H. Schlörb

Highly coercive electrodeposited FePt films by postannealing in hydrogen

The properties of electrodeposited films subsequently annealed in H2 are reported and compared with those of vacuum-annealed samples. Annealing in hydrogen reduces the oxygen content incorporated during electrodeposition, resulting in significantly higher magnetization values. Phase formation is enhanced by hydrogen and L10 ordering starts at temperatures as low as 350°C. In addition, grain growth is hindered. These effects contribute to the high coercivity of 1.1T achieved after annealing at 600°C in H2.

Highly ordered palladium nanodots and nanowires from switchable block copolymer thin films

We demonstrate a new approach to fabricate highly ordered arrays of nanoscopic palladium dots and wires using switchable block copolymer thin films. The surface-reconstructed block copolymer templates were directly deposited with palladium nanoparticles from a simple aqueous solution. The preferential interaction of the nanoparticles with one of the blocks is mainly responsible for the lateral arrangement of the nanoparticles inside the pores of the templates in addition to the capillary forces. A subsequent stabilization by UV-irradiation followed by pyrolysis in air at 450 degrees C removes the polymer to produce highly ordered metallic nanostructures. We extended this approach to micellar films to obtain metallic nanostructures. This method is highly versatile as the procedure used here is simple, eco-friendly and provides a simple approach to fabricate a broad range of nanoscaled architectures with tunable lateral spacing, and can be extended to systems with even smaller dimensions.

Phase formation, microstructure, and hard magnetic properties of electrodeposited FePt films

FePt films of different compositions have been electrodeposited on Cu coated Si substrates. After deposition films were annealed at temperatures up to 900 °C for 10 min. Phase formation, microstructure, and magnetic properties are analyzed. From these measurements, necessary conditions to obtain good hard magnetic properties are concluded. Remanence reaches a maximum value at 800 °C annealing temperature, while coercivity continuously increases for annealing temperatures up to 900 °C. A coercivity of 0.42 T has been achieved in this case.

Electrodeposition of Fe-based Magnetic Alloy Nanowires

Abstract The electrochemical fabrication of nanowires of the Fe-based magnetic alloys Co-Fe, Fe-Ga and Fe-Pd is reported. Co-Fe is deposited potentiostatically from a simple sulfate electrolyte. The influence of chemical composition, diameter and length of the wires on the magnetic properties of both single wires and arrays of wires embedded in the AAO matrix is discussed. The more challenging Fe-Ga and Fe-Pd alloys required the development of new electrolyte compositions. In order to obtain homogeneous nanowires with the desired composition and length alternating potential pulses were applied and the pulse potentials had to be adjusted.

Remanence enhancement in nanoscaled electrodeposited FePt films

L10 FePt films with a small grain size of around 13nm have been prepared by electrodeposition and postannealing at a low annealing temperature of 400°C. A high remanence of up to 0.9T is achieved due to remanence enhancement by exchange coupling between the nanosized grains. Coercivity increases with longer annealing time as the L10 order becomes more complete. The resulting maximum energy product reaches a high value of 70kJ∕m3 after annealing for 120min, exceeding the maximum energy products so far obtained in electrodeposited hard magnetic films.

Nanowire filled polymer films for 3D system integration

In the present paper, the use of high aspect ratio metallic nanowires (NWs) as functional interconnects between three-dimensionally stacked chips is proposed. First practical preparation steps, as there are the preparation of templates and the deposition of Ag-NWs, are presented. Later on, the applicational technology for vertically aligned NWs is proposed as their embedment into a polymer matrix resulting in an anisotropically conductive composite film. This is discussed in both technological and functional aspects.

