Ulrich Rüdiger

Large-Scale Synthesis of Single-Crystalline Iron Oxide Magnetic Nanorings

We present an innovative approach to the production of single-crystal iron oxide nanorings employing a solution-based route. Single-crystal hematite (alpha-Fe2O3) nanorings were synthesized using a double anion-assisted hydrothermal method (involving phosphate and sulfate ions), which can be divided into two stages: (1) formation of capsule-shaped alpha-Fe2O3 nanoparticles and (2) preferential dissolution along the long dimension of the elongated nanoparticles (the c axis of alpha-Fe2O3) to form nanorings. The shape of the nanorings is mainly regulated by the adsorption of phosphate ions on faces parallel to c axis of alpha-Fe2O3 during the nanocrystal growth, and the hollow structure is given by the preferential dissolution of the alpha-Fe2O3 along the c axis due to the strong coordination of the sulfate ions. By varying the ratios of phosphate and sulfate ions to ferric ions, we were able to control the size, morphology, and surface architecture to produce a variety of three-dimensional hollow nanostructures. These can then be converted to magnetite (Fe3O4) and maghemite (gamma-Fe2O3) by a reduction or reduction-oxidation process while preserving the same morphology. The structures and magnetic properties of these single-crystal alpha-Fe2O3, Fe3O4, and gamma-Fe2O3 nanorings were characterized by various analytical techniques. Employing off-axis electron holography, we observed the classical single-vortex magnetic state in the thin magnetite nanorings, while the thicker rings displayed an intriguing three-dimensional magnetic configuration. This work provides an easily scaled-up method for preparing tailor-made iron oxide nanorings that could meet the demands of a variety of applications ranging from medicine to magnetoelectronics.

Rashba Effect in the Graphene/Ni(111) System

We report on angle-resolved photoemission studies of the electronic pi states of high-quality epitaxial graphene layers on a Ni(111) surface. In this system the electron binding energy of the pi states shows a strong dependence on the magnetization reversal of the Ni film. The observed extraordinarily large energy shift up to 225 meV of the graphene-derived pi band peak position for opposite magnetization directions is attributed to a manifestation of the Rashba interaction between spin-polarized electrons in the pi band and the large effective electric field at the graphene/Ni interface. Our findings show that an electron spin in the graphene layer can be manipulated in a controlled way and have important implications for graphene-based spintronic devices.

Direct Observation of Domain-Wall Configurations Transformed by Spin Currents

Direct observations of current-induced domain-wall propagation by spin-polarized scanning electron microscopy are reported. Current pulses move head-to-head as well as tail-to-tail walls in submicrometer Fe20Ni80 wires in the direction of the electron flow, and a decay of the wall velocity with the number of injected current pulses is observed. High-resolution images of the domain walls reveal that the wall spin structure is transformed from a vortex to a transverse configuration with subsequent pulse injections. The change in spin structure is directly correlated with the decay of the velocity.

Graphene-protected iron layer on Ni(111)

Here we report a photoemission study of the Fe intercalation underneath a graphene layer on Ni(111). The process of intercalation was monitored by means of x-ray photoemission of corresponding core levels as well as ultraviolet photoemission of the graphene-derived π states in the valence band. Thin fcc Fe layers (2–5 ML thickness) at the interface between a graphene capping layer and Ni(111) form epitaxial films passivated from the reactive environment.

Size dependence of the exchange bias field in NiO/Ni nanostructures

NiO/Ni wires have been investigated as a function of their width in order to investigate the size dependence of exchange bias. The samples have been prepared by e-beam lithography and ion milling of ion beam sputtered thin films. For NiO/Ni wires narrower than 3 μm, the exchange bias field significantly depends on the wire width. A NiO/Ni film shows an exchange bias field of −78 Oe whereas the exchange bias field of wires narrower than 200 nm is reduced to approximately −40 Oe. The coercive field of the NiO/Ni film is 28 Oe and increases to 210 Oe for the narrowest wires. The decrease of the exchange bias field for the narrowest wires is consistent with a recent microscopic model of exchange bias where the appearance of a unidirectional anisotropy in ferromagnet/antiferromagnet bilayers has been attributed to the presence of antiferromagnetic domains in the bulk of the antiferromagnet. A possible onset of a transition from a multidomain to a single-domain state of the antiferromagnet as a function of the ...

