Sebastian Brück

Orbital reflectometry of oxide heterostructures

The occupation of d orbitals controls the magnitude and anisotropy of the inter-atomic electron transfer in transition-metal oxides and hence exerts a key influence on their chemical bonding and physical properties. Atomic-scale modulations of the orbital occupation at surfaces and interfaces are believed to be responsible for massive variations of the magnetic and transport properties, but could not thus far be probed in a quantitative manner. Here we show that it is possible to derive quantitative, spatially resolved orbital polarization profiles from soft-X-ray reflectivity data, without resorting to model calculations. We demonstrate that the method is sensitive enough to resolve differences of ~3% in the occupation of Ni e(g) orbitals in adjacent atomic layers of a LaNiO(3)-LaAlO(3) superlattice, in good agreement with ab initio electronic-structure calculations. The possibility to quantitatively correlate theory and experiment on the atomic scale opens up many new perspectives for orbital physics in transition-metal oxides.

XMCD studies on Co and Li doped ZnO magnetic semiconductors

ZnO doped with a few per cent (<10%) of magnetic ions such as Co exhibit room temperature (RT) ferromagnetism, transforming it into a very promising candidate for future spin electronic applications. We present x-ray magnetic circular dichroism (XMCD) spectroscopy, which has been used in total electron yield, total fluorescence yield, and reflection mode to investigate the origin of ferromagnetism in such diluted magnetic semiconductor materials in a surface, bulk and interface sensitive way, respectively. We investigated three different types of samples: ZnO doped with 5% Co, artificially layered films, and layered films with additional co-doping of 10% Li. These films are prepared by pulsed laser deposition (PLD) and characterized by superconducting quantum interference device (SQUID) magnetometry. Extensive XMCD measurements at the Co L2,3 and the O K edges, done with a very good signal-to-noise ratio, have revealed only cobalt paramagnetism and non magnetic oxygen signatures at low and high temperatures. We do not see any element specific signature for ferromagnetism at all. By exclusion, our results suggest oxygen vacancies as the intrinsic origin for RT ferromagnetism in doped ZnO.

Strain and composition dependence of orbital polarization in nickel oxide superlattices

A combined analysis of x-ray absorption and resonant reflectivity data was used to obtain the orbital polarization profiles of superlattices composed of four-unit-cell-thick layers of metallic LaNiO3 and layers of insulating RXO3 (R=La, Gd, Dy and X=Al, Ga, Sc), grown on substrates that impose either compressive or tensile strain. This superlattice geometry allowed us to partly separate the influence of epitaxial strain from interfacial effects controlled by the chemical composition of the insulating blocking layers. Our quantitative analysis reveal orbital polarizations up to 25%. We further show that strain is the most effective control parameter, whereas the influence of the chemical composition of the blocking layers is comparatively small.

Magnetic proximity effect in YBa2Cu3O7 / La2/3Ca1/3MnO3 and YBa2Cu3O7 / LaMnO3+δ superlattices

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa2Cu3O7 and ferromagnetic-metallic La0.67Ca0.33MnO3 or ferromagnetic-insulating LaMnO(3+δ). We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic La0.67Ca0.33MnO3 and almost absent for ferromagnetic-insulating LaMnO(3+δ). We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature.

Element Specific Monolayer Depth Profiling

The electronic phase behavior and functionality of interfaces and surfaces in complex materials are strongly correlated to chemical composition profiles, stoichiometry and intermixing. Here a novel analysis scheme for resonant X-ray reflectivity maps is introduced to determine such profiles, which is element specific and non-destructive, and which exhibits atomic-layer resolution and a probing depth of hundreds of nanometers.

Fe3O4/ZnO: A high-quality magnetic oxide-semiconductor heterostructure by reactive deposition

We demonstrate the epitaxial growth of Fe3O4 films on ZnO by a simple reactive deposition procedure using molecular oxygen as an oxidizing agent. X-ray photoelectron spectroscopy results evidence that the iron-oxide surface is nearly stoichiometric magnetite. X-ray diffraction results indicate monocrystalline epitaxy and almost complete structural relaxation. Scanning transmission electron micrographs reveal that the microstructure consists of domains which are separated by antiphase boundaries or twin boundaries. The magnetite films show rather slow magnetization behavior in comparison with bulk crystals probably due to reduced magnetization at antiphase boundaries in small applied fields.

Magnetic and electronic properties of the interface between half metallic Fe3O4 and semiconducting ZnO

We have investigated the magnetic depth profile of an epitaxial Fe3O4 thin film grown directly on a semiconducting ZnO substrate by soft x-ray resonant magnetic reflectometry (XRMR) and electron energy loss spectroscopy (EELS). Consistent chemical profiles at the interface between ZnO and Fe3O4 are found from both methods. Valence selective EELS and XRMR reveal independently that the first monolayer of Fe at the interface between ZnO and Fe3O4 contains only Fe3+ ions. Besides this narrow 2.5 A interface layer, Fe3O4 shows magnetic bulk properties throughout the whole film making highly efficient spin injection in this system feasible.

An advanced magnetic reflectometer

A new experimental setup dedicated to the measurement of soft-x-ray magnetic absorption spectroscopy and soft-x-ray resonant magnetic reflectometry (soft-XRMR) is presented. XRMR is the combination of standard x-ray reflectometry with x-ray magnetic circular dichroism which provides chemical and magnetic depth profiles of layered thin-film samples. This new diffractometer is optimized for a broad variety of sample systems. Therefore a balanced design focusing on high magnetic fields, low temperatures, and full freedom of rotation has been realized in UHV. First experimental results obtained on a NiCoO/Co bilayer sample are presented showing the potential of the setup.

The temperature-dependent magnetization profile across an epitaxial bilayer of ferromagnetic La2/3Ca1/3MnO3 and superconducting YBa2Cu3O7−δ

Epitaxial bilayers of ferromagnetic (FM) La2/3Ca1/3MnO3 (LCMO) and superconducting YBa2Cu3O7?? (YBCO) have been grown on single-crystalline SrTiO3 (STO) substrates by pulsed laser deposition. The manganese magnetization profile across the FM layer has been determined with high spatial resolution at low temperatures by x-ray resonant magnetic reflectivity (XRMR) performed at the BESSY II synchrotron light source of the Helmholtz Zentrum Berlin. It is found that not only the adjacent superconductor but also the substrate underneath influences the magnetization of the LCMO film at the interface at low temperatures. Both effects can be investigated individually by XRMR.

Absence of element specific ferromagnetism in Co doped ZnO investigated by soft X-ray resonant reflectivity

On the quest for the intrinsic origin of ferromagnetism (FM) in ZnO doped with a few percent transition metal, we show detailed X-ray resonant magnetic reflectivity (XRMR) measurements, performed at the Co L2,3 and the O K edges of pulsed laser deposition (PLD) prepared samples. These samples show ferromagnetism at room temperature (RT) (QUID: about 2μB /Co). But in contrast to the QUID measurements, element specific reflection measurements as a function of angle (θ-2θ scans) and energy (const. qz) do not show any sign of ferromagnetism. Therefore, we can exclude without doubt Co as a possible origin for FM in this system. Our results are in perfect agreement with earlier published XMCD data and strongly support the new proposed grain boundary based model for oxygen vacancies related RT-FM.