TY  - THES
AU  - Schöffmann, Patrick
TI  - Stoichiometric control and magnetoelectric coupling in artificial multiferroic heterostructures
VL  - 247
PB  - RWTH Aachen
VL  - Dissertation
CY  - Jülich
M1  - FZJ-2021-01966
SN  - 978-3-95806-575-8
T2  - Schriften des Forschungszentrums Jülich. Reihe Schlüsseltechnologien / Key Technologies
SP  - vii, 176 S.
PY  - 2021
N1  - Dissertation, RWTH Aachen, 2021
AB  - The demand for smaller and faster information storage media, new types of sensors and multifunctional devices has lead to a rush in research on transition metal oxides (TMO's). Strong electronic correlations in TMO's lead to a wealth of new and interesting effects. Particularly the use of artificial multiferroic heterostructures using advanced thin film growth techniques, has attracted a lot of interest, as they enable the tailoring of the physical properties of individual materials and hold the promise of new combined functionality. The subject of this thesis is control of the magnetisation in thin TMO films using the ferroelectric substrate [Pb(Mg$_{1/3}$Nb${_{2/3}}$)O$_{3}$]$_{0.7}$-[PbTiO$_{3}$]$_{0.3}$ (PMN-PT). The first part of this thesis is about the fabrication and investigation of SrCoO$_{3-{\delta}}$, which has different magnetic and conductive properties depending on the oxygen content. Importantly, the antiferromagnetic, insulating SrCoO$_{2.5}$ can be transformed reversibly in the ferromagnetic, metallic SrCoO$_{3}$. To grow epitaxial and stoichiometric Sr$_{1}$Co$_{1}$O$_{2.5}$ films by molecular beam epitaxy (MBE), the deposition parameter have to be precisely controlled. To determine the Co-to-Sr ratio $\textit{in-situ}$ and optimise it, Reflection High Energy Electron Diffraction (RHEED) can be used. The RHEED scattering pattern changes depending on the Co/Sr ratio and can be used to determine Co excess or deficiency and to grow stoichiometric Sr$_{1}$Co$_{1}$O$_{2.5}$ samples. High quality SrCoO$_{2.5}$ can be transformed to SrCoO$_{3}$ by heating in oxygen flow and the resulting films remain stable in vacuum. SrCoO$_{3-{\delta}}$ films (with $\delta$< 0.25) form three distinct magnetic phases and show an exchange bias effect. The magnetic phases are distributed uniformly throughout the film, as can be determined by Polarised Neutron Reflectometry (PNR). Due to a mismatch of lattice constants, SrCoO$_{2.5}$ films can't be grown directly on PMN-PT. However, using an La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ buffer layer, crystalline SrCoO$_{2.5}$/La$_{0.67}$Sr$_{0.33}$MnO$_{3}$/PMN-PT heterostructures can be grown. The second part of the dissertation is about the magnetoelectric coupling in Fe$_{3}$O$_{4}$/PMN-PT(001) and Fe$_{3}$O$_{4}$/PMN-PT(011) heterostructures grown by pulsed laser deposition. The strain and polarisation of the substrate with applied electric field mediate the coupling to the magnetisation in the ferrimagnetic layer. Using a qualitative model, the strength of the different contributions can be estimated. The substrate cut and orientation of the sample in the magnetic field has a strong influence on the magnetoelectric behaviour of the layer.
LB  - PUB:(DE-HGF)3 ; PUB:(DE-HGF)11
UR  - https://juser.fz-juelich.de/record/892131
ER  -