Ole Albrecht

Experimental evidence for an angular dependent transition of magnetization reversal modes in magnetic nanotubes

We report on the experimental and theoretical investigation of the magnetization reversal in magnetic nanotubes that have been synthesized by a combination of glancing angle and atomic layer deposition. Using superconducting quantum interference device magnetometry the angular dependence of the coercive fields is determined and reveals a nonmonotonic behavior. Analytical calculations predict the crossover between two magnetization reversal modes, namely, the movement of different types of domain boundaries (vortex wall and transverse wall). This transition, already known in the geometrical dependences of the magnetization reversal in various nanotubes, is found within one type of tube in the angular dependence and is experimentally confirmed in this work.

Structural, magnetic, and transport properties of Permalloy for spintronic experiments

Permalloy (Ni80Fe20) is broadly used to prepare magnetic nanostructures for high-frequency experiments where the magnetization is either excited by electrical currents or magnetic fields. Detailed knowledge of the material properties is mandatory for thorough understanding its magnetization dynamics. In this work, thin Permalloy films are grown by dc-magnetron sputtering on heated substrates and by thermal evaporation with subsequent annealing. The specific resistance is determined by van der Pauw methods. Point-contact Andreev reflection is employed to determine the spin polarization of the films. The topography is imaged by atomic-force microscopy, and the magnetic microstructure by magnetic-force microscopy. Transmission-electron microscopy and transmission-electron diffraction are performed to determine atomic composition, crystal structure, and morphology. From ferromagnetic resonance absorption spectra the saturation magnetization, the anisotropy, and the Gilbert damping parameter are determined. Co...

Regulated Oxidation of Nickel in Multisegmented Nickel–Platinum Nanowires: An Entry to Wavy Nanopeapods†

Oxidation of solid metal nanoobjects is a versatile approach to generating hollow metal oxide nanostructures. The mechanism for the solid-to-hollow conversions has been attributed to the Kirkendall effect, which describes an unbalanced interdiffusion of a thermal diffusion couple. When a metal nanoobject is exposed to oxygen at elevated temperatures, the outward diffusion of the metal cations is much faster than the inward diffusion of the oxygen anions through the oxide layer. As a result, a flux of vacancies is injected into the interior and gradually accumulates into a single void. The hollow metal oxide nanostructures formed in most cases are symmetric with a uniform wall thickness. The oxidation behavior of some metal elements is exceptional. For example, the complete oxidation of Ni nanoparticles in air resulted in hollow NiO nanospheres with an off-centered cavity. Similarly, irregular nanotubes with a fluctuant wall thickness were obtained by oxidizing Ni nanowires at high temperatures. 6a] It is verified that the sufficient mobility of vacancies in Ni induces the localization of limited void nuclei at random positions, which disequilibrate the following outward mass transfer of unconsumed Ni cores. The difficulty in forming homogeneous hollow NiO nanostructures hints that the rapid self-diffusion of injected vacancies in the Ni cores is a rather uncontrolled process. To date, it is very difficult only by oxidation to convert solid Ni nanoobjects into hollow NiO nanostructures of high uniformity. However, morphological regularity is of extreme importance for the reliable and reproducible applications of NiO nanostructures in fields ranging from resistive switching memory to sensors. Herein, we report a novel route to fabricate highly uniform wavy Pt/NiO hollow nanopeapods by regulating the oxidation of Ni in predesigned multisegmented Ni/Pt nanowires. It is revealed that the Ni/Pt interface is an efficient platform for manipulating the oxidation behavior of Ni that intrinsically presents a random manner. Moreover, this interface-tailored low-temperature oxidation strategy can be employed as a unique but general entry to novel wavy metal/ oxide nanopeapods relative to the approaches existent for nanopeapod construction. The multisegmented Ni/Pt nanowires with tailored parameters were prepared in a three-electrode electrochemical cell by pulsed electrodeposition using porous anodic aluminum oxide membranes as templates (see the Supporting Information). After the removal of the alumina templates by alkaline etching, dispersed straight nanowires with a smooth surface were obtained (Figure 1a). A magnified SEM view (Figure 1b) displays that the nanowires consist of

Local modes and two magnon scattering in ordered permalloy antidot arrays

Antidot arrays consisting of periodically arranged holes in continuous magnetic films exhibit periodic demagnetization field distributions, which have large impact on the magnetic structure and properties such as enhanced coercivity. The authors report on the investigation of the statics and the dynamics of ordered square antidot arrays with 180 nm period and 70 nm antidot diameter that have been prepared by vapor deposition of 20 nm Permalloy on porous anodic alumina substrates. Using superconducting quantum interference device magnetometry, magneto-optical Kerr effect measurements and angular variable ferromagnetic resonance (FMR) spectroscopy at 9.2 GHz, together with micromagnetic simulations several local magnetization modes, were identified. The enhanced FMR linewidth with increasing out-of-plane angle of the magnetic field is not due only to demagnetization effects, but is rather attributed to two magnon scattering processes. This feature is assumed to originate from the surface roughness [wrms(L=1...

Electronic Coupling through Intramolecular π–π Interactions in Biscobaltocenes: A Structural, Spectroscopic, and Magnetic Study

The paramagnetic dinuclear complexes 1,8-bis(cobaltocenyl)naphthalene (2) and 1,8-bis[(pentamethyl-η(5)-cyclopentadienyl)(η(5)-cyclopentadiendiyl)cobalt(II)]naphthalene (4) were synthesized. The molecular structures were characterized by X-ray structure analysis and consisted of two cobaltocenes linked through a distorted naphthalene clamp. Electronic interactions between the two cobalt atoms were observed by cyclic voltammetric studies. Superconducting quantum interference device (SQUID) measurements of the pure compounds and diluted in their diamagnetic iron derivatives, as well as variable-temperature NMR spectroscopy experiments in solution are presented. Magnetic measurements revealed an antiferromagnetic coupling of the electrons in complexes 2 and 4. From NMR spectroscopy experiments, Curie behavior in the temperature range from -60 to +60 °C can be deduced. The electronic structure and magnetic behavior is supported by results of broken-symmetry DFT and multireference calculations along with UV/Vis spectroscopic data, which revealed an intramolecular through space π-π interaction between the cobaltocene units.