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Abstract: Transition metal oxides exhibit a variety of physical properties due to their high tunability of spin, orbital, and charge degrees of freedom. This thesis studies two complex, transition metal oxide thin film/substrate systems, the NiFe2O4/Nb:SrTiO3 (NFO/STO) and the La0.3Sr0.7MnO3/Nb:SrTiO3 (LSMO/ STO). NFO is a ferrimagnetic insulator. LSMO is a ferromagnetic half metal and below a critical thickness transitions to an insulator. However, the physical properties of those complex oxides heterostructures are often directly linked to the quality of the thin oxide film and the interactions between the oxide thin film and its substrate at the interface. In this thesis the ultra thin NFO film growth via pulsed laser deposition is optimized. First those optimized, smooth and single crystalline NFO films are studied with lab measurement methods to ensure a high film quality. In a next step their stoichiometry and crystalline occupancy is studied via hard X-ray photoemission (HAXPES) and X-ray absorption spectroscopy, which were recorded at the large scale synchrotron BESSY II, PETRA III and the DIAMOND Light source. The stoichiometry and occupancy is found to match the theoretical expectations of the NFO oxide in the inverse spinel structure. The magnetic response of the NFO/STO heterostructure is researched with the element selective X-ray magnetic circular dichroism (XMCD). Due to the proximity effect the Ti signal mimics the ferrimagnetic response of the NFO thin film. The electronic properties of Au/NFO/STO and Pt/LSMO/STO devices were investigated via in-operando HAXPES measurements. In-operando HAXPES is recording HAXPES spectra during a voltage is applied to the sample. The band alignment and band bending are investigated by applying Kraut' s method to the HAXPES spectra.

Open Access

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Abstract: Magnetic skyrmions are vortex-like spin textures, which are usually treated as two-dimensional objects. In their lattice state, they form well-ordered, hexagonal structures, which have been studied in great detail. To obtain a three-dimensional (3D) skyrmion crystal, these planes can be envisaged to be stacked up forming skyrmion strings in the third dimension. Here, we report the observation of a 3D skyrmion phase in Cu2OSeO3 by carrying out reciprocal space mapping in resonant elastic x-ray scattering. We observe regions in the magnetic field-cooling phase diagram in which the skyrmion phase apparently coexists with the conical phase. However, such a coexistence is forbidden due to symmetry arguments. Instead, the skyrmion strings themselves are periodically modulated along their axes, as confirmed by micromagnetic simulations. The periodic modulation is in fact a necessary consequence of the evolution of the skyrmion phase out of the conical state and should therefore be a universal property of skyrmion strings in chiral helimagnets.

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Abstract: We report a comprehensive study on the magnetic ground state of La 1.5 Ca 0.5 Co O 4 combining single crystal neutron diffraction and resonant magnetic x-ray scattering at the Co L 2 , 3 edges. Three single-crystal samples obtained from the same boule were investigated exhibiting magnetic phase transitions from a high-temperature paramagnetic phase to an antiferromagnetic phase at T N ≈ 52 K . Single crystal neutron diffraction reveals that the crystal structure at room temperature shows an orthorhombic A -centered lattice but with a and b axes almost equal in length. The structural phase transition (charge-ordering-like) from the parent tetragonal cell takes place above 523 K into the space group A 2 m m where two nonequivalent compressed and expanded Co O 6 octahedra are ordered showing a checkerboard pattern in the a b plane. The charge segregation between the nonequivalent Co sites is about 0.4(1) electrons. Resonant magnetic x-ray reflections indexed as ( 1 / 4 , 1 / 4 , 0 ) t , ( 1 / 4 , 1 / 4 , 1 ) t , and ( 1 / 4 , 1 / 4 , 1 / 2 ) t in the parent tetragonal cell were observed at low temperature at the Co L 2 , 3 -edge energy range. The resonant spectral shape, with a noticeable absence of any resonant enhancement at the Co L 2 edge, indicates that only Co 2 + -like ions participate in the magnetic ordering. The polarization analysis discloses that the orientation of Co magnetic moments is the same for the three magnetic orders and they are long-range ordered along the diagonal in the a b plane of the parent tetragonal cell with a slight tilt in the c axis. Despite the onset temperatures for the three resonant magnetic reflections being the same, ≈ 55 K , different thermal behavior is observed between ( 1 / 4 , 1 / 4 , 1 / 2 ) t and ( 1 / 4 , 1 / 4 , L ) t ( L = integer ) reflections whose intensities maximize at different temperatures, suggesting the coexistence of two magnetic arrangements. Moreover, the intensity of the ( 1 / 4 , 1 / 4 , 1 / 2 ) t magnetic reflection is at least ten times larger than that of the ( 1 / 4 , 1 / 4 , L ) t ( L = integer ) ones. On the other hand\, neutron diffraction measurements only detect a single type of antiferromagnetic ordering following the propagation vector k = ( 1 / 4 , 1 / 4 , 1 / 2 ) t that involves half of the Co atoms in the unit cell. We conclude that the bulk magnetic order in La 1.5 Ca 0.5 Co O 4 corresponds then to this propagation vector k = ( 1 / 4 , 1 / 4 , 1 / 2 ) t while ( 1 / 4 , 1 / 4 , 0 ) t and ( 1 / 4 , 1 / 4 , 1 ) t magnetic reflections correspond to a minority magnetic phase that must be due to changes in the oxygen stoichiometry near the surface.

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Abstract: Over the past four years, the magnetism of 2D magnets has been extensively studied by the full arsenal of probing techniques. 2D magnets can be incorporated to form heterostructures with clean and sharp interfaces, which gives rise to exotic phenomena as a result of the interfacial proximity effect. Here we report a detailed study of the spin (ms) and orbital (ml) moments of an epitaxial (CrSb/Fe3GeTe2)6 superlattice. The synchrotron-radiation based x-ray magnetic circular dichroism (XMCD) technique was performed to probe the microscopic magnetic properties of the superlattices in an elemental resolved manner. We unambiguously obtained a bulk-like moment of Fe3GeTe2 i.e., ms = 1.58 ± 0.2 μB/Fe and ml = 0.22 ± 0.02 μB/Fe. Future works to explore the tuning of the spin polarized band structure of 2D ferromagnetic superlattices will be of great interest and can have strong implications for both fundamental physics and the emerging spintronics technology.

Open Access

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Abstract: Magnetic skyrmions are topologically non-trivial, swirling magnetization textures that form lattices in helimagnetic materials. These magnetic nanoparticles show promise as high efficiency next-generation information carriers, with dynamics that are governed by their topology. Among the many unusual properties of skyrmions is the tendency of their direction of motion to deviate from that of a driving force; the angle by which they diverge is a materials constant, known as the skyrmion Hall angle. In magnetic multilayer systems, where skyrmions often appear individually, not arranging themselves in a lattice, this deflection angle can be easily measured by tracing the real space motion of individual skyrmions. Here we describe a reciprocal space technique which can be used to determine the skyrmion Hall angle in the skyrmion lattice state, leveraging the properties of the skyrmion lattice under a shear drive. We demonstrate this procedure to yield a quantitative measurement of the skyrmion Hall angle in the room-temperature skyrmion system FeGe, shearing the skyrmion lattice with the magnetic field gradient generated by a single turn Oersted wire.