M. Weisheit

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.

Textured growth of highly coercive L10 ordered FePt thin films on single crystalline and amorphous substrates

FePt films have been prepared by pulsed laser deposition at a temperature of 800 °C onto (001) oriented single crystalline (MgO, SrTiO3) as well as amorphous substrates (Si3N4, glass) with and without a (001) textured MgO buffer layer. Although all films have been prepared under identical conditions, significant differences in coercivity are observed. Very high values of up to 5.6 T have been measured for the epitaxial films, while on the amorphous substrates the coercivities range between 0.8 and 2.0 T. We analyze whether changes in intrinsic or extrinsic properties due to a modified microstructure are responsible for these differences.

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.

The interface of laser deposited Cu/Ag multilayers: evidence of the ‘subsurface growth mode’ during pulsed laser deposition

Abstract Using Cu/Ag multilayers, we show that the deposition of a significant number of highly energetic ions, characteristic of pulsed laser deposition, leads to resputtering of the already-deposited film and implantation into the existing film. The consequent film growth is described by a ‘subsurface growth mode’, in which the material does not grow on top of the already-deposited film, but rather within the topmost few monolayers. This hypothesis is confirmed using three in situ diagnostic methods: rate monitoring which was found to exhibit a non-linearity at the beginning of each new layer, a result of resputtering of the growing film; resistance measurements showing a drop in conductance after beginning a new Cu layer, an effect attributed to implantation and alloying at the interface; and electron diffraction to determine the variation in lattice parameters during growth, which can be interpreted as a consequence of alloying.

H- and D distribution in metallic multilayers studied by 3-dimensional atom probe analysis and secondary ion mass spectrometry

Abstract Pd/Nb- and Fe/V multilayers were prepared by either ion sputtering or laser ablation. They were deposited on tungsten tips for field ion microscopy using the 3-dimensional atom probe. D was added by doping from the gas phase. In agreement with thermodynamic data D is accumulated in the metal with the more negative heat of solution, i.e. V or Nb. However, SIMS depth profiles of H concentration measured at room temperature show a pronounced enrichment in the top regions of both Nb- and V layers. This is also observed in a deuterium depth profile obtained by the 3d atom probe measured at 60 K despite the moderate removal of atoms by field evaporation which does not give rise to the formation of lattice defects when compared to sputtering. Thus we assume that surface segregation is the reason for H/D accumulation in surface near regions. The effect vanishes if measurements by the atom probe were made at 20 K where the D mobility is too low to allow D transport to the surface during the time of measurements. With the superior depth resolution of the 3d atom probe being less than 0.5 nm we were able to show that at both the V/Fe and the Nb/W interface depletion of deuterium occurs in a range of about 0.5 nm within the Nb- or V layer due to intermixing of adjacent metals.

Influence of hydrogen on the growth of FePt thin films

FePt films have been prepared by pulsed laser deposition on MgO(100) in a forming gas atmosphere of 5% hydrogen in argon. At a pressure and temperature combination of 0.1 mbar and 800 °C, smooth continuous and well ordered L10 phase films can be obtained. The anisotropy field is close to the bulk value, while the coercivity is comparatively low at 0.15 T. This is in contrast to films deposited in vacuum, which are granular in nature and show huge coercivity. It is proposed that nucleation centers for island growth introduced by hydrogen on the MgO surface are responsible for this difference in growth mode. The temperature and pressure dependence of the surface morphology are explained by this assumption.

Magnetization Reversal of Highly Coercive FePt Examined With Pulsed Microcoils

The switching of ultra-high coercivity FePt thin films has been studied by pulsed magnetic fields of up to 25 T, generated with microcoils of 50 mum diameter and using a fast magneto-optical polar Kerr effect setup. Whereas under static measurements, the coercive field reaches 5.5 T, under pulsed magnetic field it approaches 8 T. An approximation of the Landau-Lifshitz-Gilbert (LLG) equation was used to calculate the magnetization response to the field pulse. Good agreement between experiment and simulation is observed if a value of 0.1 is assumed for the damping constant

Fe–Pd thin films as a model system for self-organized exchange coupled nanomagnets

In equilibrium the Fe–Pd system on the iron rich side of the phase diagram demixes into Fe and L10-ordered FePd. Here, we examine the suitability of the demixing process for self-organized formation of exchange coupled thin film magnets. In this way the benefit of the high magnetization of Fe is combined with the high magnetocrystalline anisotropy of FePd. By using combinatorial methods the influence of composition and thickness on structure, microstructure, and magnetic properties is analyzed. Experiments show the different thermodynamic and kinetic conditions required for demixing and ordering. In particular, for nanostructures the interface energy during demixing must be considered.

Alloy–ceramic oxide multilayer mirrors for water-window soft x rays

Alloy-ceramic oxide multilayers of Ni(80)Nb(20)- MgO with 3.56-nm period have been made by use of the pulsed-laser deposition technique and characterized by means of grazing-incidence x-ray reflectivity. The interface roughness was found to be only ~0.35 nm at the two interfaces, Ni(80)Nb(20)- MgO and MgO-Ni(80)Nb(20), leading to a peak reflectivity of ~38% at the first order. The atomic structure in the two individual layers, Ni(80)Nb(20) and MgO, is found to be amorphous, in agreement with the deposition conditions. These multilayers can be used as mirrors for soft x rays in the angular range 18 degrees to 39 degrees , depending on the actual wavelength of radiation in the water-window region.