Interatomic repulsion softness directly controls the fragility of supercooled metallic melts.

We present an analytic scheme to connect the fragility and viscoelasticity of metallic glasses to the effective ion-ion interaction in the metal. This is achieved by an approximation of the short-range repulsive part of the interaction, combined with nonaffine lattice dynamics to obtain analytical expressions for the shear modulus, viscosity, and fragility in terms of the ion-ion interaction. By fitting the theoretical model to experimental data, we are able to link the steepness of the interionic repulsion to the Thomas-Fermi screened Coulomb repulsion and to the Born-Mayer valence electron overlap repulsion for various alloys. The result is a simple closed-form expression for the fragility of the supercooled liquid metal in terms of few crucial atomic-scale interaction and anharmonicity parameters. In particular, a linear relationship is found between the fragility and the energy scales of both the screened Coulomb and the electron overlap repulsions. This relationship opens up opportunities to fabricate alloys with tailored thermoelasticity and fragility by rationally tuning the chemical composition of the alloy according to general principles. The analysis presented here brings a new way of looking at the link between the outer shell electronic structure of metals and metalloids and the viscoelasticity and fragility thereof.The support of the Technische Universität München Institute for Advanced Study, funded by the German Excellence Initiative and the European Union 7th Framework Programme under Grant Agreement 291763, is acknowledged.This is the author accepted manuscript. The final version is available from PNAS via http://dx.doi.org/10.1073/pnas.150374111

Strain Rate Induced Amorphization in Metallic Nanowires

Using molecular dynamics simulations with a many-body force field, we studied the deformation of metal alloy nanowires subjected to various strain rates. For all strain rates, the Ni nanowire is elastic up to of 7.5 % strain with a yield stress of 5.5 GPa, far above that of bulk Ni. At high strain rates, the crystalline phase transforms continuously to an amorphous phase, exhibiting a dramatic change in atomic short-range order. The amorphization transition is associated with the vanishing of the tetragonal shear elastic constant perpendicular to the tensile direction

Spin polarization in half-metals probed by femtosecond spin excitation

Müller GM, Walowski J, Djordjevic M, et al. Spin polarization in half-metals probed by femtosecond spin excitation. NATURE MATERIALS. 2009;8(1):56-61.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, Co2MnSi, Fe3O4, La0.66Sr0.33MnO3 and CrO2