K. Samwer

Local elastic properties of a metallic glass

The nature of non-crystalline materials causes the local potential energy of a cluster of atoms or molecules to vary significantly in space. Different configurations of an ensemble of atoms in a metallic glass lead therefore to a distribution of elastic constants which also changes in space. This is totally different to their crystalline counterparts, where a long-range order exists in space and therefore a much more unified elastic modulus is expected. Using atomic force acoustic microscopy, we present data which show that the local so-called indentation modulus M indeed exhibits a wide distribution on a scale below 10 nm in amorphous PdCuSi, with ΔM/M≈30%. About 10(4) atoms are probed in an individual measurement. Crystallized PdCuSi shows a variation that is 10-30 times smaller and which is determined by the resolution of the microscope and by the polycrystalline structure of the material.

Giant magnetoresistance in melt spun Cu‐Co alloys

Cu1−xCox alloys with x=0.1 and x=0.2 have been prepared by conventional melt spinning. The rapid solidification process results in an extended solubility of Co in Cu although some Co precipitates already during quenching. In the as‐quenched ribbons, the magnetoresistance (MR) is only of the order of 1.5%. It increases dramatically with the controlled nucleation and growth of Co precipitates from the supersaturated Cu matrix. The highest MR of 11% at 300 K occurs for Cu90Co10 after an aging at about 440 °C when the Co clusters are superparamagnetic. Saturation is possible only after a higher annealing or at lower measuring temperatures. For the optimally annealed samples the MR increases to 36% at 30 K.

CHEMICAL VAPOR SYNTHESIS AND LUMINESCENCE PROPERTIES OF NANOCRYSTALLINE CUBIC Y2O3:EU

Nanocrystalline europium doped yttria was synthesized using a chemical vapor technique. The powder was characterized by x-ray diffraction, transmission electron microscopy, and ultraviolet spectroscopy. For the first time it was possible to obtain single phase Y2O3:Eu nanoparticles crystallized in the cubic structure with an average particle size of only 10 nm. The reflection, excitation, and emission spectra were studied. The nanoparticles show blue shifted absorption bands with respect to coarse grained material.

Beating Crystallization in Glass-Forming Metals by Millisecond Heating and Processing

Resistive heating can be used to rapidly heat a bulk metallic glass without inducing crystallization. The development of metal alloys that form glasses at modest cooling rates has stimulated broad scientific and technological interest. However, intervening crystallization of the liquid in even the most robust bulk metallic glass-formers is orders of magnitude faster than in many common polymers and silicate glass-forming liquids. Crystallization limits experimental studies of the undercooled liquid and hampers efforts to plastically process metallic glasses. We have developed a method to rapidly and uniformly heat a metallic glass at rates of 106 kelvin per second to temperatures spanning the undercooled liquid region. Liquid properties are subsequently measured on millisecond time scales at previously inaccessible temperatures under near-adiabatic conditions. Rapid thermoplastic forming of the undercooled liquid into complex net shapes is implemented under rheological conditions typically used in molding of plastics. By operating in the millisecond regime, we are able to “beat” the intervening crystallization and successfully process even marginal glass-forming alloys with very limited stability against crystallization that are not processable by conventional heating.

Spin polarization in half-metals probed by femtosecond spin excitation

Knowledge of the spin polarization is of fundamental importance for the use of a material in spintronics applications. Here, we used femtosecond optical excitation of half-metals to distinguish between half-metallic and metallic properties. Because the direct energy transfer by Elliot-Yafet scattering is blocked in a half-metal, the demagnetization time is a measure for the degree of half-metallicity. We propose that this characteristic enables us vice versa to establish a novel and fast characterization tool for this highly important material class used in spin-electronic devices. The technique has been applied to a variety of materials where the spin polarization at the Fermi level ranges from 45 to 98%: Ni, Co(2)MnSi, Fe(3)O(4), La(0.66)Sr(0.33)MnO(3) and CrO(2).

