Direct measurement of intrinsic atomic scale magnetostriction.

Using differential x-ray absorption spectroscopy (DiffXAS) we have measured and quantified the intrinsic, atomic-scale magnetostriction of Fe81Ga19. By exploiting the chemical selectivity of DiffXAS, the Fe and Ga local environments have been assessed individually. The enhanced magnetostriction induced by the addition of Ga to Fe was found to originate from the Ga environment, where lambda;{gamma,2}( approximately (3/2)lambda_{100}) is 390+/-40 ppm. In this environment, 001 Ga-Ga pair defects were found to exist, which mediate the magnetostriction by inducing large strains in the surrounding Ga-Fe bonds. For the first time, intrinsic, chemically selective magnetostrictive strain has been measured and quantified at the atomic level, allowing true comparison with theory.

Verifying DiffEXAFS measurements with differential X-ray diffraction.

Differential EXAFS (DiffEXAFS) is a novel technique for measuring atomic perturbations on a local scale. Here a complementary technique for such studies is presented: differential X-ray diffraction (DiffXRD), which may be used to independently verify DiffEXAFS results whilst using exactly the same experimental apparatus and measurement technique. A test experiment has been conducted to show that DiffXRD can be used to successfully determine the thermal expansion coefficient of SrF(2).

Probing atomic displacements with thermal differential EXAFS.

Differential extended X-ray absorption fine structure (DiffEXAFS) is a novel technique for the study of small atomic strains. Here the development of this technique to the measurement of thermally induced strain is presented. Thermal DiffEXAFS measurements have been performed on alpha-Fe and SrF(2), yielding alpha = (11.6 +/- 0.4) x 10(-6) K(-1) and (19 +/- 2) x 10(-6) K(-1), respectively. These are in good agreement with accepted values, proving the viability of the technique. Analysis has revealed sensitivity to mean atomic displacements of 0.3 fm.

Effect of pressure on magnetoelastic coupling in 3d metal alloys studied with x-ray absorption spectroscopy.

Using x-ray absorption spectroscopy, we have studied the effect of pressure on femtometer-scale bond strain due to anisotropic magnetostriction in a thin FeCo film. At 7 GPa local magnetostrictive strain is found to be larger than at ambient, in agreement with spin-polarized ab initio electronic structure calculations, but contrary to the expected effect of compression on bond stiffness. The availability of high pressure data on local magnetostrictive strain opens new capabilities for validating theoretical predictions and can lead to the development of materials with the desired properties.

Calibration of spectra from dispersive XAS beamlines.

The DXAS Calibration computer program provides a quantitative and automated solution to the problem of calibrating spectra from dispersive XAS beamlines. Such spectra, obtained in arbitrary energy units, are calibrated with respect to the absorption features of a supplied reference spectrum, which has been obtained under similar conditions on a calibrated beamline. In addition to basic energy coordinate transformation parameters, DXAS Calibration supplies instrument corrections to compensate for mismatches in instrument response functions between the dispersive and reference beamlines.

Extended X-ray absorption fine structure (EXAFS) investigations of model compounds for zinc enzymes.

A test of the ability of extended X-ray absorption fine structure (EXAFS) to determine structural information with specific reference to zinc sites in enzymes has been made. X-ray absorption spectra of 18 compounds of zinc have been measured and the nearest-neighbour scattering has been interpreted using a Fourier transform and an ab initio technique. Empirical Zn-N, Zn-O, Zn-S and Zn-Cl amplitude and phase functions have been extracted from Zn(C3H4N2)4(ClO4)2, ZnO, Zn(S2COC2H5)2 and [N(CH3)4]2[ZnCl4], respectively and tabulated as a function of the wavevector with respect to 9660.0 eV X-ray energy. These amplitude and phase functions were then tested with respect to the other 14 compounds. For a single species of atoms in the first coordination shell the interatomic distances can be established to +/- 0.5 pm (+/- 5 x 10(-3) A) whilst when mixed shells exist errors in distances are +/-4 pm (+/- 40 x 10(-3) A). Coordination numbers are given to +/- 16% for the single species case a and +/- 25% for the mixed coordination case. Using the theoretical amplitude and phase functions of McKale et al. [(1988) J. Am. Chem. Soc. 110, 3763-3768] the deduced distances are systematically too small by an average of 0.6 pm (6 x 10(-3) A). The errors in the coordination numbers are 18%.