Dietmar Baither

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.

Plastic deformation of zirconia single crystals: a review

Abstract The high-temperature deformation behaviour of zirconia single crystals stabilised with yttria is reviewed. Cubic or fully stabilised zirconia (FSZ), which is considered the matrix of high-strength partially stabilised zirconia (PSZ), deforms plastically down to 400°C without confining hydrostatic pressure. The relevant deformation behaviour above about 1200°C is characterised by athermal dislocation motion mainly on cube slip planes and diffusion-controlled recovery. Tetragonal polydomain zirconia or t′ zirconia consists of plate-like tetragonal domains alternately stacked to form large colonies. These colonies are arranged in a characteristic way to fill the whole crystal volume, t′ zirconia shows ferroelastic behaviour preceding dislocation plasticity. In tension, a tetragonal single crystal forms containing residual defects. The data available indicate that the coercive stress strongly depends on temperature. In situ straining experiments in a high-voltage electron microscope show an instantaneous switching of the individual tetragonal domains with the domain boundaries moving sidewise through the domains. Dislocations moving after the ferroelastic deformation are strongly bowed. The nature of the pinning agents is not clear yet. PSZ crystals are thought to consist of a cubic matrix and precipitates of the tetragonal phase of a structure similar to that of the colonies in t′ zirconia. Complete dislocations in the cubic matrix moving on cube planes are partial dislocations in two of the three tetragonal variants of the precipitates. They have then to produce a stacking fault or antiphase boundary like defect. The strong age-hardening and overageing experimentally observed can be explained by a decreasing width of the matrix channels between the precipitates and an increasing width of the domains within the colonies. Recent in situ studies in a high-voltage electron microscope have shown that the precipitates in PSZ may undergo ferroelastic deformation, too. Besides, in a number of cases the matrix of PSZ crystals turned out to be tetragonal rather than cubic. Thus, the formation of a tetragonal single crystal containing residual defects as during the ferroelastic deformation in t′ zirconia should affect the subsequent dislocation plasticity. The stacking fault or antiphase boundary like defects have experimentally been observed on 110 planes, however, not on the usual 100 slip planes. Thus, in spite of numerous experimental results a number of questions are still open particularly concerning the plastic deformation of PSZ crystals.

Double strengthening of copper by dissolved gold-atoms and by incoherent SiO2-particles: how do the two strengthening contributions superimpose?

Abstract Copper single crystals have been simultaneously strengthened by solved gold-atoms and by incoherent SiO 2 -particles. The critical resolved shear stress (CRSS) τ s of a binary copper–gold solid solution and the CRSS τ t of the same solid solution additionally strengthened by SiO 2 -particles have been measured in the temperature range 90–283 K. From the experimentally established temperature dependences of τ s and τ t and from the theoretically known one of τ p (=CRSS if the SiO 2 -particles are the only strengtheners present), the function τ t ( τ s , τ p ) has been derived: τ t =( τ k s + τ k p ) 1/ k , with k ≈1.8. This result is at variance with a linear relationship suggested earlier by Ebeling and Ashby [Phil. Mag. 13 (1966) 805]. The bearings of the present findings on the evaluation of experimental data on dispersion strengthening are evident.

Quantification of the detrimental effects of precipitate free zones on the yield strength of a superalloy

Abstract Precipitate free zones along grain boundaries reduce the yield strength σ y of precipitation hardened polycrystals. This reduction Δ σ y has been measured as a function of the width w of the zones in the γ ′ -strengthened superalloy NIMONIC PE16. The maximum values of Δ σ y / σ y and w amount to 22% and to 440 nm, respectively.

Strengthening of an L12-ORDERED γ'-intermetallic by disordered γ-particles. Part II : Measurements of the CRSS and tem observations of dislocation processes in Ni69Co9Al18Ti4

Abstract Strengthening of single crystals of an L1 2 -long-range ordered γ′-phase by coherent disordered spherical f.c.c. γ-precipitates has been experimentally investigated. The overall composition of the material is (at.%): Ni 69, Co 9, Al 18, Ti 4. The critical resolved shear stress (CRSS) has been measured at 253 K for homogeneous, i.e. γ-free specimens and for specimens strengthened by three different γ-precipitate dispersions. The experimental results for the CRSS are in good agreement with those obtained by computer simulations (Part I) and are satisfactorily represented by Friedels relation. The relevant configurations of dislocations have been studied by transmission electron microscopy.

