Emmanuel Kentzinger

While stoichiometric SrRuO3 (SRO) is a metallic itinerant ferromagnet with relatively homogeneous magnetization, Ru deficiency provides a powerful route to alter its electronic transport and depth-dependent magnetic properties. Ru-deficient SRO thin films grown by radio frequency high-oxygen pressure sputtering were investigated using a combination of x-ray reflectivity, polarized neutron reflectometry, off-specular neutron scattering, scanning transmission electron microscopy with energy-dispersive x-ray spectroscopy, electrical transport, and magnetometry. Structural and compositional analyses reveal that Ru deficiency is intrinsic to the films, with an enhanced deficiency at the interfaces. As a result, coherent electronic transport is suppressed and saturation magnetization is reduced, while the Curie temperature remains largely unaffected, placing Ru-deficient SRO in a regime consistent with ferromagnetic insulator-like behavior. Depth- and lateral-resolved magnetic measurements further indicate that the interfacial regions remain ferromagnetic, with a reduced in-plane magnetic projection consistent with enhanced perpendicular magnetic anisotropy that keeps the local magnetization predominantly out-of-plane. Our results establish Ru deficiency as a key control parameter governing transport, magnetization, and anisotropy in SRO thin films and highlight defect and interface engineering as powerful routes to tailor interfacial magnetism in correlated oxide heterostructures.

Heterostructures based on superconductors and ferromagnets show strong potential for innovating device applications in spintronics and quantum computing. SrRuO₃ (SRO) has recently attracted much attention among transition metal oxides because it is the only 4d oxide to exhibit itinerant ferromagnetism and metallic conductivity. YBa₂Cu₃O_7−x (YBCO) is one of the most studied high critical temperature (high-T _c) superconductors with a wide range of potential applications. We report morphological, structural, magnetic and magnetotransport characterization of YBCO/SRO (HS-YS) and SRO/YBCO (HS-SY) heterostructures grown on low miscut SrTiO₃ (001) single crystals by high oxygen pressure sputtering. All samples exhibit epitaxial growth with good crystal quality and sharp interfaces. The heterostructures exhibit T _c of 87 K and 57 K for HS-YS and HS-SY, respectively, both reduced compared to bulk YBCO (91 K). The reduction in T _c and intriguing features in magnetoresistance measurements around the onset of superconductivity are robust indicators that a proximity effect takes place in such heterostructures, and inspire further theoretical and experimental investigations.

The quest to realize new kinds of data storage devices has motivated recent studies in the field of magnetoelectric heterostructures. One of the most commonly investigated systems is Fe₃O₄/[Pb(Mg 1 / 3 Nb 2 / 3 ) O₃]_0.7–[PbTiO₃]_0.3 (PMN-PT), however, the interplay between different coupling mechanisms is not yet well understood. To disentangle the role of strain and polarisation influence in Fe₃O₄/PMN-PT, we report on magnetoelectric coupling measurements for different orientations of the applied magnetic field and for two different substrate cuts, PMN-PT(001) and PMN-PT(011). For Fe₃O₄/PMN-PT(011), having the sample aligned such that the magnetic field is parallel to the [01 1 ‾ ] easy axis leads to a remanent increase of the magnetisation for each electric field cycle. On the other hand, for the magnetic field along the [100] hard axis, the magnetisation follows a butterfly-like loop characteristic of strain coupling imparted by the substrate. For Fe₃O₄/PMN-PT(001), the magnetoelectric effect is a superposition of the observed behaviour of both in-plane directions in Fe₃O₄/PMN-PT(011). The magnetisation shows an initial remanent increase followed by a butterfly like loop. Polarised neutron reflectometry measurements on Fe₃O₄/PMN-PT(011) shows no difference between the behaviour at the interface and the bulk of the film and no decline of the interaction further away from the shared interface. Our results demonstrate the role of strain and polarisation on the magnetisation of the Fe₃O₄ layer and provide a clear step towards the design of future magnetoelectric systems.

