Thermal differential EXAFS

Differential EXAFS (DiffEXAFS) is a new and novel technique for the study of small atomic strains. It relies on examining tiny differences in x-ray absorption spectra - taken under high-stability, low-noise conditions - generated by unit modulation of some sample bulk parameter. Initial experiments conducted by Pettifer et al. [64] to measure the magnetostriction of FeCo, revealed a sensitivity to atomic displacements of the order of one femtometre (10−15m). This was two orders of magnitude more sensitive than thought possible, based on conventional EXAFS techniques [16] [2]. The mandate for this thesis was to extend DiffEXAFS to the case of samples undergoing temperature modulation - to develop Thermal Differential EXAFS - and in doing so, demonstrate that DiffEXAFS is a generally applicable technique for studying small atomic strains. Topics covered here include the nature of Thermal DiffEXAFS signals, the design, manufacture, and characterisation of apparatus for Thermal DiffEXAFS experiments, and new analysis techniques developed to extract information from DiffEXAFS data. Thermal expansion coefficients have been determined for Fe and SrF2, for temperature modulation of the order of one Kelvin, proving the viability of the technique. Numerically, these were a Fe = (11.6±0.4)×10−6K−1 and a SrF2 = (19±2)×10−6K−1 respectively, which agreed with published values [52] [74]. In these measurements sensitivity to mean atomic displacements of about 0.3 femtometres was achieved. The more interesting case of thermally induced phase transitions has also been studied, with DiffEXAFS measurements taken through the Martensitic phase transition of the Heusler alloy Ni2MnGa. These revealed a hardening of the lattice as the transition was approached in the Martensite phase, agreeing with published trends [93][56], and an accompanying lattice contraction not seen previously

Determination of the magnetostrictive atomic enviroments in FeCoB alloys

The atomic environments of Fe and Co involved in the magnetostriction effect in FeCoB alloys have been identified by differential extended x-ray fine structure (DiffEXAFS) spectroscopy. The study, done in amorphous and polycrystalline FeCoB films, demonstrates that the alloys are heterogeneous and that boron plays a crucial role in the origin of their magnetostrictive properties. The analysis of DiffEXAFS in the polycrystalline and amorphous alloys indicates that boron activates magnetostriction when entering as an impurity into octahedral interstitial sites of the Fe bcc lattice, causing its tetragonal distortion. Magnetostriction would be explained then by the relative change in volume when the tetragonal axis of the site is reoriented under an externally applied magnetic field. The experiment demonstrates the extreme sensitivity of DiffEXAFS to characterize magnetostrictive environments that are undetectable in their related EXAFS spectra

Burdekin, K.: Swastika night. Old Westbury, NY 1985

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)(approximate to (3/2)lambda(100)) is 390 +/- 40 ppm. In this environment, 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