Nucleation and growth of nickel nanoclusters on graphene Moiré on Rh(111)

Regularly sized Ni nanoclusters (NCs) have been grown on a graphene Moire on Rh(111). Using scanning tunneling microscopy, we determine that initial growth of Ni at 150 K leads to preferential nucleation of monodispersed NCs at specific sites of the Moire superstructure. However, a defined long-range ordering of NCs with increasing coverage is not observed. Room temperature Ni deposition leads to the formation of flat triangular-shaped islands which are well-matched to the Moire registry.

Direct observation of domain-wall pinning at nanoscale constrictions

In a combined experimental and numerical study, we determine the details of the pinning of domain walls at constrictions in permalloy nanostructures. Using high spatial-resolution (<10nm) electron holography, we image the spin structure of geometrically confined head-to-head domain walls at constrictions. Low-temperature magnetoresistance measurements are used to systematically ascertain the domain-wall depinning fields in constrictions down to 35 nm width. The depinning fields increase from 60 to 335 Oe with decreasing constriction width and depend on the wall spin structure. The energy barrier to depin the wall from the constriction is quantitatively determined and comparison with the depinning field strength allows us to gauge the energy barrier height of the pinning potential.

Magnetotransport and magnetic domain structure in compressively strained colossal magnetoresistance films

We have studied the magnetoresistance (MR) of compressively strained La0.7Sr0.3MnO3 (LSMO) films in various magnetic states in order to understand the role of magnetic domain structure on magnetotransport. In thin films of LSMO on (100) LaAlO3, the perpendicular magnetic anisotropy results in perpendicularly magnetized domains with fine scale ∼200 nm domain subdivision, which we image directly at room temperature using magnetic force microscopy. The main MR effects can be understood in terms of bulk colossal MR and anisotropic MR. We also find evidence for a small domain wall contribution to the MR, which is an order of magnitude larger than expected from a double exchange model.

Observation of thermally activated domain wall transformations

The spin structure of head-to-head domain walls in Ni80Fe20 structures is studied using high-resolution photoemission electron microscopy. The quantitative phase diagram is extracted from these measurements and found to exhibit two phase boundaries between vortex and transverse domain walls. The results are compared with available theoretical predictions and micromagnetic simulations and differences to the experiment are explained, taking into account thermal excitations. Temperature-dependent measurements show a thermally activated transformation of transverse to vortex domain walls in 7 nm thick and 730 nm wide structures at a transition temperature between 260 °C and 310 °C, which corresponds to a nucleation barrier height for a vortex wall between 6.7×10−21J and 8.0×10−21J.

Domain wall resistivity in epitaxial thin film microstructures

This article reviews our recent experimental studies of domain wall (DW) resistivity in epitaxial transition metal ferromagnetic thin film microstructures with stripe domains. The results are presented and analysed in the context of models of DW scattering and conventional magnetoresistance (MR) effects in ferromagnetic metals. Microstructures of progressively higher magnetic anisotropy and thus smaller DW widths have been studied, including; bcc Fe, hcp Co and L1◦ FePt. The magnetic domain structure of these materials have been investigated using magnetic force microscopy and micromagnetic simulations. In Fe and Co the dominant sources of low-field MR are ferromagnetic resistivity anisotropy, due to both anisotropic MR (AMR) and the Lorentz MR. In Fe, at low temperature, a novel negative DW contribution to the MR has been found. Hcp Co microstructures show a greater resistivity for current perpendicular to DWs than for current parallel to DWs, that is consistent with a small (positive) DW resistivity and a Hall effect mechanism. High anisotropy L1◦ FePt microstructures show strong evidence for an intrinsic DW contribution to the resistivity. Related studies and future directions are also discussed. (Some figures in this article are in colour only in the electronic version; see www.iop.org)