The miniaturization, high performance, energy efficiency, and new added functionalities are the essential drivers of modern information data storage and processing technologies. Polyoxometalates (POMs) characterized by atomically well‐defined structures with discrete energy levels and the ability to undergo redox transformations are viewed as promising active components for the integration into the next‐generation (beyond‐CMOS) hybrid nanoelectronics. Herein, new fundamental insights into the application of organically augmented POMs on conducting surfaces are offered. Three key findings resulting from scanning probe investigations combined with integral spectroscopic methods used to explore tris(alkoxo)‐ligated, vanadium‐containing Wells‐Dawson‐type POM structures on Au(111) are reported on. First, it is shown how the (OCH₂)₃C–R ligands, depending on the structurally exposed R group (R = CH₂SMe and NHCOC₆H₄SMe), influence the self‐assembly behavior of the synthesized POMs on gold. Second, the impact of the employed (OCH₂)₃C–R ligands and the determined assembly characteristics on the relative position of POM's electronic band structure against the Fermi level of the gold surface are explained. Third, the on‐surface conductance switching of single POM structures due to external electrical stimuli is demonstrated. The author's experimental efforts enable to discover highly sought‐after multi‐level resistive switching orchestrated by electrically accessible V(3d) states in the POM single‐molecules at room temperature in a narrow voltage range.

Using AFM and X-ray scattering technique (GTSAXS), the morphological transformation of supramolecular nanocomposite thin films induced by incorporated nanoparticles is revealed as a function of the nanoparticle concentration.

Cluster crystals are periodic structures with lattice sites occupied by several, overlapping building blocks, featuring fluctuating site occupancy, whose expectation value depends on thermodynamic conditions. Their assembly from atomic or mesoscopic units is long-sought-after, but its experimental realization still remains elusive. Here, we show the existence of well-controlled soft matter cluster crystals. We fabricate dendritic-linear-dendritic triblock composed of a thermosensitive water-soluble polymer and nanometer-scale all-DNA dendrons of the first and second generation. Conclusive small-angle X-ray scattering (SAXS) evidence reveals that solutions of these triblock at sufficiently high concentrations undergo a reversible phase transition from a cluster fluid to a body-centered cubic (BCC) cluster crystal with density-independent lattice spacing, through alteration of temperature. Moreover, a rich concentration-temperature phase diagram demonstrates the emergence of various ordered nanostructures, including BCC cluster crystals, birefringent cluster crystals, as well as hexagonal phases and cluster glass-like kinetically arrested states at high densities.

Magnetoelectric (ME) coupling in La_0.7Sr_0.3MnO₃/Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ (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.

Accurate characterization of the nanocrystal shape with high statistical relevance is essential for exploiting the strongly shape-dependent properties of cuboidal nanoparticles towards applications. This work presents the development of a new small-angle scattering form factor based on the superball geometry. The superball quantifies the characteristic rounding of corners and edges of cuboidalnanoparticles with a single parameter. Applied to small-angle scattering data of sufficiently monodisperse nanoparticles, the superball form factor enables differentiation between the effects of extendedparticle size distribution and irregular particle shape. The quantitative application of the superball form factor is validated against microscopy data for a series of monodisperse nanoparticles and implemented into the user-friendly, open source software Sasview.

Iron oxide nanoparticles are presently considered as main work horses for various applications including targeted drug delivery and magnetic hyperthermia. Several questions remain unsolved regarding the effect of size onto their overall magnetic behavior. One aspect is the reduction of magnetization compared to bulk samples. A detailed understanding of the underlying mechanisms of this reduction could improve the particle performance in applications. Here we use a number of complementary experimental techniques including neutron scattering and synchrotron X-ray diffraction to arrive at a consistent conclusion. We confirm the observation from previous studies of a reduced saturation magnetization and argue that this reduction is mainly associated with the presence of antiphase boundaries, which are observed directly using high-resolution transmission electron microscopy and indirectly via an anisotropic peak broadening in X-ray diffraction patterns. Additionally small-angle neutron scattering with polarized neutrons revealed a small non-magnetic surface layer, that is, however, not sufficient to explain the observed loss in magnetization alone.

We present results of a comprehensive investigation of two phenomena arising in superconductor(S)/ferromagnet(F) heterostructures of Nb on FePd with a lateral magnetic domain pattern: domain-superconductivity and spin-triplet Cooper pair generation. Resistivity measurements in a magnetic field applied out-of-plane to a Nb/FePd (S/F) sample with high magnetocrystalline anisotropy give evidence of stray field generated domain-wall- and reverse-domain-superconductivity. A corresponding bilayer comprising low magnetocrystalline anisotropy exhibits spin-triplet Cooper pair generation and a notable high variation of the S critical temperature due to spin-triplet generation (ΔT _c) of 100 mK in an in-plane applied field. Using reference samples we can clearly distinguish stray field from proximity effects. The relevance of the characteristic S and F length scales related to the observed proximity effects is discussed.