Thermally-induced Relaxation Research Articles

Structurally Oriented Nano-Sheets in Co Thin Films: Changing Their Anisotropic Physical Properties by Thermally-Induced Relaxation.

We show how nanocrystalline Co films formed by separated oblique nano-sheets display anisotropy in their resistivity, magnetization process, surface nano-morphology and optical transmission. After performing a heat treatment at 270 °C, these anisotropies decrease. This loss has been monitored measuring the resistivity as a function of temperature. The resistivity measured parallel to the direction of the nano-sheets has been constant up to 270 °C, but it decreases when measured perpendicular to the nano-sheets. This suggests the existence of a structural relaxation, which produces the change of the Co nano-sheets during annealing. The changes in the nano-morphology and the local chemical composition of the films at the nanoscale after heating above 270 °C have been analysed by scanning transmission electron microscopy (STEM). Thus, an approach and coalescence of the nano-sheets have been directly visualized. The spectrum of activation energies of this structural relaxation has indicated that the coalescence of the nano-sheets has taken place between 1.2 and 1.7 eV. In addition, an increase in the size of the nano-crystals has occurred in the samples annealed at 400 °C. This study may be relevant for the application in devices working, for example, in the GHz range and to achieve the retention of the anisotropy of these films at higher temperatures.

Implantation-induced disorder in amorphous Ge: Production and relaxation

Extended X-ray absorption fine structure and Raman spectroscopy have been utilised to measure implantation-induced micro-structural modifications in amorphous Ge including increases in bond length, broadening of the bond-angle distribution, and non-Gaussian static disorder as functions of ion dose. The resulting evolution of the inter-atomic distance distribution, over an ion dose range extending two orders of magnitude beyond that required for amorphisation, demonstrates the influence of implant conditions on amorphous phase structure. Results are attributed to increased fractions of three- and fivefold coordinated atoms as a means of accommodating implantation-induced point defects in the amorphous phase. In contrast, a common, ion-dose-independent structure is apparent following low-temperature, thermally-induced relaxation as consistent with the annealing of point defects in the amorphous phase. Structural relaxation is manifested by reductions in both bond-length and bond-angle distortion and the relaxation enthalpy for each component has been calculated separately.

Ligand binding to heme proteins: the effect of light on ligand binding in myoglobin.

Extended illumination slows the rebinding of CO to myoglobin after photodissociation at cryogenic temperatures. Two types of models have been put forward to explain the effect: motions of the CO within the heme pocket or conformational transitions of the protein. To resolve this ambiguity, we have studied the effect of extended illumination on ligand binding to horse and sperm whale myoglobin (hMb and swMb) with temperature-derivative spectroscopy, monitoring the reaction in the CO stretch bands in the infrared and the conformation-sensitive band III near 760 nm. The experiments show that the stretch frequency of the photodissociated CO does not change upon illumination, implying that the slowing of the CO rebinding is caused by conformational relaxation of Mb from the bound state toward the deoxy structure. The light-induced relaxation (LIR) depends on the number of photons absorbed but not on the light intensity or duration separately. LIR occurs on photon absorption in either the bound or photodissociated state and depends on the temperature at which the MbCO is illuminated. The LIR proceeds in jumps through a small number of conformational substates. The effective barrier for rebinding increases with each step. The substates populated are similar to those found in the thermally-induced relaxation (TIR) that is observed above 160 K. LIR depends markedly on the structural details; it differs for swMbCO and hMbCO and even for the three A substates of swMbCO. Pronounced differences exist between the effects in MbCO and MbO2. The similarity of LIR and TIR leads to a revised model for ligand binding to swMbCO and hMbCO, in which the relaxation is crucial for the escape of the ligand from the pocket, as was first suggested by Friedman [Friedman, J. M. (1985) Science 228, 1273-1280].