TEM investigations on the local microstructure of electrodeposited galfenol nanowires

The local microstructure of Fe-Ga nanowires is investigated considering dependence on the deposition technique. Using a complexed electrolyte, smooth and homogeneous Fe80Ga20 nanowires are deposited into anodic aluminum oxide templates by either applying pulse potential or potentiostatic deposition technique. At optimized deposition conditions the wires show the desired composition of Fe80±2Ga20±2 without a gradient along the growth direction. Composition distribution, structure and microstructure are examined in detail and reveal only minor differences. Line EELS and crystal lattice measurements reveal a negligible oxygen content for both preparation routines. Neither Fe/Ga oxides nor hydroxides were found. Both potentiostatically deposited as well as pulse deposited nanowires exhibit a preferred 〈110〉orientation, the latter with slightly larger crystals. Different contrast patterns were found by TEM that appear more pronounced in the case of pulse deposited wires. High resolution transmission electron microscopy analysis and comparison of differently prepared focused ion beam lamellas reveal that these contrasts are caused by defects in the alternating potential deposition itself and are not induced during the TEM preparation process. The alternating potential mode causes periodic growth thereby inducing different layers with reduced wire thickness/defects at the layer interfaces.

Quasi-periodic magnetization reversal of ferromagnetic nanoparticles induced by torsional oscillations in static magnetic fields

In order to reverse the magnetization of small ferromagnetic particles it is necessary to overcome an energy barrier, which is mainly defined by the magnetic anisotropy. Usual reversal stimuli include the application of static or time-dependent external magnetic fields, thermal activation, spin transfer torque, or combinations thereof. Here, we report on repeated, quasi-periodic magnetization reversal in single-domain particles that are exposed to a constant magnetic field perpendicular to the magnets easy axis. The continuous sequence of reversals is induced by torsional oscillations of the magnets anisotropy landscape, which are caused by angular oscillations of the magnets body. In our experiments, a nickel nanowire constitutes both a mechanical resonator and a nanomagnetic sample with uniaxial anisotropy. We measure the transient flexural vibration behavior by electron beam based methods and find strong signatures of periodic magnetization switching between two magnetic states of the nanowire. Our system can be modeled as a driven damped harmonic oscillator under the influence of switchable magnetostatic interactions.

Fe-Based Magnetic Alloy Electrodeposition for Thin Films and Template Based Nanostructures

The electrochemical fabrication of the Fe-based soft magnetic CoFe alloy and the shape memory active Fe-Pd alloy is reported. We examine the influence of electrolyte composition and deposition regime or post treatment on morphology, composition and structural development of the deposits. Up to 10 µm thick CoFe films with 55 at.% Fe were achieved from simple and additive adjusted electrolytes by potentiostatic or alternating potential deposition regime. Micropatterned deposits of smooth surface morphology and high saturation magnetization applicable as magnetic field gradient generator can be prepared best by the combination of SDS additive and alternating potential regime. The Fe-Pd alloy has been deposited and heat treated to achieve a composition with 30 at.% Pd and to adjust the crystal structure. For thin films and nanowires the transition from bcc to fcc phase is accomplished. This represents a good starting point towards shape memory active materials and their further physical characterization.

Electric field control of magnetization and anisotropy in ultrathin Fe und FePt/Fe films

The control of magnetic properties by an electric field is exciting both from the point of view of applications and fundamental research. Low-power electrical switching of magnetic properties is e.g. discussed as promising alternative to temperature control as in heat assisted magnetic recording. Magnetoelectric coupling is known for one- or two-phase multiferroic materials and magnetic semiconductors but is, in these cases, restricted to low temperatures or a fixed substrate. More and more studies report on an influence of electric field on the magnetism also of nanostructured metals [1-6]. The advantage here is that an effect can be exploited at room temperature and may be very large due to the strong ferromagnetism. The electric field can be applied either by using solid dielectric layers or by liquid electrolytes. Many magnetic properties as e.g. coercivity, saturation magnetization, anisotropy and Curie temperature have been shown to be electric field dependent and large effects have been achieved already. Materials studied include Fe, Co, FePt, FePt, CoPt ultrathin films or nanoporous structures. In almost all cases, however, the simple picture of an increase/decrease of DOS at the metal surface is not sufficient to explain the dependencies. Enlightening the underlying mechanism requires a detailed interface analysis to understand the influence of interface chemistry, stress and strain for the distinct sample and gating architecture. The role of oxygen at the interface has been found to be crucial. Reversible potential dependent oxidation and reduction reactions as well as oxygen hybridization have been discussed as origin for magnetic property changes [2-6]. Our concept follows this argumentation and seeks to by intention exploit reversible electrochemical surface reactions involving magnetic species. This allows addressing surface magnetism by an external voltage, opening the way to controlled electrical switching of magnetic properties.