Guido Schmitz

Magnetic nano-oscillator driven by pure spin current

With the advent of pure-spin-current sources, spin-based electronic (spintronic) devices no longer require electrical charge transfer, opening new possibilities for both conducting and insulating spintronic systems. Pure spin currents have been used to suppress noise caused by thermal fluctuations in magnetic nanodevices, amplify propagating magnetization waves, and to reduce the dynamic damping in magnetic films. However, generation of coherent auto-oscillations by pure spin currents has not been achieved so far. Here we demonstrate the generation of single-mode coherent auto-oscillations in a device that combines local injection of a pure spin current with enhanced spin-wave radiation losses. Counterintuitively, radiation losses enable excitation of auto-oscillation, suppressing the nonlinear processes that prevent auto-oscillation by redistributing the energy between different modes. Our devices exhibit auto-oscillations at moderate current densities, at a microwave frequency tunable over a wide range. These findings suggest a new route for the implementation of nanoscale microwave sources for next-generation integrated electronics.

Phosphorus segregation in nanocrystalline Ni–3.6 at.% P alloy investigated with the tomographic atom probe (TAP)

Abstract The microstructures of electroless plated and thermally aged nanocrystalline nickel–3.6 at.% phosphorus layers were investigated on an atomic scale with a tomographic atom probe (TAP). After heat treatments at 250 and 400°C, a continuous P-segregation in the grain boundaries of the nanocrystalline structure was directly proved for the first time. This segregation effect explains the comparatively high thermal stability of the material. Assuming the existence of a metastable equilibrium, a simple mass balance calculation, which uses experimentally determined data exclusively, makes it possible to predict grain sizes of other NiP alloys within the thermal stability region.

On the mechanism of the binary Cu/Sn solder reaction

The solder reaction of Cu and molten pure Sn is studied by scanning and transmission electron microscopy. Similar as reported for SnPb solder, the intermetallic Cu6Sn5 product is formed in a scallop-like morphology and a growth kinetic proportional to the cube root of time is found. Size distributions and shape of the scallops are determined experimentally. Comparing the binary reaction couple Cu/Sn with the technical combination Cu/SnPb, a significant difference in the length to width aspect ratio of individual scallops is noticed. We demonstrate by transmission electron microscopy that neighbored scallops are not separated by channels of solder, but by grain boundaries. Thus, grain boundary transport is the rate controlling step and the observed variation in scallop shape is due to differences in the interface tensions of the two reaction couples.

On the field evaporation behavior of dielectric materials in three-dimensional atom probe: a numeric simulation.

As a major improvement in three-dimensional (3D) atom probe, the range of applicable material classes has recently been broadened by the establishment of laser-assisted atom probes (LA-3DAP). Meanwhile, measurements of materials of low conductivity, such as dielectrics, ceramics, and semiconductors, have widely been demonstrated. However, besides different evaporation probabilities, heterogeneous dielectric properties are expected to give rise to additional artifacts in the 3D volume reconstruction on which the method is based. In this article, these conceivable artifacts are discussed based on a numeric simulation of the field evaporation. Sample tips of layer- or precipitate-type geometry are considered. It is demonstrated that dielectric materials tend to behave similarly to metals of reduced critical evaporation field.

Design of a laser-assisted tomographic atom probe at Münster University

To benefit from the latest technical improvements in atom probe analysis, a new tomographic atom probe has been built at the University of Münster, Germany. The instrument utilizes a femtosecond laser system with a high repetition rate combined with the ability of using a micrometer-sized extraction electrode and a wide angle configuration. Since field evaporation is triggered by laser pulses instead of high-voltage pulses, the instrument offers the ability to expand the range of analyzed materials to poorly conducting or insulating materials such as oxides, glasses, ceramics, and polymeric materials. The article describes the design of the instrument and presents characterizing measurements on metals, semiconductors, and oxide ceramic.

Laser-assisted atom probe tomography of oxide materials.