Intrinsic inhomogeneities in manganite thin films investigated with scanning tunneling spectroscopy

Thin films of La0.7Sr0.3MnO3 on MgO show a metal insulator transition and colossal magnetoresistance. The shape of this transition can be explained by intrinsic spatial inhomogeneities, which give rise to a domain structure of conducting and insulating domains at the submicrometer scale. These domains then undergo a percolation transition. The tunneling conductance and tunneling gap measured by scanning tunneling spectroscopy were used to distinguish and visualize these domains.

Texture and phase transformation of sputter-deposited metastable Ta films and Ta/Cu multilayers

Two different structures of Ta are known : the bulk bcc structure of α-Ta (a=0.33058 nm) and the metastable tetragonal β-Ta phase (a=0.534 nm, c=0.994 nm). The metastable phase is mainly observed in sputter-deposited thin films and a dependence on film thickness and deposition conditions have been reported. In our study we investigated the structure and the crystallographic orientation of d.c.-magnetron sputtered Ta films using wide-angle X-ray diffraction after preparation and after annealing. Ta films were deposited onto different underlayer materials (Al, SiO 2 and Cu). For Ta/Cu multilayers the single layer thickness and the sputter parameters are varied. It is observed that the Ta structure after deposition mainly depends on the substrate or underlayer material. β-Ta is always observed after sputtering onto SiO 2 or (111)-textured Cu, with the orientation of the Ta grains depending on the Ta-layer thickness, whereas α-Ta is observed on an (111)Al underlayer. Therefore, the bulk equilibrium phase only seems to be forced by an epitaxial relation. During annealing up to 700°C the (100)-oriented grains of β-Ta change their orientation and the metastable β-phase transforms into the equilibrium α-phase with (110) texture. The temperature for structural transformation decreases with increasing Ta layer thickness.

Intrinsic giant magnetoresistance of mixed valence La‐A‐Mn oxide (A=Ca,Sr,Ba) (invited)

A large intrinsic magnetoresistance has been found near the ferromagnetic transition of metallic manganese oxides with perovskite‐type crystal structure. The magnetic and transport properties were measured on bulk and thin‐film La1−xAxMnO3+δ with A=Ca,Sr,Ba. Assuming the double‐exchange model proposed by Zener [Phys. Rev. 81, 440 (1951); 82, 403 (1951)], the strong dependence of the transport properties on the magnetic field and also on the chemical composition is attributed to the mixed Mn3+/Mn4+ valence.

Microstructure-controlled magnetic properties of the bulk glass-forming alloy Nd60Fe30Al10

We report a combination of analytical transmission electron microscopy, small angle neutron scattering, and studies of magnetic properties of the glass-forming alloy Nd60Fe30Al10. These investigations show the existence of an in situ formed finely dispersed nanocrystalline Nd-rich phase embedded in a Fe-rich glassy matrix of a bulk sample. The crystalline phase forms an extended network over the whole sample but its volume fraction is small compared to that of compact phase. Small angle neutron scattering data exhibit power law behavior with an exponent of −2.5 indicating the formation of a mass fractal. The microstructure observed may be related to phase separation in the undercooled liquid which induces a microstructure that can explain the hard magnetic behavior of such an intrinsic composite.

Orientation relationships of epitaxial oxide buffer layers on silicon (100) for high‐temperature superconducting YBa2Cu3O7−x films

The preparation parameters of epitaxially grown buffer layers on silicon (100) wafers were investigated. We found that an in situ removal of the native amorphous SiO2 layer from the Si surface is possible, avoiding the etching of the wafer prior to the deposition. YSZ and Y2O3 were chosen as buffer layers for subsequent YBa2Cu3O7−x thin‐film deposition. The orientation of the thin films during the deposition process was analyzed by RHEED. Different orientations on the substrates are obtained depending on the evaporation parameters. TEM studies of the interfaces, x‐ray diffraction analysis, and measurements of the superconducting properties were made after the deposition of the films.