K. Leistner

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.

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.

Thermodynamics and kinetics during pulsed laser annealing and patterning of FePt films

Laser annealing using 25ns pulses of a KrF excimer laser was applied to electrochemical and pulsed laser deposited FePt films of 700nm and 40 or 80nm thicknesses, respectively. The dependence of phase formation and magnetization behavior on the film thickness and the as-deposited state are studied. Models of the film heating are compared to the experimentally observed results. Furthermore a time temperature transformation (TTT) diagram for the ordering of FePt is proposed, summarizing the phase formation in dependence on annealing time and temperature. In agreement with the TTT diagram, laser annealing leads to a disordering of L10-ordered films. By only partially disordering the L10 phase, this approach provides control over the coercivity of hard magnetic FePt films. Local disordering is applied for magnetic patterning of the FePt films with dot patterns in the micrometer size range.

Electrochemically driven variation of magnetic properties in ultrathin CoPt films

Controlled variation of magnetism in ultrathin metallic films by external electric fields is a promising route towards advanced multifunctional devices. In situ magnetic measurements of ultrathin CoPt films in a liquid electrolyte (LiClO4 in dimethyl carbonate-ethylene carbonate) have been performed, dependent on the applied external voltage. Huge hysteretic changes of coercivity (above 200%) and smaller variations of saturation magnetisation (∼4 %) have been identified. The commonly considered concept of changing the electron density of states by double layer charging is not applicable as the changes observed are mainly irreversible. Rather, a strong influence of redox processes, namely surface oxide reduction and Co dissolution, is considered. We argue that by exploiting these redox reactions much higher changes of the magnetic properties are achievable than for double layer charging.

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.

Initial preorder as condition for L10 ordering in ultrathin CoPt films

Materials with high magnetocrystalline anisotropy have been of interest for magnetic data storage for a long time. It is, however, still challenging to achieve sufficient L10 order when dimensions are reduced to the nanometer range needed for these applications. We report on formation of L10 order in continuous epitaxial CoPt films with a thickness below 3 nm. The ordering mechanism for pulsed laser deposited films has been elucidated by a systematic analysis of the thermal and depositional conditions. In addition to deposition temperature and post annealing, deposition rate is found to have a major influence on ordering. High order is achieved at very high and very low deposition rates. We propose that this rate dependent ordering is due to an initial short range order formed by surface diffusion during the deposition step. The realisation of such a preorder is found to be decisive for the progression of ordering during post annealing.

Axial EBIC oscillations at core/shell GaAs/Fe nanowire contacts

Electron beam induced current (EBIC) measurements were carried out in situ in the scanning electron microscope on free-standing GaAs/Fe core-shell nanowires (NWs), isolated from the GaAs substrate via a layer of aluminum oxide. The excess current as a function of the electron beam energy, position on the NW, and scan direction were collected, together with energy dispersive x-ray spectroscopy. A model that included the effects of beam energy and Fe thickness predicted an average collection efficiency of 60%. Small spatial oscillations in the EBIC current were observed, that correlated with the average Fe grain size (30 nm). These oscillations likely originated from lateral variations in the interfacial oxide thickness, affecting the resistance, barrier potentials, and density of minority carrier recombination traps.

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.

A comparison of highly coercive FePt films prepared by pulsed laser deposition and electrodeposition

Two alternative preparation routes for FePt films are compared: pulsed laser deposition (PLD) and electrodeposition. It is described how an optimal microstructure can be obtained for best magnetic properties, and the advantages of both preparation routes are discussed. The low slope of the initial magnetization curve annealed at optimum conditions indicates that coercivity in electrodeposited granular films is controlled by pinning. In contrary, epitaxial films prepared by PLD on MgO(100) show that domain reversal in FePt films reaching the highest coercivity is governed by nucleation.