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Dissertation, RWTH Aachen University, 2021; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen (2021). = Dissertation, RWTH Aachen University, 2021, 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 $\delta$, Published by RWTH Aachen University, Aachen

Proximity effects in superconductor/ferromagnet thin film heterostructures are ahighly topical issue due to their potential application in superconducting spin valvesor fluxonic devices [1, 2]. Physical properties can be controlled by an appliedmagnetic field and emerge for example as stray-field generated domainsuperconductivityor spin-triplet correlations. Our goal is to investigate their interplayand tunability by an external magnetic field. We use a heterostructure system ofNb/FePd with varying strength of magnetocrystalline anisotropy and a lateral domainstructure, grown by molecular beam epitaxy.On the one hand, macroscopic magnetoelectric transport measurements reveal aconfined superconducting state due to the stray fields of L10-ordered FePd. On theother hand, direct proximity effects at the Nb/FePd interface with a non-collinearmagnetization presumably lead to the generation of spin-triplet Cooper paircomponents with long penetration depth within the ferromagnetic layer [3]. PolarizedGrazing-Incidence Small-Angle Neutron Scattering (GISANS) probe exchangemechanisms on the microscopic scale and reveal a change in the ferromagneticdomain pattern by an onset of domain-wall-superconductivity. This mechanismcannot be revealed by macroscopic magnetization measurements, which makesGISANS the method of choice for detecting inverse superconducting proximityeffects.

New journal of physics 23(6), 063043 (2021). doi:10.1088/1367-2630/ac04c7, Published by Dt. Physikalische Ges., [Bad Honnef]

Magnetoelectric (ME) coupling in La0.7Sr0.3MnO3/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (LSMO/PMN–PT (001)) has been probed in the past years to identify the underlying mechanism behind it. PMN–PT, which is well known for its excellent piezoelectric properties, also exhibits ferroelectricity. This motivates our interest to differentiate which effect is dominant for this 'voltage control of magnetism'. Here, we present results for the ME coupling at different temperatures: 300 K and 80 K. In this article we discuss and explain, how the nature of ME coupling is influenced by different parameters such as magnetic field, electric field, directional dependence (hard axis, easy axis) and temperature. Owing to large lattice mismatch between LSMO and PMN–PT, the strain-mediated coupling is strongly prevalent, however the change in strain behaviour from butterfly loop to linear loop is observed as a function of temperature. ME measurements are performed along hard axis [100] and easy axis [110] of LSMO in the presence of remanent magnetic field which showcases the pure influence of electric field on the system, resulting in a combination of strain- and charge-mediated coupling. The magnetic depth profile is probed by polarized neutron reflectometry as a function of electric field which demonstrates the existence of an interlayer with reduced nuclear scattering length density and reduced magnetic scattering length density at the interface. From transmission electron microscopy, stoichiometric variations are observed due to the presence of Mn3O4 particles at the interface.

Transition metal oxides are an extremely interesting class of materials, exhibiting a wide range of properties, from ferromagnetism to antiferromagnetism, frominsulating to conducting, superconductivity, multiferroicity, and many more. One special system is strontium cobaltite (SrCoO$_{x}$). In its fully oxidized state SrCoO$_3$ it is a ferromagnetic, conducting perovskite. If, however, the oxygen content is changed slightly to SrCoO$_{2.5}$, the oxygen vacancies form channels, changing the crystalline structure to a brownmillerite, and the magnetic and conductive properties reverse to an antiferromagnetic insulator. These two crystal structures can be topotactically transformed into one another without destroying the crystallinity. This property makes SrCoO$_x$ suitable for a large variety of applications, e.g. as a cathode material and catalyst for redox reactions in fuel cells or magnetic switches via epitaxial strain. To take advantage of the unique possibilities of SrCoO$_x$ , particularly in the field of thin film devices, it is imperative to be able to grow Sr$_1$Co$_1$O$_x$ in the correct stoichiometry. The aim of this thesis is to find the correct deposition conditions for the growth of stoichiometric SrCoO$_x$ thin film samples via molecular beam epitaxy (MBE), like substrate temperature, cooling speed, pressure, oxygen power, and especially the Sr and Co deposition rates. Because the samples are prepared via MBE, which does not use a target with an already defined stoichiometry like sputter deposition or pulsed laser deposition, but rather the evaporation of elemental material, the stoichiometry of the samples depends on several factors. The individual growth rates for Sr and Co do not only depend on the amount of evaporated material, but also on the sticking coeficient of the material on the substrate. As the sticking coeficient is also temperature dependent, there is a large parameter space that needs to be investigated. Therefore, samples with varying Co/Sr deposition rate ratios at different deposition temperatures were produced. The stoichiometry was analysed by Rutherford backscattering spectroscopy (RBS). The crystallinity of the samples was studied by low energy electron diffraction (LEED), reflection high energy electron diffraction (RHEED) and X-ray diffraction (XRD). To investigate the surface topography, atomic force microscopy (AFM) was performed. X-ray reflectometry (XRR) was used to determine the global surface roughness and film thickness.