Atom probe tomography provides a chemical analysis of nanostructured materials with outstanding resolution. However, due to the process of field evaporation triggered by nanosecond high voltage pulses, the method is usually limited to conductive materials. As part of recent efforts to overcome this limitation, it is demonstrated that the analysis of thick NiO and WO3 oxide layers is possible by laser pulses of 500 ps duration. A careful analysis of the mass spectra demonstrates that the expected stoichiometries are well reproduced by the measurement. The reconstruction of lattice planes proves that surface diffusion is negligible also in the case of thermal pulses.

Decomposition of an AlLi alloy—The early stages observed by HREM

Abstract Using HREM the decomposition of an Al7 at.% Li alloy during isothermal aging at 463 K was studied. Ordered precipitates with radii down to 0.5 nm were resolved. The size distributions of the precipitates and the decomposition parameters such as mean radius, particle density, and precipitated volume fraction were measured. The time dependence of these parameters is well described by a modified Langer-Schwartz model. Thus, at the aging conditions chosen the decomposition follows a classical nucleation and growth mechanism. The analysis of the decomposition process yields some hints for a congruent ordering reaction, which takes place during quenching.

Mechanism of Intermediate Temperature Embrittlement of Ni and Ni-based Superalloys

Ni-based superalloys play an important role in aircraft engine propulsion. However, many experiments demonstrated that these alloys as well as Ni always show ductility loss at intermediate temperature, which constrains further development. A comparison of published papers by various authors reveals considerable differences in understanding the mechanism of intermediate temperature embrittlement. To clarify this situation, the present article first confirms the generality of intermediate temperature embrittlement of Ni and Ni-based alloys by the experimental results reported in the literature. The existing interpretations of the mechanism are then outlined. Based on the generality, these interpretations are discussed through the representative investigations on intermediate temperature embrittlement. It is shown that the mechanism of intermediate temperature embrittlement has not been satisfactorily explained yet and “nonequilibrium interface segregation” of impurities taking into account the effect of strain rate may be the origin of intermediate temperature embrittlement of Ni and Ni-based superalloys. Future research directions aiming at the reason of abnormal fracture mode, the effect of the state of applied stress, the influence of strain rate, and the development of the theory nonequilibrium grain boundary segregation, are suggested to provide a complete understanding of intermediate temperature embrittlement.

A full-scale simulation approach for atom probe tomography

A versatile approach for simulation of APT measurements is presented. The model is founded on a Voronoi cell partition of 3D space. The partition is used in dual role: First, the atomic structure of the field emitter is depicted in a one to one relationship by single Wigner-Seitz cells. Second, the construction of an adaptive tetrahedral mesh enables solving the Poisson equation on length scales covering seven orders of magnitude. Ion trajectories are computed in full-length comparable to experiments. Contrary to former simulation approaches the sequence of desorbing atoms is determined by field-induced polarization forces. Both results for cubic lattices in <001>, <011>, and <111> orientation are presented and the simulation of an APT measurement of a complex crystalline/amorphous layer structure is demonstrated. The example of a grain boundary addresses the new possibility of constructing models with structural defects. In this case, the simulation reveals strong artifacts in the reconstruction even if homogenous evaporation threshold is assumed.

Triple junction transport and the impact of grain boundary width in nanocrystalline Cu.

Triple junctions (TJ), singular topological defects of the grain boundary (GB) structure, get a dominant role for grain growth and atomic transport in nanocrystalline matter. Here, we present detailed measurements by atom probe tomography, even of the temperature dependence of TJ transport of Ni in nanocrystalline Cu in the chemical regime of interdiffusion. An unexpected variation of the effective width of merging GBs with temperature is detected. It is demonstrated that proper measurement of TJ transport requires taking into account this remarkable effect. TJ diffusion is found to be a factor of about 200 faster than GB diffusion. Its activation energy amounts to only two-thirds of that of the GB.