The presented thesis explores the magnetoelectric (ME) coupling in multiferroic thin film multilayers of BaTiO3 (BTO) and BiFeO3 (BFO). Multiferroics possess more than one ferroic order parameter, in this case ferroelectricity and anti-ferromagnetism. Cross-coupling between these otherwise separate order parameters promises great advantages in the fields of multistate memory, spintronics and even medical applications. The first major challenge in this field of study is the rarity of multiferroics. Second, most known multiferroics, both intrinsic and extrinsic in nature, possess very low ME coupling coefficients. In previous studies conducted
by our group, BTO-BFO multilayers deposited by pulsed laser deposition (PLD) showed a ME coupling coefficient αME enhanced by one order of magnitude, when compared to single-layers of the intrinsic multiferroic BFO. However, the mechanism of ME coupling in such heterostructures is poorly understood until now. In this thesis, we used a selection of structural, chemical, electrical and magnetic measurements to maximize the αME-coefficient and shed light on the origin of this enhanced ME effect.
The comparison of BTO-BFO multilayers over single-layers revealed not only enhanced ME-coupling, but also reduced mosaicity, roughness and leakage current density in multilayers. Following a parametric sample optimization, we achieved an atomically smooth interface roughness and vast improvements in the ferroelectric properties by introducing a shadow mask in the PLD process. We measured the highest αME-value so far of 480 Vcm-1Oe-1 for a multilayer with a double-layer thickness of only 4.6 nm, two orders of magnitude larger than the coefficient of 4 Vcm-1Oe-1 measured for BFO single-layers. The αME-coefficient in these multilayers stands in an inverse correlation with the double-layer thickness ddl. The influence of oxygen pressure during growth and BTO-BFO ratio on αME was shown to be neglible in comparison to that of ddl. From the characteristic dependencies of αME on magnetic bias field, temperature and ddl, we concluded the existence of an interface-driven coupling mechanism in BTO-BFO multilayers.:1 Introduction
2 Theory of Multiferroic Magnetoelectrics
2.1 Primary Ferroic Properties
2.2 Magnetoelectric Coupling
3 Materials
3.1 The General Structure of Perovskites ABX3
3.2 Strontium Titanate SrTiO3
3.3 Barium Titanate BaTiO3
3.4 Bismuth Ferrite BiFeO3
3.5 Heterostructures Based on BiFeO3
4 Experimental Section
4.1 Thin Film Fabrication
4.2 X–Ray Diffraction
4.3 Microscopic Techniques
4.4 Chemical Analysis Techniques
4.5 Ferroelectric Characterization
4.6 Magnetic Property Measurements
4.7 Measurement of the Magnetoelectric Coupling Coefficient
5 BaTiO3–BiFeO3 Heterostructures
5.1 General Properties of Single-Layers and Multilayers of BTO and BFO
5.2 PLD–Growth of BaTiO3–BiFeO3 Multilayers
5.3 Manipulation of Multilayer Properties through Design
5.4 Effectiveness of Eclipse–PLD
5.5 Enhanced ME Effect in BaTiO3–BiFeO3 Multilayers
6 Summary and Outlook
A Magnetoelectric Measurement Setup
B Magnetic Background Measurements
C Polarized Neutron Reflectometry
Literature
Own and Contributed Work
Acknowledgement
Erratum
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:73842 |
| Date | 12 February 2021 |
| Creators | Hohenberger, Stefan |
| Contributors | Universität Leipzig |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/acceptedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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