivocally shown, that the material represents a quasi-one-dimensional spin-chain compound. In particular, the competition of ferro- and antiferromagnetic interactions in the chain has been evidenced. The applicability of a spin-wave model for analysis of this low-dimensional spin-1=2 system is shown. The magnetic phase diagram of Li2CuO2 is studied by specific heat, thermal expansion and magnetostriction measurements as well as magnetization measurements in both static and pulsed magnetic fifields. The phase diagram is discussed with respect to the exchange interactions. With its simple crystallographic and magnetic structure, Li2CuO2 may serve as a worthwhile model system in the class of spin-chain compounds with competing ferromagnetic and antiferromagnetic interactions.:1. Motivation 9
I. Introduction 13
2. Li2CuO2 15
2.1. ... as a cuprate 15
2.2. ... as a quasi-one dimensional magnet 17
2.3. Literature review on magnetic properties 21
2.3.1. Crystallographic structure 21
2.3.2. Magnetic structure and magnetic properties 21
3. Employed experimental techniques 25
3.1. Thermodynamic studies 25
3.2. Inelastic magnetic neutron scattering 28
4. Sample properties and characterization 33
4.1. A sample for INS 33
4.2. Crystal growth and characterization 36
II. Inelastic neutron scattering studies 41
5. Magnon excitations & spin-wave analysis 43
5.1. Linear spin-wave model 44
5.2. Experimental setup 46
5.3. Magnon dispersion for q || b* 46
5.3.1. Thermal neutrons 46
5.3.2. Cold neutrons 49
5.4. Magnon dispersion for q b* 52
5.5. Magnon dispersion at the zone boundary 55
5.6. Spin-wave analysis 58
5.7. Frustration of inter-chain couplings 64
6. Low energy excitations 69
III. Studies on thermodynamic properties 79
7. The magnetic phase diagram 81
7.1. High temperature short range order 83
7.2. The antiferromagnetic phase 85
7.3. The meta-magnetic transition and intermediate phase 88
7.4. Low temperature anomalies 93
7.4.1. Weak ferromagnetism 94
7.4.2. Anomalous magnetization at low T 100
8. Magnetization studies 105
8.1. Magnetization M(T) 105
8.2. Magnetization M(H) 110
9. Analysis of the magnetic specific heat 117
9.1. Estimate of phononic specific heat 118
9.2. Fluctuations, correlations near TN 120
9.3. Entropy 124
9.4. Specific heat at low temperature 129
10. Magneto-elastic coupling 133
10.1. Remarks on the measurement setup 133
10.2. Uniaxial pressure dependence of TN 135
10.3. Exploration of the easy axis magnetic phase diagram 139
10.3.1. Low temperature magnetostriction 139
10.3.2. Comprehensive survey of thermal expansion and magnetostriction data 141
10.3.3. A phenomenological model 146
10.4. Thermal expansion in magnetic fifields along the hard axes 150
IV. Conclusion 153
11.Summary and Outlook 155
V. Appendix 159
A. Supplementary data 161
A.1. Excitation spectrum in applied magnetic fifield 161
A.2. Low temperature specific heat 163
A.3. Pressure dependence of TN for H||a = 12T 163
A.4. Pressure dependence of magnetization 164
Bibliography 180
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:25633 |
| Date | 27 June 2011 |
| Creators | Lorenz, Wolfram |
| Contributors | Büchner, Bernd, Klingeler, Rüdiger, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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