Rejuvenation of metallic glasses by non-affine thermal strain.

When a spatially uniform temperature change is imposed on a solid with more than one phase, or on a polycrystal of a single, non-cubic phase (showing anisotropic expansion-contraction), the resulting thermal strain is inhomogeneous (non-affine). Thermal cycling induces internal stresses, leading to structural and property changes that are usually deleterious. Glasses are the solids that form on cooling a liquid if crystallization is avoided--they might be considered the ultimate, uniform solids, without the microstructural features and defects associated with polycrystals. Here we explore the effects of cryogenic thermal cycling on glasses, specifically metallic glasses. We show that, contrary to the null effect expected from uniformity, thermal cycling induces rejuvenation, reaching less relaxed states of higher energy. We interpret these findings in the context that the dynamics in liquids become heterogeneous on cooling towards the glass transition, and that there may be consequent heterogeneities in the resulting glasses. For example, the vibrational dynamics of glassy silica at long wavelengths are those of an elastic continuum, but at wavelengths less than approximately three nanometres the vibrational dynamics are similar to those of a polycrystal with anisotropic grains. Thermal cycling of metallic glasses is easily applied, and gives improvements in compressive plasticity. The fact that such effects can be achieved is attributed to intrinsic non-uniformity of the glass structure, giving a non-uniform coefficient of thermal expansion. While metallic glasses may be particularly suitable for thermal cycling, the non-affine nature of strains in glasses in general deserves further study, whether they are induced by applied stresses or by temperature change.This research was supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan, by NSF China and MOST 973 China, and by the Engineering and the Engineering and Physical Sciences Research Council, UK (Materials World Network project). Y.H.S. acknowledges support from a China Scholarship Council (CSC) scholarship.This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/nature1467

Exchange-coupling of c-axis oriented L1[sub 0]–FePd and Fe in FePd/Fe thin films

This letter reports the control of the c-axis orientation and the magnetic properties of L10 FePd in the well-annealed FePd/Fe thin films Fe(20 nm)/FePd(10 nm)/Fe(5 nm)/SiO2-substrate and Fe(20  nm)/FePd(5 nm)/Fe(5 nm)/SiO2-substrate, with the compositions of dual phase region in phase equilibrium. After annealing at 550 °C for 24 h, the c-axis of L1[sub 0]FePd is controlled to be oriented in the normal direction in the soft Fe matrix. These thin films exhibit the exchange-coupling of hard FePd with soft Fe, and a characteristic spring-magnet behavior showed up reversibly as a significant jump from positive to negative in the magnetization reversal