Plastic deformation of cubic zirconia at temperatures between 1150 and 700°C

Abstract The plastic deformation of cubic ZrO 2 single crystals containing 11 mol.% Y 2 O 3 , which were considered to be brittle below about 1100°C, was studied in compression along [1¯12¯] between 700 and 1150°C. Single slip was observed on a (001)[11¯0] slip system, which is the primary slip system at higher temperatures. Screw dislocations of zig—zag shape in narrow slip bands dominate the microstructure. The strain rate and temperature sensitivities of the flow stress at low temperatures can be interpreted by the interaction between dislocations and small precipitates. At 1150°C, serrated flow occurs and the deformation has athermal character.

Plastic Deformation of Cubic Zirconia Single Crystals at 1400 °C

Cubic zirconia single crystals stabilized with 11 mol% yttria were deformed in air at 1400 °C and around 1200 °C at different strain rates along [11-2] and [100] compression directions. The strain rate sensitivity of the flow stress was determined by strain rate cycling and stress relaxation tests. The microstructure of the deformed specimens was investigated by transmission high-voltage electron microscopy, inlcuding contrast extinction analysis for determining the Burgers vectors as well as stereo pairs and wide-angle tilting experiments to find the active slip planes. At deformation along [11-2], the primary and secondary slip planes are of {100} type. Previous experiments had shown that the dislocations move easily on these planes in an athermal way. During deformation along [100], mainly dislocations on {110} planes are activated, which move in a viscous way by the aid of thermal activation. The discussion of the different deformation behaviours during deformation along [11-2] and [100] is based on the different dynamic properties of dislocations and the fact that recovery is an essential feature of the deformation of cubic zirconia at 1400 °C. The results on the shape of the deformation curve and the strain rate sensitivity of the flow stress are partly at variance with those of previous authors.

Superellipsoids: A unified analytical description of the geometry of nanoscale second-phase particles of any shape

An equation for the analytical description of the geometry of nanoscale second-phase particles of any shape (spheres, cubes, plates, cylinders, and fibers) is presented: they are elegantly described as superellipsoids. The respective equation involves four parameters, which can be experimentally derived from just one micrograph.

In-situ transmission electron microscopy study of dislocation processes at precipitate-free zones in a γ ′-strengthened superalloy

In precipitation-strengthened polycrystals, precipitate-free zones (PFZs) often form along grain boundaries. These PFZs lower the yield strength. In this investigation, thin foils of the commercial γ′-strengthened nickel-based superalloy Nimonic PE16 have been strained inside a transmission electron microscope and the relevant dislocation processes in the PFZs and in the γ′-strengthened material next to them have been observed under load. Since the PFZs are only solid solution strengthened, they are softer than the interior of the γ′-strengthened grains. Many different slip systems are activated in the PFZs even at relatively low external stresses. Multiple slip allows for compatible deformation of neighbouring grains. Extensive cross-slip and double cross-slip in the PFZs lead to a high dislocation multiplication rate. Easy creation of dislocations in the PFZs and pile-ups at the border between the PFZs and the γ′-strengthened interior of the grains enhance the propagation of slip across grain boundaries and thus lower the yield strength of the material.

The effects of precipitation free zones on order strengthening in NIMONIC PE16 bi-crystals

Bi-crystals of the nickel-base superalloy NIMONIC PE16 have been used as model systems to study the influence of precipitation free zones (PFZ) at grain boundaries on the critical resolved shear stress. Analyses of the slip lines and dislocation configurations have been performed. After ageing for different times, the samples have been compression tested. In addition to the primary slip systems in the interior of the grains, secondary ones were activated in a grain boundary affected zone. They allowed the compatible deformation. Influences of the PFZs were attributed to an easier penetration of slip bands into the neighbouring grains.